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The Rock Cycle
IGNEOUS ROCKS
C
A
B
Earth’s Changing Face:
The basics of plate tectonics
A
C
SEDIMENTARY
ROCKS
A
B
METAMORPHIC
ROCKS
MELTING OF PROTOLITH AND COOLING OF MAGMA
B
UPLIFT AND EROSION OF PROTOLITH, TRANSPORT AND
DEPOSITION OF GRAINS/PRECIPITATES
C
DEEP BURIAL, HEATING AND COMPRESSION OF
PROTOLITH, SOLID STATE ALTERATIONS
The changing face of the Dynamic Earth, as well as the Rock
Cycle are primarily products of the process known as “Plate
Tectonics”.
The Dynamic Earth
Earth is a very dynamic planet, both externally and internally
(but of course, all geological processes that we can actually
observe are at the surface).
This process explains large-scale movements of portions of the
Earth’s crust over geological time.
It is due to this dynamic nature of Earth that the three rock
classes (Igneous, Sedimentary and Metamorphic) exist at all!
Evidence for the presence of continents at considerably different
latitudes and longitudes in the distant past (continental drift), the
opening and closing of ocean basins, and the formation of
mountains is easily rationalized under such a model, as is the
concentration of earthquake and volcanic activity in certain
portions of the crust.
At the same time, it is important to note that the Earth is also
the ultimate recycler – any of the three rock classes can be
changed into any of the other rock classes.
These tranformations are commonly illustrated as a ROCK
CYCLE.
The processes which drive the Rock Cycle, namely uplift, sea
level change, deep burial and melting of crustal rocks are also
directly tied to plate tectonic activities.
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First Principles of Plate Tectonics
The Earth’s lithosphere is not a continuous sheet, but rather, is
broken into multiple plates. These rigid, solid plates rest upon the
underlying semi-fluid asthenosphere.
The Mechanics of Plate Tectonics and Tectonic Forces
Due to their lower density, the lithospheric plates essentially float
on the denser asthenosphere, and will shift laterally when acted
upon by tectonic forces.
A Brief Reminder: Asthenosphere and Lithosphere
Another Food Analogy: Scum on Soup
If you have ever made Cream of Mushroom Soup or soups of
a similar consistency, you will probably have noticed a relatively
thick scum forming on top which usually breaks up into smaller
pieces once you have heated it for a while. This may be a
reasonably good model for understanding plate tectonic
processes, assuming that the pieces of scum act in a similar
manner to tectonic plates and the underlying liquid soup
approximates the behaviour of the asthenosphere.
Of all the material in the mantle, the asthenosphere flows most
readily.
If you remember the analogy of the jelly sandwich “Jelly between
the bread”, the asthenosphere relates to the jelly, whereas the
overlying lithosphere and underlyihg mesosphere correspond to
the two slices of bread.
Remember that the lithosphere
consist of both the uppermost,
brittle part of the mantle
(lithospheric mantle) and the
crust.
Scum (Lithosphere)
Plates of Scum
Liquid Soup (Asthenosphere)
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Tectonic Forces: Convection Model
Slab-Pull Model
Convection involves the movement of currents (usually circular) within
fluids (liquids and gasses) as a result of density and/or temperature
gradients within the fluid. The effects of convection are clearly seen in
soup when material bubbles to to surface in certain areas and is seen to
be carried downward in other locations.
One might visualize plate movement as being controlled by convection,
as convection currents in the asthenosphere drag the scum (lithospheric)
plates) along.
Scum
slab (plate)
Scum slab (plate)
Convection
cell
This would cause the old, outer edge of a plate to sink in a typical
subduction zone setting.
In effect, the slab of scum (lithosphere) could be pulled by the sinking,
leading edge of the plate (which is subducted when it meets an adjacent
slab). A trench marks the area where the plate begins its descent
beneath the adjacent plate)
Scum
slab (plate)
Scum slab (plate)
Upward flow
(new scum created)
old scum
destroyed
As a lithospheric plate cools, it thickens a bit, but more importantly, it
increases greatly in density (thermally dense).
trench
Scum
slab (plate)
old scum
destroyed
Convection
cell
Downward
flow
Downward
flow
“Slab pull”
old scum
destroyed
Liquid soup
(asthenosphere)
Ridge-Push Model
Liquid soup
(asthenosphere)
Scum
slab (plate)
“Slab pull”
old scum
destroyed
It appears that the neither convection nor “ridge push” are strong
enough on their own to move entire plates.
However, it is probable that convection and “ridge push” contribute to
plate movement to at least a minor degree (note that all models are
compatible). Otherwise it may be difficult to explain processes such as
obduction which involve
the thrusting of more
dense oceanic crust on
top of less dense
continental crust (at least
in some cases), and the
rifting of continents.
“Ridge Push”
Scum
slab (plate)
Scum slab (plate)
Injection of liquid soup
(magma)
between spreading plates
(new scum created)
old scum
destroyed
Liquid soup (magma) passively
infills space between spreading plates
(new scum created)
So Which Model is Correct ?
Note that a ridge is formed at the site of spreading because the newly
solidified material is still warm (thermally buoyant) and floats higher than
the older, cooler crustal material farther away from the site of
spreading…hence the term “ridge-push”
Scum slab (plate)
Scum slab (plate)
At the present time, it is thought that the majority of plate movement is
due to slab-pull.
Alternatively, perhaps plates are actively pushed from sites of spreading
as new liquid soup (magma) is injected and solidifies.
Scum
slab (plate)
trench
Scum slab (plate)
old scum
destroyed
Liquid soup
(asthenosphere)
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Types of Plate Boundaries
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Differentiation of Oceanic and Continental Lithosphere
Plate movement is manifested in two principal types of plate boundaries:
1. A divergent boundary is a boundary between two plates that are moving
apart (diverging). New lithosphere is formed at divergent boundaries via
seafloor spreading.
As already alluded to, the
lithosphere underlying the
oceans and that underlying
the continents differ in
density.
2. A convergent boundary is a boundary between two plates that are
moving toward one another (converging). Lithosphere is destroyed at
convergent boundaries via subduction.
Convergent
boundary
sub
Convergent
boundary
Divergent
boundary
n
ctio
du
Lithosphere is
destroyed here
New lithosphere
forms here
(seafloor spreading)
As we will see, this is why
we have ocean basins and
continents in the first place!
sub
duc
tion
Significant differences in
other characteristics are
also noted.
Lithosphere is
destroyed here
Creation and Destruction of Lithosphere
Oceanic lithosphere versus Continental Lithosphere
Features on the ocean floor
show sites of plate
divergence and plate
convergence.
The mantle part of the lithosphere
(lithospheric mantle) is fairly uniform
in composition throughout, being
composed of ultramafic rocks.
Divergent boundaries are
represented by mid-ocean
ridges (e.g. East Pacific Rise,
Mid-Atlantic Ridge) in later
stages, and rift valleys (East
Africa Rift) in early stages of
their development.
However, the crustal part of the
lithosphere varies in composition.
Two types of crust are recognized:
Oceanic crust (mafic crust which
underlies the oceans).
Convergent boundaries are
represented by trenches
associated with subduction
zones (e.g., Peru-Chile
trench).
Continental crust (intermediate to
felsic crust that underlies the
continents).
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Effects of lithospheric density and thickness
Effects of Lithospheric Thickness: Modelling using water and blocks of wood.
Oceanic Crust
Average Density: 3.0 g/cm3
Average Thickness: 7 km
Maximum Age: 180 million years
Continental Crust
Average Density:2.7 g/cm3
Average Thickness: 35-40
Maximum Age: 4.0 billion years
The top of thick block of a buoyant material will stand higher in the water
column than a thin block of the same material. However, the proportion of the
block standing above and below the surface of the medium in which it is
floating is constant. This concept is termed ISOSTASY (“equal standing”).
For example the top
surface of thick block of
wood stands higher above
water level than that of
thin block of wood.
However, the ratio of
material above and below
water level is the same for
all of these blocks.
Note: Average Density of Lithospheric Mantle is 3.3 g/cm3
Both oceanic and continental crust are welded to lithospheric mantle (the
hard, brittle, uppermost part of mantle to form lithospheric plates).
Bottom line: oceanic lithosphere (oceanic crust + lithospheric mantle) is
heavy and thin whereas continental lithosphere (continental crust +
lithospheric mantle) is light and thick!
Likewise, the great thickness of continental lithosphere allows it to
stand very high above the surface of the asthenosphere. Oceanic
lithosphere, tending to be very thin, stands low.
Ocean basins and continents manifest the combined
effects of density and isostasy.
Summary
Effects of Density:
Due to its lower overall density, a block of continental
lithosphere will sit higher in the asthenosphere than a block of
oceanic lithosphere of the same dimensions
Oceanic crust:
3.0 g/cm3
Lithospheric mantle
~ 3.3 g/cm3
The combined effects of
higher density and lesser
thickness cause oceanic
lithosphere to sit lower in
the asthenosphere,
forming relatively
depressed areas of the
crust (ocean basins).
Continental crust:
2.7 g/cm3
Lithospheric mantle
~ 3.3 g/cm3
The combined effects of
lower density and greater
thickness cause
continental lithosphere to
sit higher on the
asthenosphere, producing
relatively elevated areas
of the crust (continental
land masses).
Asthenosphere (semi-fluid part of mantle)
This serves well to explain (in part) why ocean basins form areas of relative
depression of the Earth’s crust, while continental plates form areas of relative
elevation with respect to the centre of the Earth. There is, however, another
factor involved.
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Where subduction is the dominant plate tectonic process, ocean
basins close, and adjacent continental plates move closer to one
another (e.g. Pacific Ocean).
Why does it matter that continental lithosphere is
different from oceanic lithosphere ?
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•
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Oceanic lithosphere is constantly being formed and destroyed (by
seafloor spreading and subduction). Therefore, ocean basins are
transient features.
Continental lithosphere is too buoyant to be destroyed by subduction –
in effect, plates of continental lithosphere “go along for the ride” as
oceanic lithosphere is created and destroyed. Therefore continents
are relatively permanent features (although they can change
configuration).
Where seafloor spreading is the dominant plate tectonic
process, oceanic basins increase in size and adjacent
continents move farther apart (e.g. Atlantic Ocean)
To better appreciate plate tectonic processes, we should
look at plate boundaries in greater detail.
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Mature Divergent Plate Boundaries: Mid-Ocean Ridges
Mid-ocean ridges are the best-known form of divergent plate boundary and
are the area where seafloor spreading occurs; this produces new oceanic
lithosphere. For this reason, these are also known as “Constructive
boundaries”.
Red Sea-Gulf of Aden: An ocean basin in the making
Upper crust of
oceanic lithosphere
is made of the
volcanic rock basalt
(aphanitic mafic
rock, cooled at
surface).
East African
Rift will
probably
stop
spreading
and become
a “failed arm”
Future
ocean
basin
Lower crust of
oceanic lithosphere
is made of the
plutonic rock gabbro
(phaneritic mafic
rock, cooled at
depth)
Incipient Divergent Plate Boundaries: Continental Rift Valleys
Early evidence of seafloor spreading
Considerable evidence for the mobility of the continents was first brough forth
by Alfred Wegener (1880-1930). Largely due to his own curiousity, he made the
following observations and conclusions which led to the development of the theory
of “Contiental Drift”.
Hot plume in mantle upwarps
lithosphere of continent (thermal dome)
1. Jigsaw puzzle fit of continents (first noted by Wegener in 1910)
Cracks develop in this dome generally
in the form of a triple junction (incipient
rift valley).
Mafic magma is generated by
decompression melting. Magma that
cools at depth forms gabbro of lower
crust. Magma that cools at surface
forms basalt.
Two of the arms continue to spread,
forming oceanic lithosphere of an
ocean basin. The remaining “failed arm”
stops spreading and becomes filled
with sediment (Bay of Fundy basin
represents one such failed arms of the
Atlantic).
This jigsaw puzzle fit of continents suggested the former existence of a
supercontinent (a continent composed of several or all of the known continents).
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Early evidence of seafloor spreading
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Early evidence of seafloor spreading
3. Distribution of old mountain belts and coal deposits.
2. Fossil evidence (noted by Alfred Wegener in 1915).
Still, not all of his colleages were
convinced!
Wegener also brought forth strong
evidence for the linkage and common
formation of well-known mountain
ranges on either side of the Atlantic
Ocean, as well as continuity of coal
deposits in South America and Africa.
Old mountains belts (Appalachians
and Caledonides) are now
separated, but if the continents are
fit together, these mountain chains
form a continuous belt.
Fossils of land organisms (of Permian-Triassic periods) such as the reptiles
Mesosaurus and Lystrosaurus and the fern Glossopteris were distributed over
several continents: how did they get from one continent to another? Some of
Wegener’s opponents suggested land bridges that later subsided as the answer,
but he showed that this was a preposterous and unnecessary conclusion.
More recent evidence of seafloor spreading
Symmetry of magnetic stripes (defined by polarity of magnetic minerals
in basaltic rock of seafloor-first discovered in1940s)
Symmetrical pattern
of normal and
reverse polarities on
either side of a
divergent boundary
can only be
explained if magnetic
polarity fluctuates as
new crust is
continually being
formed.
On joining up the “jigsaw pieces” such that there was a continuous linkage
of fossil occurrences, Wegener convincingly demonstrated that there was
reasonably good evidence for the former connection of the continents as
such.
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More recent evidence of seafloor spreading
The linear arrangement and sequential development of hotspot volcanoes
(volcanoes produced by movement of a tectonic plate over a stationary magma
plume originating in the mantle). The Hawaiian Islands and Emperor Seamount
Chain are prime examples of this.
Types of Convergent Plate Boundaries
Oceanic-oceanic convergence
-subduction of oceanic lithosphere under
another plate of oceanic lithosphere
-magma produced by hydration melting rises
to surface to produce a volcanic island arc
(e.g. Japan)
Hawaii (“the big island”) is
the latest in the series of
these volcanic islands. It’s
position on the Pacific Plate
is directly above the mantle
plume at the current time.
However, this will change as
the plate continues to move.
New islands will be formed to
the SE of Hawaii, unless the
plate changes its direction of
motion.
Oceanic-continental convergence
-subduction of oceanic lithosphere under a
plate of continental lithosphere
-magma produced by hydration melting rises
to surface to form a continental volcanic arc
(e.g. Cascades with Mt. St. Helens)
Continent-continent collision
-where two pieces of continental lithosphere
meet (intervening ocean becomes completely
closed)
-continental lithosphere can’t be subducted,
so basically collides, shortens and thickens
-Earth’s highest mountain belts produced in
this way (e.g. Himalayas)
Note: The heights of hotspot volcanoes decrease with increasing distance (and time) from the point
of active volcanism (due to cooling and sinking of lithospheric material). Also note the “kink” in
Hawaiian-Emperor volcanic island/seamount chain, indicating change in the direction of plate
movement.
Evidence of Subduction
Convergent Plate Boundaries
As you know, these are zones where lithospheric plates move toward one another and
where oceanic lithosphere is consumed back into the mantle. For this reason, these are
also known as “Destructive plate boundaries”
•
The process of crustal “destruction”, i.e. subduction, ensures that the Earth retains a
constant volume. Otherwise, the Earth would be continually expanding in size-we know
this isn’t happening.
Existence of ocean trenches (deepest areas of the ocean)
-a trench is the surface expression of a subduction zone
where oceanic lithosphere begins its descent.
Explosive volcanoes (hydration melting produces thick,
viscous intermediate or felsic magma). Extrusive igneous
rocks andesite or rhyolite (and lots of pyroclastic materials).
Intrusive rocks include diorite and granite.
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Other geologic consequences of plate tectonics
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Another Minor Complication: Transform Plate Boundaries
Siliciclastic sediments are derived from the erosion of mountains that ultimately
owe their existence to uplift associated with the convergence of plates.
If a mountain chain is close to the sea, a sedimentary “clastic wedge” can form
through differential sediment transport (mainly by water) and the segregation of
sedimentary grains.
Transform plate boundaries occur where lithospheric plates move beside one
another without significant collision or subduction. These are basically “offsets”
that form within moving plates. No oceanic lithosphere is created or destroyed
(these are also know as “strike-slip” boundaries).
Poorly sorted conglomerates generally occur on land, close to the mountains.
Better sorted sands are found close to the shoreline.
Mud (silt + clay) is generally deposited offshore.
Beyond the reach of mud (i.e. where water is clear), limestone can be deposited
on a “carbonate platform”.
Mountains (on land)
Sea
These boundaries occur most commonly in oceanic lithosphere of
ocean basins lateral to divergent plate boundaries.
They also occur in continental lithosphere (e.g. San Andreas fault).
conglomerate
sandstone siltstone/shale
limestone
No magma is generated in this type of boundary.
Other geologic consequences of plate tectonics
Implications of Plate Tectonics
Mountains
(without volcanoes in this case)
So…Oceanic lithosphere
is constantly being
created at divergent plate
boundaries, destroyed at
convergent plate
boundaries, and offset at
transform plate
boundaries
slate
schist
gneiss
Again, oceans are
temporary features.
In the past 600 million
years, Atlantic has
opened, closed and
reopened (we are now
witnessing only the latest
opening event).
The convergence of lithospheric (particularly continental) plates and resultant
crustal thickening is the necessary prelude for regional metamorphism
(metamorphic grade increases with depth). Contact metamorphism can also
occur around chambers of magma generated by hydration melting).
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Continental Drift: 750 Millon years ago to present
Artist’s depiction of
early stages of
breakup of the
supercontinent
Rodinia about 750
Ma (million years
before present).
This supercontinent
is believed to have
formed about 1300
Ma.
End of lecture
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