Download Plate Tectonics

Plate Tectonics
Continental Drift
• Continents can be made to fit together like pieces
of a picture puzzle
• The similarity of the Atlantic coastlines of Africa
and South America has long been recognized
• In the early 1900s Alfred Wegner made a strong
case for continental drift: he noticed that Africa,
South America, India, Antarctica and Australia
had almost identical late Paleozoic rocks and
fossils
• Wegner assembled the continents to form a giant
supercontinent Pangaea
Palaeomagnetism
• The Earth has a natural magnetic field with a
north and south pole, like a magnet.
• It is this field to which the north arrow of a
compass needle aligns to point northwards.
• When rock is molten (such as lava) any
magnetic minerals also align to this natural
magnetic field.
• When the rock cools, these miniature
"compasses" are frozen in that direction
• When geoscientists look at igneous rocks, they
find that some show that the Earth's magnetic
field is opposite to what it is today.
• In these rocks the minerals show that the north
magnetic pole was once at the south magnetic
pole, and the south magnetic pole at the north
magnetic pole.
• This change in poles is known as Polar Reversal
and has taken place many times over the age of
the earth
• The ages of many Polar Reversals are well
known through studies of basalts making up
the ocean floors
• palaeomagnetic methods allow an understanding
of changing continental positions over time.
• Geoscientists can use this magnetic information
to locate the position of the Earth's magnetic
pole at the time the lava cooled.
• When the pole is plotted relative to a continent
over time, it appears that the pole has moved.
• This phenomena is known as "polar wander".
• Geoscientists assume that the pole has been
reasonably fixed and that polar wander is really
the result of continent drift.
Plate Tectonics
A relatively recent theory that the Earth's
crust is composed of rigid plates that move
relative to one another.
Plate movements are on the order of a few
centimeters/year - about the same rate as
your fingernails grow!
There are 3 types of plate
boundaries:
1. divergent
2. convergent
3. transform
What is Plate Tectonics?
• If you look at a map of the world, you may notice that some of the
continents could fit together like pieces of a puzzle.
Plate Tectonics
• The Earth’s crust is divided into 12 major plates
which are moved in various directions.
• This plate motion causes them to collide, pull
apart, or scrape against each other.
• Each type of interaction causes a characteristic set
of Earth structures or “tectonic” features.
• The word, tectonic, refers to the deformation of
the crust as a consequence of plate interaction.
Modern Plate Map
What are tectonic plates made of?
• Plates are made
of rigid
lithosphere.
The lithosphere is made up
of the crust and the upper
part of the mantle.
What lies beneath the tectonic plates?
• Below the
lithosphere
(which makes up
the tectonic
plates) is the
asthenosphere.
Plate Movement
• “Plates” of lithosphere are moved around by the
underlying hot mantle convection cells
What happens at tectonic plate
boundaries?
Three types of plate boundary
• Divergent
• Convergent
• Transform
Divergent Boundaries
• Spreading ridges
– As plates move apart new material is erupted to fill
the gap
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Oceanic Divergent Boundary
Example: Mid-Atlantic Ridge
Iceland: An example of continental rifting
• Iceland has a divergent plate
boundary running through its
middle
Continental Divergent Boundary
Example: Red Sea / E. African Rift
Convergent Boundaries
• There are three styles of convergent plate
boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
Continent-Continent Collision
• Forms mountains, e.g. European Alps, Himalayas
Himalayas
Continent-Oceanic Crust Collision
• Called SUBDUCTION
Subduction
• Oceanic lithosphere subducts
underneath the continental
lithosphere
• Oceanic lithosphere heats and
dehydrates as it subsides
• The melt rises forming
volcanism
• E.g. The Andes
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the other
which causes it to sink into the mantle forming a subduction
zone.
• The subducting plate is bent downward to form a very deep
depression in the ocean floor called a trench.
• The worlds deepest parts of the ocean are found along
trenches.
– E.g. The Mariana Trench is 11 km deep!
Transform Boundaries
• Where plates slide past each other
Above: View of the San Andreas
transform fault
Continental Transform Boundary - Example: San Andreas
Volcanoes and Plate Tectonics…
…what’s the connection?
Pacific Ring of Fire
Volcanism is
mostly focused at
plate margins
Volcanoes are formed by:
- Subduction - Rifting - Hotspots
Pacific Ring of Fire
Hotspot
volcanoes
What are Hotspot Volcanoes?
• Hot mantle plumes breaching the surface in
the middle of a tectonic plate
The Hawaiian island chain are examples
of hotspot volcanoes.
Photo: Tom Pfeiffer / www.volcanodiscovery.com
The tectonic plate moves over a fixed hotspot forming a chain of
volcanoes.
The volcanoes get younger from one end to the other.
Earthquakes and Plate Tectonics…
…what’s the connection?
• As with volcanoes, earthquakes are not
randomly distributed over the globe
Figure showing the
distribution of
earthquakes
around the globe
• At the boundaries between plates, friction
causes them to stick together. When built up
energy causes them to break, earthquakes occur.
Plate Tectonic Theory
• Plate boundaries are marked by
– Volcanic activity
– Earthquake activity
• At plate boundaries
– plates diverge,
– plates converge,
– plates slide sideways past each other
Plate Tectonic Theory
Influence on geological sciences:
• Revolutionary concept
– major milestone, comparable to Darwin’s theory of
evolution in biology
• Provides a framework for
– interpreting many aspects of Earth on a global scale
– relating many seemingly unrelated phenomena
– interpreting Earth history
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
Plate Tectonics Summary
• The Earth is made up of 3 main layers (core,
mantle, crust)
• On the surface of the Earth are tectonic plates
that slowly move around the globe
• Plates are made of crust and upper mantle
(lithosphere)
• There are 2 types of plate
• There are 3 types of plate boundaries
• Volcanoes and Earthquakes are closely linked to
the margins of the tectonic plates
PLATE TECTONICS
We are concerned primarily with plate collision and
orogenesis:
Harry Williams, Historical Geology
44
OROGENESIS AT CONVERGENT BOUNDARIES
1. Passive Margins: prior to orogenesis, the continental boundary is a PASSIVE margin.
Sedimentation at passive margins reflects the progressive increase in water depth. Nearshore
deposits are coarser - sand grading to silt and clay; further out on the continental shelf in clean
shallow water, carbonate reefs form in tropical regions. A common feature of continental
margins undergoing extensive sedimentation is SUBSIDENCE, due to the weight of sediment
ISOSTATICALLY DEPRESSING the crust; in this way shallow water deposits (e.g. 100’ depth) can
build up thicknesses of 1000's of feet.
Harry Williams, Historical Geology
45
Andean-Type Orogenesis
1. Passive stage (pre-convergence) -> marginal deposits form.
hills
mountains
cliffs
ridges
Harry Williams, Historical Geology
46
2. early subduction -> marginal deposits are deformed by compression. valleys
Folds and thrust
faults are formed.
3. Volcanic arc forms.
4. Lateral growth by accretion; emplacement of igneous masses; metamorphism; further
deformation of marginal deposits.
canyons
deltas
beaches
Harry Williams, Historical Geology
47
5. Continued uplift and deformation results from continuing plate convergence.
Examples = Andes of western south America; Rockies of North America (older, inactive
mountain belt).
Harry Williams, Historical Geology
48
The Sierra Nevada and the Coast
Range are good examples of
inactive Andean-type
orogenesis. The Sierra Nevada
batholith is a remnant of a
continental volcanic arc. The
Coast Range consists of
accretionary wedge sediments
that have been deformed and
uplifted by plate convergence
(Note: the subduction zone
between the Pacific and North
American plates that formed
these features has since changed
into a transform fault – the San
Andreas).
Harry Williams, Historical Geology
49
Continental Collision-Type Orogenesis
About 20 million years ago - India was separated from Asia by a progressively narrowing
ocean basin - the Tethys Sea. The “collision” begun with the subduction of the oceanic plate
beneath the Tethys Sea. This caused the onset of orogenesis in Tibet (uplift, folding,
faulting, metamorphism, volcanism).
Initial growth was similar to Cordilleran-type orogenesis; however, when the continents
collide one of them can not be subducted (too thick and buoyant), therefore the plates are
welded together forming a SUTURE ZONE and producing a large mountain chain, containing
sedimentary, igneous and metamorphic rock.
17.23a
Harry Williams, Historical Geology
50
The period of mountain building is termed an
OROGENY. The Himalayas are very high because they
are very young (geologically). Uplift continues and
erosion hasn’t had long to wear the mountains down.
Harry Williams, Historical Geology
51
Suture zones usually have symmetrical patterns of rock types (folded sedimentary,
metamorphic, granite) and the high rugged mountains resulting from orogenesis are
subject to intense erosion, shedding sediment to both sides of the mountain range.
sediment
sediment
Harry Williams, Historical Geology
52
The present-day relief of mountain belts is a result of erosion: in North America,
the Appalachians being much older than the Rockies are more subdued and lower
and consist mainly of eroded folds.
Appalachians
Rockies
Harry Williams, Historical Geology
53
folds
Harry Williams, Historical Geology
54
Young (relatively) mountain belts like the Rockies are higher and more rugged.
Fault block mountains are also common
UPLIFT
Harry Williams, Historical Geology
55
The Grand Tetons of Wyoming are an example of a block of crust
uplifted along a fault.
faults
Harry Williams, Historical Geology
56
All orogenies have common features: 1. compression, buckling and
uplift at the edge of the continent. 2. emplacement of igneous rock
masses 3. folding, faulting and metamorphism 4. erosion of the
growing mountains and deposition adjacent to the mountains
(including the back-arc basin).
5. scraping off,
folding and uplift of
sediment
sea floor rock and
sediments onto the
edge of the
continent - resulting
in:
Harry Williams, Historical Geology
57
Continental Growth By Accretion
Caused by the "plastering on" of
material at the edge of a continental
plate adjacent to a subduction zone.
The material that accumulates in this
fashion can be small continental
masses MICROCONTINENTS or oceanic
features such as volcanic arcs and
seamounts submarine volcanoes). The
Seychelles Bank in the Indian Ocean is
an example of a microcontinent, which
has
apparently become detached from Africa. These larger masses of
rock are scraped off the subducting plate and plastered onto the
continent to form ALLOCHTHONOUS or EXOTIC TERRANES
(meaning they had originated elsewhere).
Harry Williams, Historical Geology
58
Example: Wrangellia consists
of basaltic island arc volcanic
rocks, deep marine shales and
shallow marine limestones.
This material originated in the
area presently around New
Guinea in Triassic time (225
million years ago) - travelling
some 10,000 km to its present
location. The collision of these
smaller masses with the larger
continent also causes orogenic
activity, resulting in coastal
mountain ranges.
Harry Williams, Historical Geology
59
11.3 Mountain Formation
Mountain Building at Convergent
Boundaries
 Ocean-Ocean Convergence
• Ocean-ocean convergence mainly produces volcanic mountains.
 Ocean-Continental Convergence
• The types of mountains formed by ocean-continental
convergence are volcanic mountains and folded mountains.
• An accretionary wedge is the accumulation of different
sedimentary and metamorphic rocks with some scraps of ocean
crust.
Ocean-Ocean Convergence
Ocean-Continental Convergence
11.3 Mountain Formation
Mountain Building at Convergent
Boundaries
 Continental-Continental Convergence
• At a convergent boundary between two plates carrying
continental crust, a collision between the continental fragments
will result and form folded mountains.
Continental-Continental Convergence
11.3 Mountain Formation
Mountain Building at Divergent
Boundaries
 The mountains that form along ocean
ridges at divergent plate boundaries are
fault-block type mountains.
Mountain Building by
Continental Accretion
11.3 Mountain Formation
Non-Boundary Mountains
 Not all mountains are formed by plate
boundaries. Some are formed by hot spots
or regional extension or stretching.
11.3 Mountain Formation
Continental Accretion
 Accretion is a process that occurs when
crustal fragments collide with and stay
connected to a continental plate.
 Terranes
• Terranes are any crustal fragments that have a geologic history
distinct from that of the adjoining fragments.
• Terranes occur along the Pacific Coast.
Accretion in Western North America
11.3 Mountain Formation
Principles of Isostasy
 Isostatic Adjustment for Mountains
• Isostasy is the concept that Earth’s crust is floating in
gravitational balance upon the material of the mantle.
• Because of isostasy, deformed and thickened crust will undergo
regional uplift both during mountain building and for a long period
afterward.
• Isostatic adjustment is the process of establishing a new level
of gravitational equilibrium.
Isostatic Adjustment
Isostatic Adjustment in Mountains