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Introduction
The Theory of Continental Drift has had a long and turbulent history since it was first proposed
by Alfred Wegener in 1910. Vigorously challenged yet widely ignored, the theory languished for
half a century, primarily due to its lack of a plausible mechanism to support the proposed drift.
With the discovery of sea-floor spreading in the late 1950's and early 60's, the idea was
reinvigorated, this time as the Theory of Plate Tectonics.
Plate tectonics is now almost universally accepted, its mechanisms plausible and to a degree
demonstrable. However, many details of the mechanism are yet to be worked out, and many
theories involving various details of plate tectonics rest on some questionable assumptions. This
set of pages attempts to define some of the basic principles of the mechanism, and to examine
their effect on the creation of landforms.
What follows is NOT a summary of the current thinking about plate tectonics and its
mechanisms; rather, many new, and probably highly controversial, ideas are presented for
consideration. What IS presented is a broad analysis of the basic principles that should apply to
the movements of plates, some new hypotheses about how they apply to convection and
landform formation, and some expected scenarios for differing tectonic events.
For those unfamiliar with the theory of plate tectonics, a separate page - The Basics of Plate
Tectonics - is provided. This summary offers a brief condensation of the basic principles of Plate
Tectonics. A much more comprehensive explanation of Plate Tectonics can be found on the
USGS Web Site.
Lesson 1 The Basics of Plate Tectonics
Introduction: Plates and Boundaries.
The surface of the Earth is divided into approximately six large 'plates', plus a number of smaller
ones. Where they underlie the World's oceans, these plates are composed of the same material as
the fluid mantle, but are supercooled into solid rock, and are typically around 6 to 10 miles (1016 km) thick according to current estimates. (Where the plates contain continental material, their
thickness can vary considerably, as will be discussed in a later chapter.)
The thickness of oceanic plates is defined by the insulating properties of this 'frozen' mantle
material. The surface of the plates is maintained at 39°F (+4°C) - the temperature of deep ocean
water, while the underside is right at the melting point of the mantle material, currently estimated
at around 1100°F (~600°C). Thus a thermal gradient exists vertically across the plates, much like
the ice on the surface of a frozen pond: the colder the surface temperatures, the thicker the ice.
(Ice is not, other than this particular example, a good analogy for the behavior of frozen mantle
material. Most solids, including mantle material, are heavier than and will sink into their liquid
counterpart; ice is less dense than water, and floats.) As the surface temperature of oceanic plates
and, in most cases, the mantle temperature beneath them are held constant, it stands to reason
that plates are generally of a uniform thickness. (Exceptions where elevated undersurface
temperatures occur will be discussed later.)
Continents and islands are made up of a mixture of lighter materials imbedded within this frozen
mantle. As the plates move, any land masses that are imbedded in them are carried passively
along.
The plates are bounded by an interconnected network of ridges, transform faults, and trenches.
Ridges, also called spreading centers, occur where two plates are moving away from each other.
As the plates separate, hot molten mantle material flows up to fill the void. The increased heat
resulting from this flow reduces the density of the plates, causing them to float higher in the
molten mantle, thus elevating the boundaries by many thousands of feet above the colder
surrounding sea floor. Ridges on the ocean floor form the longest continuous ranges of
mountains on the planet, but only in a very few places on the Earth do these 'mountains' rise
above the ocean surface.
Ridge segments invariably form perpendicular to the relative motion of the two separating plates.
However, the general trend of the boundary between two spreading plates need not be, and in
fact usually is not, perpendicular to the motion of the plates. Ridges therefore tend to be made up
of small spreading segments offset from each other and connected by transform faults, forming a
zig-zag line, where the 'zigs' are perpendicular to the 'zags.' Along transform faults, the two
plates are sliding past each other going in opposite directions. (See figure at right, above. Arrows
indicate direction of plate movement.)
Subduction Zones:
New sea floor is constantly being created along spreading centers. Obviously somewhere else old
sea floor must be going away. This occurs in trenches, also called subduction zones. Trenches
occur along the boundary between two plates that are moving towards each other. Where this
occurs, one plate is bent downwards, typically at about a 40° angle, and plunges under the other
plate's leading edge, eventually to melt back into the liquid mantle below. Where the subducting
plate bends downward, there is left a gap in the ocean floor running along the plate boundary thus the common term 'trench.' The Marianas Trench off Guam in the western Pacific plunges to
a depth of over 36,000' or 11,000 m (compared to an average ocean depth of 13,000' or 3950 m),
and is the lowest point in the ocean floor.
As the subducting plate is heated back up to mantle temperatures, certain minerals in the plate
melt sooner than others. Minerals that melt at lower temperatures and are lighter than the
surrounding material tend to rise, melting their way up through the overriding plate to erupt as
volcanoes on the ocean floor. As these volcanoes grow, they rise above the ocean surface to form
lines of islands along the leading edge of the overriding plate. Numerous islands of Micronesia
and Melanesia in the western Pacific were created in this way.
Colliding Continents:
When a continent occupies the leading edge of an overriding plate, the differentially melted
minerals from the subducting plate will melt their way through the continental mass to erupt on
its surface, forming ranges of volcanic mountains. The entire Pacific margin of the Americas
provides abundant examples of this process. When continents occupy the leading edges of both
converging plates, neither continent can be subducted; continental material is too light, too
buoyant, to sink into the mantle (see "Buoyancy and Floating Continents" in the next lesson).
What happens instead is that one or both continents are deformed and thickened, causing them to
float higher above the molten mantle to form a range of mountains. (Which continent deforms
and the nature of the deformation will be discussed in a subsequent lesson.) After two continents
have been in collision for a while, it appears as if a range of mountains has formed in the interior
of a continent. The Himalayan Mountains, where the Indian Plate is pushing up into the Eurasian
Plate, are an example of this process in action.
Plumes:
It should be noted that all of the mountain and volcano producing events described in the
preceding paragraphs occur only on or near the margins of plates. One additional process is basic
to the theory of plate tectonics and is not confined to plate boundaries; that process is the action
of plumes. Plumes are thought to be columns of exceptionally hot material rising from deep
within the Earth's mantle, perhaps even from its bottom, where the mantle contacts the Earth's
liquid core. Plumes are hot enough to melt their way through the ocean floor or through a
continent, whichever happens to be above them. They are thought to be (relatively) stationary
within the Earth, while plates and continents move above them. Thus a volcano which forms on a
moving plate above a plume will eventually move away from the rising column, which will then
melt through at a new location and form another volcano, while the old volcano becomes extinct
(i.e. no longer capable of erupting).
The Hawaiian Islands represent a chain of volcanic islands formed in the middle of the Pacific
Plate from the actions of such a plume. Only the big island of Hawaii harbors active volcanoes;
the volcanoes which formed the other islands, now moved away from their source of molten
material, are all extinct. Yellowstone National Park in Wyoming is a caldera formed by the
catastrophic explosion of a volcano which formed over the Yellowstone Plume. The Snake River
Basalts of southern Idaho are lavas marking previous eruptions of the same plume as North
America slowly passed over it. Iceland is a volcanic island formed by a plume which happens to
exactly straddle a ridge or spreading center: the Mid-Atlantic Ridge.
Earthquakes:
Earthquakes tend to be concentrated along (but not exclusively confined to) the margins of
plates. Deep seated earthquakes, often severe in magnitude, generally occur beneath subduction
zones, as one plate is forced deep into the mantle beneath another. Shallower, but equally severe
earthquakes occur along transform faults where one plate is dragged sideways against another.
Transform fault earthquakes are most noticeable where the fault passes through a continent,
rather than the sea floor. The San Andreas Fault of California is a prime example.
Earthquakes along spreading centers tend to be very mild. One exception can occur where a new
spreading center forms beneath the interior of a continent. Such events tend to be short-lived, in
geological terms, for as the spreading continues, a rift is formed that will eventually fill with sea
water while the spreading center creates new sea floor beneath it. The Atlantic Ocean was
created in this way between 50 and 150 million years ago. The Red Sea is in a transitional stage,
already filled with sea water but not fully separated, while the Rift Valley of East Africa is in the
early stages of what will ultimately be the complete separation of east Africa from the rest of the
continent (assuming the process continues as it now is).
The Red Sea and the Rift Valley of Africa are the only documented examples of spreading
centers beneath continents today (technically, the San Andreas Fault is a stairstep of very short
spreading center segments connected by very long transform faults, all a portion of the East
Pacific Rise), but geology and oceanology tell of numerous such occurrences in the past.
Thus is the floor of the ocean continually being recycled; no sea floor is known to be older than
around 200 million years (compared to 3 1/2 BILLION years for portions of some continents).
Continent to continent collisions along subduction zones make continents larger, while rifts
along spreading centers break them apart again. Volcanoes and compressive deformation during
collisions along subduction zones makes them thicker (and hence higher), while erosion wears
them down, making them broader but flatter; all in a never ending cycle of aging and
rejuvenation. The continents we know today are but a frozen snapshot of an unending global
dance. The job of plate tectonics is to unravel the clues in that snapshot, to learn the dance steps,
to determine where the dancers have been, and, for a while, where they are going.
Lesson 2 Buoyancy and Floating Continents
Introduction: Buoyancy.
Buoyancy is what makes a piece of wood float in water. It is also what makes a battleship float
on the high seas, or a block of steel float in a pool of liquid mercury. The first principle of
buoyancy is very simple: (1) If a solid immersed in a fluid weighs less than an equal volume of
the fluid, the solid will float. Another way of saying the same thing is, if a solid has a lower
specific gravity than a fluid, the solid will float in that fluid. Specific gravity is defined as the
weight of a substance divided by the weight of an equal volume of water (specific gravity of
water = 1).
If the first condition is met, then the level at which the solid will float is determined by the
second principle: (2) A floating object will displace its own weight in a fluid. Thus, the
percentage of the solid immersed in the fluid will be equal to the specific gravity of the solid
divided by the specific gravity of the fluid. For a block of wood having a specific gravity of 0.7
floating in water (specific gravity 1.0), 70% of the block will be below the water's surface.
Continents are made up of solid rock, and float in the fluid (a very viscous fluid) of the Earth's
mantle. If the specific gravity of the mantle and the specific gravity of continental material are
known, it should be possible to calculate how continents float in the Earth's mantle.
Floating Continents:
Specific gravities of various Earth substances are listed in table 1.
TABLE I
Typical Specific Gravities of Earth Materials
Substance
Specific Gravity*
Sea Water
1.02
Limestone
2.68-2.76**
Granite
2.64-2.76**
Sandstone
2.14-2.36**
Slate
2.6-3.3**
Basalt
2.4-3.1**
Average Specific Gravity of Continents
2.7
Average Specific Gravity of SiMa (Mantle Material) 3.3
* Actual specific gravities vary slightly, depending on chemical composition
(** Source: Handbook of Chemistry and Physics)
The average continent is made up primarily of limestone, granite, or eroded granitic byproducts
such as shale, siltstone, and sandstone, as well as metamorphics like slate, schist, and gneiss.
However, most continents also have substantial amounts of andesite and basalt added in, from a
past history of various volcanic events. The addition of these denser materials raises the average
specific gravity for continental material to around 2.7. SiMa (from Silicon/Magnesium - its
principal elements) is the material of the Earth's mantle - the 'fluid' in which continents are
floating. To all appearances, this material is solid rock, but under the extreme pressure and
temperature to which it is subjected, it actually flows like a liquid, albeit very slowly. Its specific
gravity of 3.3 is high enough to insure that continents cannot sink.
Given these figures, it should be possible to calculate the thickness of an average continent.
Hypothesize a continent that is absolutely flat and whose surface is exactly at sea level.
(Substantial portions of Australia and parts of the Canadian shield come very close to this
description.) The effective 'surface' of the SiMa fluid is actually the floor of the oceans,
averaging approximately 13,000' (~4 km) below sea level. The weight of 13,000' of ocean water
pressing down on the mantle wherever continents are not present complicates the formula
somewhat. It becomes easier to think in terms of pressures, rather than the percentage of floating
object that are submerged in the fluid.
To refer back to the example in Section 1, if a rectangular block of wood 10" thick, having a
specific gravity of 0.7, is floating in water having a specific gravity of 1.0, 70% or 7" of the
block will be submerged and 3" will be above the surface. In terms of pressures, the 3" above
water is pushing down with a force proportional to 3" times its specific gravity (0.7), while the
submerged 7" are being pushed up by water pressure with a force proportional to 7" times the
difference in specific gravity between the water and the wood (1.0 - 0.7). Thus:
3" x 0.7 = 7" x 0.3
In the case of a sea level continent (e.g. the image at right), the 13,000' between the sea floor and
its surface is pushing down with a force proportional to 13,000' x 2.7, but that force is partially
offset by the force of 13,000' of sea water pushing down elsewhere on the mantle, 13,000' x 1.0.
Thus the effective downward force from the continent is 13,000' x 1.7. This is equal to the force
generated by the buoyancy of that portion of continent submerged into the mantle, S, times the
difference in specific gravities (3.3 - 2.7, or 0.6). Thus:
1.7 x 13,000 = 0.6 x S
Then S = 36,833, or ~ 37,000', and the total thickness of our hypothetical continent is 37,000' +
13,000', approximately 50,000' or ~15 km.
Continental material extending above sea level is not, of course, buoyed by the weight of the sea
water, and adds thickness to the continent at a rate of
1 + (2.7/(3.3 - 2.7))
or 5.5 feet in thickness for every foot of elevation. Therefore, when erosional sediments are
being deposited above sea level, for every 5.5' of sediments deposited, the continent will gain 1'
in elevation. (Of course, this may not be immediately obvious; the mantle 'fluid' is very viscous,
and flows ever so slowly.)
It is interesting to note that in many places on the Earth, marine sedimentary deposits of up to
50,000' thickness have been reported, but this author has never seen a report of more than 50,000'
of marine deposits. This is predictable from the above calculations. Sediments eroded on dry
land are ultimately washed out to sea, to be deposited on the sea floor along continental margins.
As soon as 50,000' of sediments have accumulated, the depositional surface has reached sea
level. New erosional sediments are then washed over the old deposits and farther out to sea. The
old depositional surface has effectively become dry land.
Rates of Isostacy:
The mantle flows very slowly, but it moves fast enough to stay ahead of most geological
processes. Thus in most cases isostatic equilibrium - the balance that keeps continents floating
where calculations say they should be - is maintained. The rate of isostacy can best be illustrated
in Scandinavia, which 8,000 to 10,000 years ago was covered by a large ice cap. The weight of
the ice caused Northern Europe to sink some 2,300' (700 m). In the 8,000 years since the ice
melted, the land has risen about 1,800' (550 m) and it is rising still, at a rate of up to 3' (90 cm)
per century. The average over 8,000 years is 0.225' (68 mm) per year. Put into a geological
perspective, this implies that isostacy could compensate for redistribution of continental mass at
a rate of 225,000' (68 km) per million years! Of course, no geological process known can
redistribute continental material at this rate sustained over that long a time span, but a few
processes can get ahead of isostacy for a short while.
Rates of deposition of eroded sediments rarely exceeds 20,000' (~6000 m) within a single
geological period - typically 35-60 million years, or 1/3' (10 cm) per million years or less. Over a
shorter time span, however, certain forms of deposition can exceed the rate of isostacy. One
example is the formation, or at least the melting, of continental glaciers, as mentioned above.
Another process is volcanism. Mt. Katmai in Alaska erupted in 1912, ejecting an aerosol of
molten lava at essentially the speed of sound. The "Valley of Ten Thousand Smokes" was filled
with tens of feet of tuff for 6-7 miles (~10 km) in perhaps 90 seconds! 30 to 50' (10-15 m) of
volcanic deposits aren't significant to the isostatic balance of an entire continent, but sustained
volcanic eruptions over an extended period can be.
The big island of Hawaii sits above a plume in the mantle - a rising column of hot molten
material rising from deep within the Earth. The mountain thus formed is nearly 30,000' (9000 m)
high; 15,000' (4500 m) above sea level, and nearly the same below to the ocean floor. Its
volcanos are active today, and the island continues to grow. The Pacific plate, on which Hawaii
sits, is drifting slowly to the northwest, and eventually will carry the island away from the plume,
bringing its volcanism to a halt. To the north and west are the rest of the Hawaiian island chain,
each island an extinct volcano which had previously been created by the same plume. The farther
these islands are from the big island, the older they are, and also the lower in elevation. For once
volcanism ceases, isostacy asserts itself, and the islands slowly sink back into the mantle.
Beyond the oldest island of the Hawaiian chain stretch a row of submerged seamounts - sunken
islands that have at last reached their isostatic equilibrium.
It can then be safely assumed that, barring some obvious short term perturbation like glaciation
or volcanism, most continents are stable and very close to being isostatically balanced in their
liquid mantle medium. But isostacy is a dynamic process; mountains erode, and their sediments
are washed out to sea, to accumulate as sediments along the continental shelves. Also, continents
tend to move primarily as solid blocks, so what happens in one part of a continent affects the rest
of the continent as well. Thus, Scandinavia is still rising, and Venice is sinking, as Europe rotates
slowly in response to the dictates of isostacy.
Lesson 3 Sedimentation and Continental Growth
Tectonic forces elevate the land, reducing the surface area of continental masses, while
increasing their thickness. Erosion wears away the high places, transporting its sediments to the
continents' edge and depositing them off the edge of the continental shelf onto the sea floor,
thereby reducing the thickness of continents but increasing their surface area. The form that this
"new land" takes depends on a number of factors: the age of the continental margin, the shape of
the continental shelf, as determined during the rifting process, and the presence or absence of a
subduction zone adjacent to the continental margin.
Rifts: The Making of a Continental Margin.
When a new spreading center forms beneath a pre-existing continent, a rift forms that will
eventually, if allowed to proceed normally, divide the original continent into two, with new
ocean floor being created to separate them. As the two newly formed plates begin to separate,
molten material, mostly basalt, from the mantle beneath will flow upwards into the crack. The
heat from this molten material is conducted to the continental material above, reducing its
density and causing it to float higher in the mantle, producing a ridge of mountains above the
spreading center. As spreading continues, blocks will break loose from the sides of the crack and
subside into the void, creating the characteristic "rift valley," such as that in East Africa. As
spreading continues, the rift valley will deepen, ultimately subsiding below sea level and
allowing ocean water to fill the valley. The now water-covered subsidence blocks will later
become the submerged continental shelves.
Depending on the rate of spreading and the amount of heat flowing into the rift, these continental
margins may be broad or narrow. Also depending on heat flow, which to a large degree depends
on the proximity of plumes, volcanoes may or may not form. Surface fissure flows of basaltic
lava may also occur where heat flow is high. (Plumes generally appear to be associated with the
formation of new spreading centers, a notion that will be discussed further in a later lesson.)
Continued spreading causes the complete separation of the two land masses, with new sea floor
being created beneath the ever-widening ocean. As the continental margins move farther from
the heat flow at the spreading, the mountains formed along the continental margins cool and
slowly subside back into the mantle. Depending on how much material erosion has removed
from their summits, they may sink below the waves and vanish forever.
Freshly rifted continental margins tend to have steep walls, the continental shelves plunging
nearly vertically from a few hundred feet deep to the ocean floor, many thousands of feet below.
As erosional sediments are washed off the land surface, they first cover the continental shelves,
then wash over the precipice to fall on the ocean floor below. Very old continental margins,
therefore, tend to have large accretionary wedges on the deep ocean floor piled up against the
continental margin.
The rifting process is not always as 'clean' as the above description may sound. Sometimes a
segment of spreading center may shift slightly while separation is occurring, causing some of the
subsidence blocks to be separated from both daughter continents. These messy remnants may
become submerged plateaus or 'banks' on the sea floor, coral atolls, or islands in their own right.
New Zealand, for example, is a continental fragment left behind after a long forgotten rifting
episode.
Passive and Active Margins:
The continental margins discussed in the preceding section are known as passive margins, where
the continent and the adjoining ocean floor are part of the same plate. Passive margins occur on
both sides of the Atlantic Ocean, in Europe, Africa, and North and South America. Margins
where continent and sea floor are on separate plates, which usually implies a subduction zone
adjacent to the continent, are known as active margins. Active margins occur around much of the
Pacific Rim, in North and South America, the Alaska and Kamchatka Peninsulas, the Aleutian
Islands, and Japan.
Barring a realignment of plate boundaries, passive continental margins can persist for very long
spans of time, building accretionary wedges that can extend hundreds of miles out to sea. As the
wedge of sediments thicken, the thickest portion can eventually rise above sea level, burying the
original continental shelves and becoming 'new' dry land.
Sedimentary deposition along active margins is made somewhat more complicated by the
presence of a subduction zone offshore. The rate of sedimentary deposition and the subduction
rate, as well as the age of the continental margin, all influence the type of landforms created. The
simplest case, which occurs with very young active margins, is where a range of volcanic
mountains form on the leading edge of the continent as material from the subducting oceanic
plate is heated and the lighter fractions melt their way up through the overlying continent. (The
oceanic plate is always subducted under the continental plate.) As the mountains rise, some of
the erosional sediments will be washed out to sea, eventually to tumble over the edge of the
continental shelf and down into the offshore trench. A small percentage of these sediments may
actually be carried beneath the continent, to contribute to the molten material feeding the
volcanoes. The great majority of the sediments in the trench, however, will be scraped off of the
subducting plate, to adhere to the edge of the continent, as will any extraneous material of lower
density previously residing on top of the subducting plate, such as oceanic sediments, seamounts,
or even small islands. As material is added to the continental margin, the added weight further
depresses the subducting sea floor, effectively pushing the trench farther out to sea. The western
coastline of South America remains in this state today, the Peru-Chile Trench being responsible
for the formation of the Andes Mountains.
Island Arcs and Back-Arc Basins:
When the volume of sediments is great enough relative to the rate of subduction, the trench
adjacent to an active continental margin can be overwhelmed by erosional deposits, and simply
stop subducting. Continued convergence of the two plates will cause a new trench to form some
distance offshore. Subduction at the new site will lead to the formation of a volcanic island arc,
behind which remains a now passive section of sea floor, known as a back arc basin. The
presence of an offshore subduction can also produce tension on the overriding continent, as
subducting material from the oceanic plate entrains mantle material from beneath the continent.
This can lead to rifting and crustal stretching within the back-arc basin.
Continued erosion from the mainland, plus added sediments eroded from the new volcanic
islands, will eventually fill the basin up to sea level. Subsequent sediments from the mainland
must then be washed over the now dry land, between the volcanic islands, and out into the
trench.
Over a long enough span of time, this process of new island arcs and filled up back arc basins
can be repeated many times, as a careful examination of the Pacific margin of the Asian
continent will reveal. The Kamchatka Peninsula, the Kuril Islands, and the Japanese Archipelago
are the most recent (and currently active) island arcs, while the Sea of Okhotsk and the Sea of
Japan are back arc basins that have not yet filled with sediments. The Korean Peninsula is a
former island arc, as are any number of arc-shaped mountain ranges on the Asian mainland. The
Yellow Sea is very shallow (less than 600' or 180 m. deep) but not quite completely filled with
sediments, while the Amur River and several other Manchurian rivers flow through former back
arc basins that are now high and dry. Manchuria and much of far eastern Siberia, then, are
composed of 'new' land created by erosion and island arc formation along a very old active
margin. (An explanation of the distinction between 'new' land and 'old' land (or cratons) will be
discussed in a subsequent lesson.)
Lesson 4 When Continents Collide
Introduction: Initial Conditions.
Undoubtedly the most dramatic event in plate tectonics is the collision of two continents. The
exact sequence of events in a continent to continent collision depend on a large number of
factors, and it is highly unlikely that any two collisions have ever occurred in exactly the same
manner. Nevertheless, a number of general principals can be described, and correlations made
with known collision events.
One thing that can confidently be predicted is that before two continents can collide, they must
first have an intervening ocean between them. There must also be one trench, into which the
ocean floor between them is subducted. Should such a a trench initially form in the middle of an
ocean between two continent-bearing plates, the ocean floor between the trench and one
continent will be subducted first. When all the ocean floor on the subducting plate is consumed,
and the trench reaches the continental margin, the trench will collapse. Any sediments
accumulated within the trench will be uplifted as the formerly depressed section of ocean floor
rebounds. The Coast Range of California was created by just such an uplift.
Continued convergence between the two plates will cause the formation of a new trench, most
likely forming just offshore from the former one but reversed in direction, as the now active
continental margin overrides the remainder of the intervening ocean floor. Therefore in all
continent to continent collisions, at the time of contact one continent must have a passive margin,
the other an active one. There is no evidence that a subduction zone can ever form within the
interior of a continent (a subduction zone is defined by the subduction of an oceanic plate). There
are, however, several subduction zones in the South Pacific that are a considerable distance from
any continental margin.
It can also be reasonably predicted that when two continents collide, not all adjacent points will
'hit' at the same time. Depending on the relative shapes of the two approaching continental
margins and the rate of convergence, orogeny may commence in one region tens of millions of
years before it occurs elsewhere. Where island arcs exist on the active margin, minor orogeny
may begin when an island contacts the passive mainland, while the major 'crunch' doesn't occur
until millions of years later. The Taconic Orogeny, which produced the mountains of the same
name in eastern New York state, USA, were an island arc collision around 450 to 500 MYa
(million years ago). The major crunch between Baltica (northern Europe) and Laurentia
(ancestral North America) didn't occur until around 400 MYa.
The Folding of Sedimentary Layers:
The fact that continents rather than sea floor occupy the leading edges of two converging plates
seems to have no effect on the forces driving the plates together. As continental material is too
buoyant to be subducted, the continental material is variously buckled, fractured, and/or folded
as the continents are compressed against each other. The exact nature of the deformation is
largely controlled by the physical composition of the continental margins. Accretionary wedges
of unconsolidated sedimentary material, for example, tend to compress by forming gigantic
vertical folds or pleats. (An idealized cross section is shown at right. Vertical scale is not
exagerated.) The upper folds rise up to form parallel mountain ridges, while the lower folds
penetrate deep into the mantle, becoming the 'roots' which provide buoyancy to support the
mountains above. The thickness of folded sediment mountains can be as great as 150,000' to
200,000' (45,000 to 60,000 m), producing mountains up to 27,000' (8,200 m) or more in
elevation.
The upper folds of folded mountain ranges remain poorly consolidated sediments, and erode
away rather quickly. River courses tend to follow the orientation of the folds, cutting their way
deeper and deeper into the soft material. The roots, on the other hand, are depressed so deeply
into the mantle that the increase in pressure and temperature partially melts the sedementary
material and causes it to flow much like toothpaste squeezed from a tube. As the tops of the
mountains are eroded away, the buoyancy of the roots causes them to be uplifted, until the
partially remelted material, now cooled and recrystalized as metamorphic rocks (granitite, schist,
and gneiss) are exposed at the surface. Much of this later uplift occurs as upthrust faulting, and is
frequently accompanied by intrusions of fully molten magmatic or granitic material into the
cracks and crevasses. (The Appallachian Mountains of the Eastern USA, now a mere ghost of
their former glory, illustrate nicely this stage of evolution.) Given enough time, the mountains
will be fully eroded away, leaving an essentially flat, peneplained surface of metamorphic rock.
Such surfaces, called cratons, form the cores of many of our present continents. (The Canadian
and Baltic Shields are prime examples.)
Cratons in Collision:
Generally, when two continents collide, one or both of them will have sizeable accretionary
wedges of sediment on their leading edges. These wedges, being the softest material, will
crumple first, normally into pleated folds as depicted above. Once the sediments have been
compressed as much as they can, and providing that closure between the colliding plates
continues, the next phase of deformation is determined by the composition of the interiors of the
two continents. Cratons, being composed entirely of crystaline igneous (i.e. rock that has fully
melted and re-solidified) and metamorphic rock, are very brittle, and cannot be folded. They can,
however, be fractured.
One possible scenario for the crustal shortening of a craton is overthrust faulting. It has been
reported that when the craton of India slammed into the southern coast of Asia, a large wedge of
cratonic material was sheared off the leading (northern) edge of the Indian subcontinent and
thrust up and southward over the Indian craton. The thickest (northern) edge of this wedge is said
to make up the south face of the Himalayas. A similar wedge has been reported to have thrust
eastward across Wyoming and into the Black Hills of South Dakota.
A second scenario, likely to occur when two cratons collide, was reported from echo soundings
in one section of the southern Appalachians. It appeared that both continents (Gondwanaland and
ancestral North America) sustained multiple shallow dipping fractures, allowing fingers of
cratonic material to interpenetrate, much like shuffling a deck of cards.
Conclusion: The Forest and the Trees.
It is very rare for nature to be so considerate as to simplify processes to the extent described
above. Most continents are in fact made up of a diverse variety of materials, randomly mixed
together. The composition of a continental margin can vary considerably from one mile to the
next. A typical margin might contain a basement of faulted cratonic material, thinned during the
rifting process that made it a margin in the first place, covered by a thick layer of sediments. In a
collision, the overlying sediments will be folded, while the crystaline basement will be fractured
into wedges. (This is probably what occurred in the southern Appalachians.)
A typical active margin will also likely include odd erratics, bits of debris ranging from sea floor
sediments to ancient seamounts and small islands, that were scraped off of the subducting sea
floor. Intrusive magmas, deposited during the rifting process, may or may not be present. Deep
sea sediments uplifted by the collapse of the intervening trench will likely also be found. Island
arcs may end up becoming compressed into the mess as well. Each of these pieces is likely to
behave differently under the enormous pressures that occur in a continental collision, and each
adds its own bit of complexity to the resultant structure. It is small wonder that the underlying
processes are rarely visible and are exceedingly difficult to interpret. In order to examine a
forest, one has to look at a lot of trees.
Lesson 5 The Mechanism of Plate Tectonics
Introduction; Speculation:
There appears to be no agreed upon explanation of the mechanism that drives plate tectonics.
(The notions that are presented here are original to this article, and to my knowledge have never
appeared elsewhere.) However, as was the case for premises presented in previous lessons,
various principles can be applied to impose constraints on the kind of mechanism that must be at
work.
What does appear to be accepted is that plate movement is driven by convection within the upper
mantle, powered by the temperature difference between the (relatively) cold surface and the heat
generated deep within the Earth. Whether the plates are themselves the upper part of this
convection system, with a static or reverse-flowing fluid mantle beneath, or whether they are just
rafted along on the surface of an actively convective mantle, remains unanswered.
Clearly, for the plates to move, they must either be pushed away from spreading centers, pulled
down into trenches, or dragged along by the friction of convection on their undersides.
Conventional wisdom has it that all three contribute, although the proportions contributed by
each force remains undecided.
Ridge Push is the force applied to plates at spreading centers. As plates separate, new, hotter
material is extruded into the gap between plates. The elevated temperatures lower the density of
the recently deposited material, causing it to float higher in the mantle; hence the formation of
ridges. Gravity then takes over, drawing the ridge material down and away from the spreading
center, thereby widening the gap for more hot mantle material to upwell.
Slab Pull is considered by many to be the dominant, or even the only, driving mechanism for
plate tectonics. Subduction zones typically occur a long distance away from spreading centers,
where the plates have had plenty of time to cool off. Lower temperature makes the plate more
dense than the material beneath it, causing it to subduct into the mantle. As the plate edge is
drawn under, it pulls the remainder of the plate behind it.
Plate Drag is supported in several different models of plate tectonics. In one model, the upper
mantle is separated from the lower mantle, with little or no transfer of material between them.
Small, local convection cells in the upper mantle can occur beneath the interiors of plates and
can contribute frictional drag forces to reinforce plate movement. In another model, the upper
and lower mantle are more tightly coupled, with material from plumes originating deep within
the lower mantle and contributing significantly to upper mantle convection, and hence to plate
movement.
Considerations for a Convective Mantle:
The combination of ridge push, plate drag, and local convection can insure that a plate, once set
in motion, will tend to remain in motion. The interlocking zig-zag of spreading center segments
and transform faults tends to stabilize the accretionary boundaries of moving plates; far less
energy is required to continue with the same relative plate motions than would be required to
fracture each segment and shift the direction of motion. And plates do appear to retain the same
relative motion for spans of tens of millions of years. However, rearrangements do occur. Prior
to around 40 million years ago, the Pacific plate was moving nearly due north, in contrast to its
present northwesterly movement, as evidenced by a change in direction of the Emperor
Seamounts, which trail off from the Hawaiian Islands.
Ridge push and slap pull can explain why plates remain in motion once spreading is initiated, but
do not adequately explain what initiates a rifting episode in the first place. The traditional
wisdom is that new rifts form under large continental masses, such as supercontinents, where the
greater insulating properties of continental material cause an increase in heat buildup, leading to
uplift, softening, and ultimately rifting of the continental material. It is true that the
supercontinent of Pangaea was incredibly short lived, on a geologic time scale, starting to break
up almost as it was being formed. (See the companion article "The Formation of Pangaea: The
Making of a Supercontinent", also on this site.) But Gondwanaland, which was a supercontinent
in its own right, survived for almost a billion years before it broke up during Pangaea's
formation. And there was no supercontinent, in fact no continent of any sort, over the East
Pacific Rise 40 million years ago when it began spreading in a northwest-southeast direction.
Clearly the force applied to move the plates must be capable, at least occasionally, of exceeding
the tensile strength of the plate material and of breaking a larger plate into two smaller ones. If
this were not the case, the Earth would be covered by a single spherical plate, and no relative
motion between plates could occur. Also, the force applied cannot normally be greater than the
tensile strength of the plates, or else small pieces would be breaking off all the time, and there
would be a myriad of very small plates bobbing about, rather than the very few, large plates that
we know to exist.
The obvious cantidate for a mechanism for transforming thermal energy into the movement of
plates is the action of plumes. Plumes are thought to be columns of heated mantle material rising
from deep within the mantle, possibly from the very bottom of the mantle where it contacts the
Earth's liquid core. The problem with models using plumes to power plate movements is that the
volume of material rising in the plumes is thought to be small compared to the volume of the
moving plates themselves. Yet if the plume material is hot enough, which it shows every
evidence of being, there may well be enough energy available.
The Earth's mantle is not composed of a single mineral, with a sharp melting/freezing point.
Rather, it is a mixture of a large number of different substances, each with a different melting
temperature. Thus while the upper surface of an oceanic plate is a crystalline solid, the underside
exhibits a gradual phase change, from solid to plastic to viscous liquid as the temperatures and
pressures increase with depth. Yet even at great depth, the mantle remains an exceedingly
viscous fluid, flowing ever so slowly even in response to extreme pressures. Any force
postulated to either push or pull oceanic plates would be quickly dissipated overcoming the
friction of sliding over a static fluid mantle beneath.
Furthermore, any pulling force great enough to tear a new plate off from an existing one would
surely tear off a piece immediately adjacent to where the pulling force is applied (and therefore is
the greatest, undiminished by friction), rather than creating a new rift somewhere clear across the
ocean. Nor can slab pull explain the orogenies resulting from continental collisions. Once two
continents meet, any oceanic slab separating them will have been totally consumed by
subduction, and the plate's leading edge will be made up exclusively of continental material,
which is too buoyant to be pulled down into the mantle. Yet the Indian plate has continued to
drive northward into Asia for something on the order of a thousand miles (1600 km) since its
collision with Asia. Only push or drag forces can explain the Himayalas.
Similarly, any pushing force great enough to buckle a plate and form a new trench and
subduction zone would almost certainly form one immediately adjacent to an existing spreading
center, or at least no farther away than the point at which the new plate has cooled to a normal
deep ocean temperature, rather than at the opposite side of the ocean or the middle of a continent.
These arguments support the notion of a convective mantle, with the plates being rifted apart and
transported by the accumulated forces of frictional drag on their undersides.
It should be noted that it is not necessary to postulate that the convective current beneath a
mobile plate exactly match the movement of the plate above. Any convection cell model for the
mantle that involves horizontal flow in many different directions beneath the plates will cause
motion of an overlying plate; the plate will simply move as the vector sum of all the drag forces
applied to its underside, provided that the frictional drag forces applied do not exceed the shear
strength of the plate material.
A Simple Convection Model
The simplest convection model is that of a contained layer of liquid sitting above a point heat
source. The liquid directly above the heat source will be heated, reducing its density and causing
it to rise. Upon reaching the surface, the heated liquid will spread laterally in all directions until
it reaches the edges of the container, where it will be deflected downward to the bottom of the
liquid layer, eventually to be drawn back towards the heat source. If the amount of cooling at the
surface exactly equals the heat delivered by the point source, a self-sustaining toroidal
convection cell will be created. If the cooling at the surface is less than the heat added, the
temperature will rise until an equilibrium is reached.
Add multiple heat sources of equal intensity, uniformly spaced beneath the fluid, and multiple
toroidal convection cells, all of equal size, will form. If the heat sources are of unequal size,
hotter sources will create larger toruses. This provides the basis for the proposed model for
convection in the Earth's mantle, with the heat sources lying deep within the Earth and plumes
being the rising columns at the center of (approximately) toroidal convection cells.
The Distribution of Plumes
Not all plumes are created equal. Some are energetic enough to produce conspicuous, even
spectacular, displays on the Earth's surface. Others are small and inconspicuous enough that they
can only be detected by sophisticated heat-flow measuring instruments. Several hundred plumes
have been identified, although some of the smaller ones may be questionable.
Plumes are scattered in an apparently random manner all over the surface of the globe. Many, but
not not all, of the larger plumes are situated along spreading centers, suggesting that plumes may
be instrumental in the formation of rifts and the origin of new spreading centers. A list of some
of the more prominent currently active plumes is provided in the following table:
Table II
Major Active Plumes
On Spreading Centers
Not on Spreading Centers
Afar Triangle (East Africa)
Hawaii (Mid Pacific)
Chagos Archipelago (Indian Ocean)
Yellowstone (W. North America)
Iceland (North Atlantic)
Pitcairn Island (South Pacific)
Azores (North Atlantic)
Rapa Island (South Pacific)
Ascension Island (South Atlantic)
Madeira Island (North Atlantic)
Tristan de Cunha* (South Atlantic)
St.Helena Island (South Atlantic)
Galapagos Islands (East Pacific)
*Tristan de Cunha Island is not located exactly on the Mid-Atlantic Ridge, but is close enough to
it to be contributing significantly to its activity.
Major plumes such as these leave a conspicuous trail on the surface of plates as the plates move
over them. Island archipelagos and submerged seamounts mark their trail on the sea floor, while
basaltic lava flows mark the paths of continents. From the length of the tracks and the age of the
lavas produced, it appears that an average plume has an average life expectancy of around 100
million years. It is generally assumed that plumes maintain a relatively constant position as the
plates move. It is this assumption that allows us to plot the past positions of plates and continents
in absolute latitude and longitude coordinates.
Plume-Driven Convection
Plumes are generally believed to form over localized hot spots deep within or below the mantle
layer. The amount of material directly heated by contact with the hot spot is very small, but the
heat transmitted can apparently be quite great. Thus the heated material, as it starts its migration
upwards through the mantle layer, can be much hotter than the surrounding material. Much of
the added heat will therefore diffuse outwards into the surrounding material. This elevates the
temperature of the adjacent material, making it less dense, and it, too, starts migrating upwards,
albeit at a progressively slower rate the farther it is from the hot center of the column. A
relatively small amount of material heated directly by the hot spot can therefore entrain vastly
greater volumes of mantle material, producing very large rising columns.
The material in the center of the column, being the hottest, will rise fastest, and only this hottest
material will likely reach the surface. When it does, it will spread out radially beneath the frozen
plate material, while the cooler entrained material will flow outward beneath it. Surface flow
under the plates will thus form a thermal gradient, with the hottest, fastest flowing material
nearest the surface and progressively cooler and slower moving material flowing beneath it. Only
when the surface material cools below the material beneath it will it descend into the mantle
below, ultimately to return to the heat source.
Plumes rising from less intense hot spots may well encounter hotter material from a more intense
adjacent plume spreading out above it, well before the lesser plume reaches the surface. Its
material may then never reach the surface at all, but instead be deflected laterally to flow along
with the material from the hotter plume, thereby enhancing the already existing flow. This is, of
course, pure speculation, and the degree to which such lesser plumes contribute to the movement
of plates cannot be reasonably appraised. As their flow never reaches the surface, their
contributions cannot be easily detected.
A Model for Plate Rifting
The number of large, active plumes that are found deep in the interiors of plates strongly
suggests that a single plume, even a large one, is not enough by itself to initiate a rifting episode.
The pairing of large plumes in both the North and South Atlantic (the most recently created
major ocean) suggests that two plumes working in concert just might be sufficient.
A possible convective model for how two plumes can act together to create a rift can be
illustrated using as an example the Iceland and Azores plumes, which lie roughly on a northsouth line. As the hot material from the Iceland plume reaches the surface, it spreads out radially
in all directions, gradually cooling as it goes. However, the material that flows south soon runs in
to the material flowing north from the Azores plume, well before either flow has cooled
sufficiently to sink back into the cooler mantle material below. Where the two flows meet, both
must therefore be deflected to the sides. As a result, along a line between the two plumes, all
surface flow should therefore be redirected to the east and west, perpendicular to and away from
the connecting line. This 'shear line' of opposing flows should effectively concentrate the drag
exerted on the underside of the plates to a degree that may be sufficient to initiate, or at least,
redirect a rifting episode. (The rifting episode that produced the North Atlantic appears to
actually have started far to the west, first separating what later became southern Mexico from
South America, and only later turning northward to open the North Atlantic.) A similar shear
line, also essentially north-south but offset far to the east, seems to have been created when the
Ascension and Tristan de Cunha Island plumes conspired to separate South America from
Africa.
The Atlantic ocean continues to widen, and North America is being dragged westward. However,
western North America seems to be moving westward faster than the rest of the continent. A
succession of paired mountain ranges, their steeper sides facing each other and rift valleys
separating them, progress eastward from California into Wyoming, Colorado, and New Mexico.
Starting with the Sierra Nevada and White Mountains, separated by the Owens Valley, and the
Panamint and Amargosa Mountains, separated by Death Valley, paired ranges progress across
Nevada and on into the Rocky Mountains of Wyoming and Colorado. The easternmost pair of
ranges in Wyoming are the Tetons and the Absaroka/Wind River Range, separated by Jackson's
Hole, a rift valley whose northern end is Yellowstone National Park, site of the Yellowstone
plume. In southern Wyoming and Colorado, paired ranges, offset considerably to the east,
include the Medicine Bow and Laramie Ranges, separated by North Park; the Never Summer,
Gore, and Front Ranges, separated by the upper Colorado River Valley, Middle Park, and the
Arkansas River Valley; and the San Juan and Sangre de Christo Mountains, separated by the San
Luis Valley. The rift zone ends in the Rio Grande rift valley, near a minor plume identified by
heat flow measurements near Socorro, New Mexico.
The model suggested for these features is that a shear line exists between the Yellowstone and
Socorro plumes. However, the surface flow of plume material from this line is insufficient to
overcome the westward flow from the Mid-Atlantic shear line, driven by the Iceland and Azores
plumes. Rather, all of the flow from the Yellowstone/Socorro shear line is deflected westward by
the more powerful currents from the east. Thus all of North America is being dragged westward,
but the western part is being pushed harder than the rest. As a result, a rift forms wherever the
lesser shear line occurs, but continued westward drift soon pushes the incipient rift west of the
shear line, where it becomes inactive, and a new rift is initiated farther to the east. The current
series of rifts between Yellowstone and Socorro are probably doomed to failure as surely as were
their predecessors farther west.
Triple Junctions
A common occurrence in the early stages of a rifting episode is for three rifts to come together at
a central point. As such triple junctions develop, one of the three arms becomes inactive, while
the remaining two continue to separate. One such 'failed' rift occurred at the Gulf of Guinea, near
the mouth of the Niger River, and extended north into the Sahara Desert. It is thought to
represent the failed third arm of the rift that opened the South Atlantic. Another tentatively
identified failed rift extends from the Gulf of Mexico up the Mississippi River valley into the
USA. This may represent the failed third arm of the rift that separated Mexico from South
America, and which subsequently veered north to create the North Atlantic.
The classic triple junction, however, is the Afar Triangle of East Africa. The Afar Triangle is
actually a section of sea floor uplifted by the high heat flow from the plume beneath it. It sits at
the junction of three currently active rift zones: the Gulf of Aden, the Red Sea, and the East
African Rift Valley. At the opposite end of the Carlsberg Ridge - the spreading center
responsible for opening the Gulf of Aden, is a plume beneath the Chagos Archipelago. This is
the same plume that created the Maldive and Lacadive Islands farther north, and deposited the
flood basalts of the Deccan Traps of India, when that subcontinent passed over the plume around
65 million years ago.
Two other plumes are also postulated by the proposed model; one under East Africa near the
south end of the East African Rift Valley, which, it is suggested, is responsible for the volcanos
Mt. Kilimanjaro and Mt. Kenya, and the other, perhaps a minor plume, near the north end of the
Red Sea or in the Eastern Mediterranean Sea. The proposal is that a triple junction occurs where
four plumes occur in close proximity to one another, with the three outer plumes forming a 'Y'
and the fourth plume in the center. If two plumes are insufficient to initiate a rifting episode (and
it may be that the 'shear line' proposed above is only sufficient to redirect and perpetuate a rift,
but not to initiate one), then surely four plumes in close proximity should suffice.
Triple junctions are rather rare occurrences in the geological record, but the small number of
plates into which the Earth's crust is divided testifies that the separation of plates and the
formation of new spreading centers is likewise rare. We are fortunate indeed that such an event is
playing itself out even as we watch.
Lesson 6 The Formation of Pangea – The making of a Supercontinent
Introduction:
The discovery of the one time existence of the supercontinent Pangaea was the crowning triumph
of the theory of Plate Tectonics. Books, articles, maps, and diagrams abound describing Pangaea
and documenting its ultimate breakup at the end of the Mesozoic era. But Pangaea was,
geologically speaking, a rather short-lived phenomenon. Many learned articles have been written
based on faulty assumptions of its antiquity and origin. Yet the evidence of the last round of
orogenies leading to the formation of Pangaea remain visible for anyone who has the time and
inclination to examine it.
What follows is one author's interpretation of those events during the Paleozoic Era that led up to
the formation of this Mesozoic supercontinent.
-------------------------------------------------------------------------------The Initial Setting:
At the Dawn of the Paleozoic era, the land mass known as Gondwanaland was already a
supercontinent in its own right, having formed relatively late in the Precambrian era. It sat in
what is now the South Pacific, extending from perhaps the equator to as far south as the South
Pole. It had by this time already scraped clean of all continental 'debris' a broad swatch through
what we now know as the Pacific Ocean.
Straddling the Equator on the opposite side of the globe sat the small continent Laurentia, which
later grew substantially to become North America. An unknown distance to the east, and also
near the Equator, lay another small continent, Baltica, which now comprises a major portion of
Europe. Shaped like a fat comma, Baltica's point lay along side and west of what is now the
Iberian peninsula, with its bulbous end comprising Belarus and European Russia. An island arc
bounded by a subduction zone ran along its western margin. (The northern margins of Baltica
and Laurentia remain obscure; subsequent rifting and burial by sediments during the Pleistocene
Ice Age have obliterated many traces.)
Various other continental masses, most now welded to form sections of Asia, were scattered in
places as yet unknown. There is no definitive evidence that the Colorado Plateau, the Rocky
Mountains, or Alaska were attached to the continent of Laurentia at this time. Circumstantial
evidence suggests that they may not have been.
-------------------------------------------------------------------------------First Collision - the Acadian orogeny:
During the Ordovician period, around 500 million years ago, the continents of Laurentia and
Baltica collided. First contact, in the Lower Ordovician, occurred when the southernmost of the
islands off the southern peninsula of Baltica, west of modern Portugal, bumped into western New
York state. This resulted in the Taconic orogeny, which formed the mountains of the same name
west of the Hudson River. The New England States of the USA and most of the Maritime
provinces of Canada were part of Baltica's island arc at this time.
The next impact occurred in the Lower Devonian in what is now the Maritime Provinces of
Canada. The Acadian orogeny, as this first collision episode is know, continued through the
Devonian and into the Mississippian. As the two continents ground into one another, they
obliterated most of the ocean that had once separated them. (Many of the back arc basins behind
the island arc on Baltica's western margin remain, as the North Sea, the Irish Sea, the Baltic sea,
the Grand Banks off Newfoundland, and the Gulf of St. Lawrence.) When the orogeny was over,
a range of mountains extended from the Adirondacks north through Nova Scotia, the British
Isles, Norway and the eastern margin of Greenland, and possibly on to Spitsbergen.
Meanwhile, Gondwanaland was moving across the South Pole and northward up what is now the
South Atlantic.
-------------------------------------------------------------------------------The Entrance of Gondwanaland:
The supercontinent of Gondwanaland included all of Africa, South America, India, Antarctica,
Australia, New Guinea, and New Zealand, plus assorted pieces now attached to other continents.
Along its northern or leading margin was a subduction zone, fronting an island arc that included
several large islands, which now make up much of Central Europe, Italy, the Balkan Peninsula,
Turkey, and probably the Middle East and Iran. (For a modern comparison, look at the islands of
Indonesia.) All of the Eastern Seaboard and Gulf Coast of the United States and Mexico south of
Long Island, east of the Appalachians and the Marathon Mountains of West Texas and south of
the Ozark and Smoky Mountains, were also originally part of Gondwanaland, as were the major
islands of the Caribbean.
The Pacific coast of what is now South America looked far different then. The Andes Mountains
didn't exist yet. The Gulf of Mexico was probably a back-arc basin, open either to the north or
west. (The existence of Central America at this time is doubtful. It seems likely that the Yucatan
Peninsula was attached directly to Colombia or Venezuela.) The Amazon Basin, into which the
Niger River discharged, was a large sea that opened into the Pacific.
-------------------------------------------------------------------------------Second Collision - the Appalachian orogeny:
The behemoth called Gondwanaland, comprising well over half of the world's land mass, was
moving north. The combination of Laurentia and Baltica -- now known as Laurussia -- was in its
way, and the Pennsylvanian and Permian periods were crunch time. The exact sequence of
events remains unclear, but contact appears to have occurred in Late Mississippian in Oklahoma,
in Early Pennsylvanian in the Appalachians, but not until Late Pennsylvanian in Alabama.
(Remember that the coastlines of approaching continents don't match each other like adjacent
pieces of a jigsaw puzzle. The 'bumps' tend to hit first.)
Timing of events in Europe are not known by this author, but the collision ultimately resulted in
mountain ranges extending from the Marathon Mountains of West Texas, through the Ozarks
and Ouachitas, the Great Smoky Mountains, the Appalachians, and the Cevennes of France, to
the Carpathians of Eastern Europe. Any orogenies that may have occurred at this time in the
Middle East are obscured by later events.
Third Collision - the Urals:
While this crunch was going on, but at a time difficult for the author to document precisely with
the information at hand, another small continent, here called Angaria, collided with Baltica's
eastern margin, uplifting the Urals and the mountains of Nova Zemlya. Angaria, named from the
Angara shield at its core and the Angara river that cuts through it, comprised most of Siberia
from the Ural Mountains to the Lena River. (A small wedge of land in modern Kazakhstan,
identified as a separate plate by Russian geologists, remains unaccounted for.)
The Conclusion:
If all the land masses described above were truly joined by the Permian period, then Pangaea
(from Greek, literally all of Earth) truly deserves its name. Only Eastern and Southeastern Asia,
and (arguably) Western North America, including the subcontinent of Beringia (Alaska and
Siberia east of the Lena River) were not included. However Gondwanaland was starting to break
up even as it slammed into Laurussia; further evidence is needed to determine if
Australia/Antarctica and India were still joined to the rest of the supercontinent at the time of
impact.
The fact that most of the Earth's land masses were connected during the Mesozoic era does not
mean that they actually comprised one continent in the sense in which we use that word in the
modern world. Asia today is connected to Europe across the Ural Mountains, to Africa via the
Suez, and technically, to North America via a shallowly submerged bridge of continental
material that includes the Bering and Chukchi Seas. (This 'bridge' is a minimum of 1100 miles
wide, and shallow enough that it stood high and dry during the last ice age.) North America is in
turn connected to South America by the Isthmus of Panama.
So, in a very real sense, all the Earth's land masses except Australia and Antarctica are 'joined'
today. If actual shorelines for Pangaea were known with any accuracy, we would undoubtedly be
inclined to divide it into numerous 'continents' as well. Not all the back-arc basins along Baltica's
west coast were collapsed during its collision with Laurentia; remnants remain today as the Gulf
of Saint Lawrence, the Grand Banks off Newfoundland, the Irish, North, Barents, and possibly
Baltic Seas. These 'seas' formed an interior seaway that virtually cut 'Europe' (i.e. eastern
Pangaea) off from 'America' (i.e. western Pangaea). Parts of the Mediterranean Sea, the Black,
Caspian, and Aral Seas, and perhaps the Persian Gulf represent possible remnants of similar
basins along the north coast of Gondwanaland, separating 'Europe' from 'Africa' (i.e. southern
Pangaea). The Gulf of Mexico as yet another a back-arc basin has been mentioned earlier.
Leftover Pieces:
For most of the Mesozoic era, roughly the time that Pangaea existed, Western North America
was isolated from the rest of the continent by a waterway known as the Great Interior Seaway.
Whether this was entirely made up of submerged continental crust - and thus was truly an
'interior' seaway, or whether an actual ocean basin separated the two land masses, remains
conjectural, but the Laramide Orogeny of the later Mesozoic does look suspiciously like the
result of a continental collision. It is my postulate that the Colorado Plateau, along with a
northwest to southeast trending series of island arcs which now make up much of the Rocky
Mountains and ranges westward, collided with Pangaea (Eastern North America) somewhere
around the Late Jurassic period, causing the Laramide Orogeny. This mountain-building event
occurred too far from any now apparent continental margins to be explained by known
mechanisms of plate tectonics. Upper Jurassic deposits, such as the Entrada and Morrison
formations, can be found on both sides of the orogeny zone in Colorado, but correlations
between western and eastern slope formations earlier than these are problematical at best.
It is also unlikely that the various pieces that now make up eastern Asia (modern China, Tibet,
and Mongolia), plus the subcontinent of Beringia (which includes Alaska from Anchorage to
Mackensie Bay, the Chukchi and Bering Seas, and Russia east of the Lena River) were attached
in their present positions to the Pangaean continent. Some of these pieces might have been
attached to Pangaea elsewhere, and subsequently rifted away, only to collide again in a different
position.
Index
Author: Donald L. Blanchard
Created: May 3, 1998
Last Updated: November 2004
Name of Web Site: The ABCs of Plate Tectonics
Hosted by: WebSpinners.com