plate tectonics

PLATE TECTONICS

Early Evolution of Earth• Origin of Planet Earth

– Most researchers believe that Earth and the other planets formed at essentially the same time from the same primordial material as the Sun.

– The Big Bang

– Nebular Hypothesis

• Layered structure developed by chemical segregation early in the formation of the Solar System and the Earth.

Nebular Hypothesis• Solar Nebula – Rotating cloud of

dust and gases (Hydrogen and Helium) begins to contract (~5bya).

• Material assumes a flat, disk shape. • Most of the material gravitationally

swept to the center forming the protosun (Pre-Sun).

• Planets begin to accrete from the material in the flattened disk which is orbiting the sun, due to gravitational effects.

• In time, most of the material is swept into planets/moons or lost into space.

Animation Clip

Nebular Hypothesis• Planetary segregation due to

pressure, gravitational, and temperature gradients (chemical differentiation).

• The most refractory particles survived the hottest part of the contracting nebula.

• Inner rocky planets are:– High density– Small in size– Composed of metallic and rocky

substances (Fe, Ni, Si, Ca, Na)

Nebular Hypothesis• The gases were vaporized or

swept away by the solar wind.• These materials could only

exist in the cooler outer regions of the nebula.

• Outer gaseous planets are: – Low density– Large in size– Formed primarily from ices

(H2O, CO2, CH4, and NH3-Ammonia) and gases (H2 and He)

Nebular Hypothesis

Earth’s Internal Structure• Formation of Earth’s Layered

Structure– Early period of heating due to high-velocity

impact of nebular debris and decay of radioactive elements.

– Heavy metals sank to the center (Fe, Ni).– Less dense materials rose to produce a

primitive crust – rich in O, Si, Al, lesser Ca, Na, K, Fe, Mg, and some metals with low melting points/high solubility (Ag, Pb, U).

– Physical and chemical segregation established the three basic divisions of Earth’s interior.

Earth’s Internal Structure• Earth’s Internal Layers can be Defined

by: • Chemical Composition• Physical Properties• Geophysical Properties

• Layers Defined by Composition

• Crust• Mantle• Core

Earth’s Internal Structure• Five main layers of Earth are

based on physical properties and mechanical strength: – Lithosphere – Crust – Cool, Rigid

Solid – Moves independently due to zone of melting in upper asthenosphere.

• Continental Crust– Average rock density about 2.7 g/cm3

– Average Thickness = 20 to 80 km• Oceanic Crust

– Average rock density about 3.0 g/cm3

– Average Thickness = 10 km

– Mantle: • Average rock density about 4.5 g/cm3

• Asthenosphere – Upper Mantle – Soft, Weak

• Mesosphere – Lower Mantle – Plastic Flow– Core:

• Average density is nearly 11 g/cm3 • Larger than the planet Mars• Outer Core – Fe-Ni Alloy – Liquid• Inner Core – Fe-Ni Alloy – Solid

Why Does Oceanic Crust Form Ocean Basins and Continental Crust

Form the Continents? • Buoyancy and floating of

the Earth's crust on the mantle.

• Due to differences in crustal density, age, weight, thickness.– Denser oceanic crust

floats lower, forming ocean basins.

– Less dense continental crust floats higher, forming continents.

Dynamic Earth

• The Theory of Plate Tectonics is a "great unifying theory" in geology.

• Plate tectonics explains the interrelatedness of many large scale patterns in the Earth's geological record.

• It has revolutionized the understanding of geology.

Continental Drift: An Idea Before its Time

• Alfred Wegener– First proposed his

Continental Drift Hypothesis in 1915.

– Published The Origin of Continents and Oceans

Alfred Wegener (1880-1930)

Scientific Method Exercise

• Continental Drift Hypothesis: – The continents move or "drift" relative to one

another. – Supercontinent called Pangaea began breaking

apart about 200 million years ago

The diagrams show the break-up

of the supercontinent

Pangaea (meaning “All Lands" in Greek), which

figured prominently in the

theory of continental drift -- the forerunner to the theory of plate

tectonics.

Pangaea Breakup

Continental Drift: Evidence

• Evidence used in support of Continental Drift Hypothesis:

• Fit of the continents • Fossil evidence• Rock type and geologic structural similarities • Paleoclimatic evidence • Apparent Wandering of the Earth's Polar

Regions • Geodetic evidence that Greenland was moving

away from Europe at a measurable rate

Fit of the Continental Shelves

Fit of the Continental

Shelves

The fit was made along the continental slope

(green) along the contour line for a depth of 500 fathoms (3,000 feet). Overlaps and gaps

(orange) probably result from deformation and

sedimentation after rifting.

Continental Shelf, Slope, and Riseand Abyssal Plain

Similar Rock

Types and Geologic

Structures Good correlation of the

Appalachian Mountains rocks and structures

with mountains of similar age and

structure in eastern Greenland, the British

Isles, and Norway.

These Folded mountain belts formed ~300 mya

as these landmasses collided during the

formation of the super-continent of Pangaea.

Animations\MountainRanges.mov

Rock Types and Geological Structures

• Good correlation of a line (unconformity) separating 550 million year old rocks from 2 billion year old rocks in northwestern Africa with a similar line in eastern Brazil.

Similar Rock SequencesRock sequences in South America, Africa, India, Antarctica, and Australia show remarkable similarities. Wegener showed that the same three layers occur at each of these localities. The bottom (oldest) layer is called glacial tillite and is thought to be a glacial deposit. The middle layer is composed of sandstone, shale, and coal beds. Glossopteris fossils are in the bottom and middle layers. The top (youngest) layer is lava flows. The same three layers are in the same order in areas now separated by great distances. Wegener proposed that the rock layers were made when all the continents were part of Pangaea. Thus, they formed in a smaller contiguous area that was later broken and drifted apart..

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part4.html

Fossil Evidence

Fossils of Mesosaurs, small freshwater reptiles, are found in Early Permian strata in a limited area along the Brazilian and West African coasts and nowhere else in the world.Fossil remains of this and other organisms on the African and South American continents appear to link these landmasses during the Late Paleozoic through Early Mesozoic. STOP

• Identical fossils on widely separated landmasses: – Mesosaurus – snaggle-toothed reptile– Glossopteris – tropical fern

Fossil Evidence

Fossils of the same species were found on several different continents. Wegener proposed that the species dispersed when the continents were connected and later carried to their present positions as the continents drifted. For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and Australia. If the continents are reassembled into Pangaea, the distribution of Glossopteris can be accounted for over a much smaller contiguous geographic area. The distribution of other species can also be accounted for by initially spreading across Pangaea, followed by the breakup of the supercontinent, and movement of the continents to their present positions.

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part3.html

Animations\FossilEvidence-Pangaea.mov

Gondwana Rock and Fossil Correlations

Volcanism, including basaltic lava flows near the top of each section, reflect the opening of fissures as Gondwana began to break apart.

Pangea Break-Up

Paleoclimatic Evidence

• How can rocks indicate paleoclimate?

• Examples?

Paleoclimatic EvidenceNear the end of the Paleozoic Era (~300 mya) ice sheets covered extensive areas of the Southern Hemisphere and India.Many of the glacial deposits now lie in sub-tropical areas within 30° of the equator.

Animations\AncientClimates.mov

Arrows show directions of glacial movement as inferred from glacial scouring grooves in bedrock.

Paleoclimatic Evidence

Glacial striations on quartzite bedrock.

These striations were scratched into Proterozoic quartzite in Australia during the Permian Period (250-295 mya).

STOP

Paleoclimatic Evidence

• Glaciation in South America, Africa, India, and Australia is best explained if these continents were once connected. Glaciers covered all or part of each of these continents during the same time period in the geologic past. • If the continents were in their present position, a major glaciation event that covered nearly all of the continents and extended north of the equator would be required. Geologists have found no evidence of glacial action in the northern hemisphere during this time period. In fact, during this time period, the climate in North America was warm. • Wegener proposed that the continents were adjacent to each other during the glacial event. Therefore, glaciers spread over a much smaller area in the southern hemisphere and probably did not influence the climate of the northern hemisphere.

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part5.html

Paleoclimatic Evidence

• Rocks deposited in warm arid environments often include evaporite deposits (formed when a body of water containing salts evaporates).

• It would be difficult to explain the presence of the great Permian evaportite deposits in northern Europe, the Urals, and southwestern U.S., if the continents have always remained stationary.

Paleoclimatic Evidence

• Large Paleozoic swamps (coal fields) in eastern U.S., Europe, and Siberia with fossil plants indicative of a tropical setting.

• Blue TrianglesBlue Triangles = = Glacial TillitesGlacial Tillites

• Red CirclesRed Circles = = Coal Deposits Coal Deposits

– Humid climates during Humid climates during interglacial periods, possibly interglacial periods, possibly associated with glacial associated with glacial meltwatersmeltwaters

• Irregular Green AreasIrregular Green Areas = = EvaporitesEvaporites

– Arid climatesArid climates

Fossil and Paleoclimatic

Evidence• Fossils are also paleoclimatic

indicators:• (A) Permian-aged fossil leaf in coal

from Antarctica.• (B) Reconstruction of a Permian-

aged tropical tree.• Tropical trees typically lack annual

rings that result from seasonal variations in growth.

• Tropical climates are required for the growth of these trees and the formation of coal deposits containing their fossils. Animations\TropicalSwamps.mov http://www.scotese.com/climate.htm

Paleoclimatic Evidence

Wegener used the distribution of specific rock types to determine the distribution of climate zones in the geologic past. For example, glacial till and striations (scratches on the rock), sand dunes, and coral reefs, indicate polar, desert, and tropical climates, respectively. The present climate zones are shown in the above figure. Note how the distribution of reefs, deserts, and glacial ice constrain the position of the rotational pole of earth.http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part5.html

Paleoclimatic Evidence

Using the distribution of rock types, Wegener reconstructed the distribution of climates zones at specific times in the geologic past. He found that, unlike the present distribution, in which zones parallel the equator, the past zones occupied very different positions. This implies that the rotational pole was in very different locations relative to today. Wegener proposed an alternative interpretation. He believed that the climate zones remained stationary and the continents drifted to different locations. The drift of the continents caused the apparent movement of the climate zones.http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part5.html

Paleoclimatic Evidence

Wegener used the distribution of climate zones to determine the location of the poles at different times in the geologic past. He found that the rotational pole appears to gradually change location, arriving at its present position only in the very recent geologic past. The apparent movement in the pole position over time is called polar wandering. Wegener offered an alternative explanation. He suggested that the poles remained stationary and that the continents changed their positions relative to the poles.

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part5.html

The Great Debate

• Wegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers cut through ice.

• Strong opposition to the hypothesis from all areas of the scientific community.

• Objections to the Continental Drift Hypothesis: – Lack of a mechanism for

moving continents.

Wegener’s proposal that continents broke through the ocean crust

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part6.html

The greatest shortcoming, at least in the eyes of American geologists, was the lack of an adequate mechanism for moving the continents. Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges. Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely.

Wegener’s proposal that continents broke through the ocean crust

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part6.html

• Wegener's model was not accepted by all geologists. • Some thought that dispersion by winds or ocean currents could explain the distribution of fossil species. • Other geologists thought the poles might wander and continents remain stationary. • Many geologists thought Wegener's evidence was insufficient.

The Great Debate• Continental Drift and the

Scientific Method– Wegener’s hypothesis was

correct in principle, but contained incorrect details.

– The hypothesis was all but abandoned by the 1930’s for the predominant “Shrinking Earth” Theory of the time.

Arthur Holmes’ hypothesized in 1930 that the mantle underwent convection

http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part6.html

• However, a few scientists considered Wegener’s ideas plausible and continued the search, most notably, Alexander du Toit and Arthur Holmes.• Would not be until later that this hypothesis would be proven correct.

Continental Drift and Paleomagnetism

• Very little new information was shed on continental drift from 1930 until the early 1950s.

• In the mid-1950s, renewed interest in continental drift initially came from rock magnetism.

• Magnetized minerals in rocks… • Show the direction to Earth’s magnetic poles.

• Provide a means of determining the direction and distance from the poles, and latitude of origin of the rocks during formation/magnetization.

Fossil Magnetism or Paleomagnetism

The Core Generates the Earth’s Magnetic Field

• The requirements for the core to produce Earth’s magnetic field are met in that it is

• made of material that conducts electricity and it is mobile.

• Igneous rocks contain magnetic minerals, like magnetite.

• These magnetic mineral grains in rock align with the earth's magnetic field (as if they were tiny compass needles floating freely in a liquid) and become magnetized in that direction when they cool below the Curie Point (about 580°C).

• The magnetism remains "frozen in".

How Rocks Become Magnetized

How Rocks Become

Magnetized in the Direction of the

Earth’s Magnetic North Pole

Continental Drift and Paleomagnetism• Magnetized grains record both the magnetic declination and

the inclination at the time the rock cooled through the Curie Point.

• Inclination = the angle of the magnetic field with respect to the horizontal (or the dip of the magnetic field). – Inclination = 90o at North magnetic pole– Inclination = 0o at the equator

Continental Drift and Paleomagnetism• The closer the igneous body containing the magnetic minerals is

located to the poles, the higher the magnetic inclination.• Therefore, the magnetic inclination is used to determine the

latitude at which an igneous body containing the magnetic minerals cooled and solidified.

• Thus, if either the pole or the rock moved between the time the rock formed and the present, the magnetic field locked in the rock would not be appropriate for the rock's current location.

Magnetic Inclination and Declination

Earth’s Magnetic and Geographic Poles• Magnetized grains record both the magnetic declination and

the inclination at the time the rock cooled through the Curie Point.

• Declination = the angle between where a compass needle points (magnetic north) and the true geographic north pole (axis of the Earth).

You may have noticed a

Difference Between Earth’s

Magnetic and Geographic

Poles on a Map

Earth’s Magnetic and Geographic Poles

Magnetic Inclination and Declination

In addition, the Earth’s Magnetic Field Varies Through Time

Continental Drift and Paleomagnetism

• Paleomagnetism • The study of ancient magnetic field

directions by studying the magnetism frozen into rocks of different ages.

• Polar Wandering• The apparent movement of the magnetic

poles illustrated in magnetized rocks. • Indicates that the continents have moved.

Polar-Wandering Paths for Eurasia and North America

• Polar Wandering:– A study of rocks in both

Europe and North America indicated that during the past 500 million years, the position of the North Pole had gradually wandered from points in the Pacific to its present position (termed "polar wandering").

– But what was even more surprising was that the two polar wander paths diverged more and more the further back in time they went.

Polar-Wandering Paths for Eurasia and North America

Animations\MagneticPoleDrift.movAnimations\MagneticPoleDrift200.m

ov

• Paths for North America and Europe have similar shapes but are separated by about 30 of longitude. – Differences between the paths can be reconciled if the continents are placed next

to one another.– When Europe and North America were reconstructed according to Wegener's earlier

model, the two paths coincided during the period from ~400 to 160 mya.– North America and Europe were joined together and moved relative to the poles

as part of the same continent.

A Scientific Revolution Begins• Following World War II, during the 1950s

and 1960s technological advances permitted extensive mapping of the ocean floor:

o Echo Sounder -- bathymetric mapping (mid-ocean ridges)

o Magnetometer -- mapping the magnetization of the sea floor

o Deep Sea Sampling -- mapping the age of the sea floor (found to be less than 180 million years old)

Continuously Recorded Profiles Along the Edge of the Mid-Atlantic Ridge

A new picture of the little-known ocean floor began to emerge that included great chasms, flat-topped submerged mountains, boundless abyssal plains, and globe-spanning volcanic ranges

that appeared on the new maps begged explanation.

Mid-Oceanic Ridge, Deep-Sea Trenches, and other Features of the Ocean Floors

These New Features Begged Explanation

o How did the volcanic mid-oceanic ridges originate?

o How did the deep-sea trenches originate?o Why were both so prone to earthquake

activity?o How did the submerged mountains originate? o Why was the Mid-Atlantic Ridge so nicely

centered and parallel to the coastlines of the continents on either side?

Sea Floor Spreading – A New Hypothesis

• Seafloor Spreading Hypothesis was proposed by Harry Hess in the early 1960s.

• Central Idea: Convective flow in the mantle caused the Earth’s outer shell to move.– Building on Arthur

Holmes’ idea (1930’s) – Convection currents in the mantle are the mechanism by which the continents move.

Seafloor Spreading Hypothesis

• The rising magma cools at the surface and is carried away laterally in both directions from the ridge crest in "conveyor belt" fashion.

• Tensional forces fracture or rift the crust at the ridge crest providing pathways for magma to intrude and generate new slivers of oceanic crust.

• Descending limbs of convection cells are located in the vicinity of deep-ocean trenches.

• Older seafloor consumed/recycled as it descends into the mantle.

• Proposed that oceanic ridges are located above zones of convective upwelling in the mantle.

Geomagnetic Reversals – The Other Piece of the Puzzle

• Normal Polarity (today) versus Reversed Polarity (present-day north magnetic pole becomes the south magnetic pole).

• Rock magnetic measurements on lavas of various different ages on the continents indicated that the earth's magnetic field periodically reversed polarity (geomagnetic reversal).

Geomagnetic Reversals – The Other Piece of the Puzzle

• Dates when the polarity of Earth’s magnetism changed were determined from lava flows at various localities and used to establish a magnetic polarity time scale.

Geomagnetic Reversals – The Other Piece of the Puzzle

• At the same time, magnetometers were being towed over the world's oceans to map the magnetism of the sea floor.

• Geomagnetic reversals are recorded in the ocean crust.

– It was discovered that there were alternating bands (stripes) of high-and low-intensity magnetism parallel to the ridge crests.

– It was also discovered that the patterns of stripes were mirror images of one another on either side of the ridge.

The ocean floor as a magnetic tape recorderThe high-intensity stripes were correlated with regions where the magnetism of the ocean crust was normal, and the low-intensity stripes where the magnetism was reversed.

Geomagnetic Reversals and Paleomagnetism

• These observations confirmed the formation of crust at mid-ocean ridges and the spreading away in either direction.

• Furthermore, from the polarity time scale, rates of spreading could be determined (on the order of a few centimeters per year).

Seafloor Spreading and Magnetism

Geomagnetic Reversals and Paleomagnetism

• In 1963 Vine and Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading.

• Paleomagnetism was the most convincing evidence set forth to support the concepts of continental drift and seafloor spreading.

A Scientific Revolution Begins

• In 1965, J. Tuzo Wilson provided the missing pieces needed to formulate the Theory of Plate TectonicsTheory of Plate Tectonics– Proposed that large faults connected the global

mobile belts into a continuous network dividing the Earth’s outer shell into rigid plates.

– Described three types of plate margins and how these solid blocks of Earth’s outer shell moved relative to one another.

Plate Tectonics: The New Paradigm• The Theory of Plate TectonicsThe Theory of Plate Tectonics

• Theory, called plate tectonics, has now emerged that provides geologists with the first comprehensive model of Earth’s internal workings.

• It is a unifying theory showing how a large number of diverse, seemingly-unrelated geologic facts are interrelated.

• A revolution in the Earth Sciences

Plate Tectonics: The New Paradigm

Animations\Pangaea600myears.mov

Plate Tectonics: The New Paradigm

Video Clip

Plate Tectonics: The New Paradigm• Earth’s Major Plates

• Associated with Earth's strong, rigid outer layer

– Known as the lithosphere

– Consists of uppermost mantle and overlying crust

– Overlies a weaker region in the mantle called the asthenosphere

Earth’s Major Plates: – Plates are in motion and continually changing in shape and size

– Plates move relative to each other at a very slow but continual rate

Plate Tectonic Boundaries• All major plate tectonic interactions

among individual plates occurs along their boundaries:

1) Convergent Boundary (Destructive Margins) – Two plates move together with subduction of oceanic plates or collision of two continental plates.

Plate Tectonic Boundaries2) Transform Boundaries (Conservative Margin) –

located where plates grind past each other without either generating new lithosphere or consuming old lithosphere.

Plate Tectonic Boundaries

3) Divergent Boundary (Constructive Margins) – Two plates move apart, due to upwelling of material from the mantle to create new seafloor.

Earth’s Major Plates: – Each plate is bounded by a combination of the three types of boundaries.

– Changing Boundaries – new plate boundaries can be created in response to changes in the forces acting on the lithosphere.

Divergent Plate Boundaries• Plates move apart from one another (diverge) – Rifting• Constructive Plate Margins • Most are located along the crests of oceanic ridges, but

also form on the continents• Also called spreading centers• Continental fragmentation• Decompression melting of the mantle• Igneous extrusions, commonly basalt, forming new crust –

submarine volcanism, continental volcanism, and flood basalts

• Forms rift valleys• Tensional stress• Normal and transform faulting • Shallow, lower magnitude earthquakes• Lengthening and thinning of the crust

Oceanic Ridges and Seafloor Spreading• New oceanic lithosphere is generated and the seafloor is

elevated forming oceanic ridges

Divergent Boundaries – Seafloor Spreading

Insert Mantle Melting and Plate Tectonics: Decompression Melting Animation #52

Divergent Boundaries – Seafloor Spreading

Insert Mantle Melting and Pressure-Temperature Graph Animation #53

• The rising magma cools at the surface and is carried away laterally in both directions from the ridge crest in "conveyor belt" fashion.

Animations\SeaFloorSpreading.mov

Divergent Boundaries – Seafloor Spreading

Insert Motion at Plate Boundaries: Divergent Animation #57

Divergent Boundaries – Rifting Plate Boundaries

Divergent Plate Boundaries• Spreading Rates and Ridge

Topography:– Typical rates of seafloor spreading

average 2 inches/year.– Rates are rapid enough to have

generated all of the Earth’s ocean basins within the last 180 my.

– Ridge systems exhibit topographic differences:

• These differences are controlled by spreading rates.

• Newly oceanic crust is elevated because it is hot, less dense, and occupies more volume.

• ~80 my for cooling and contraction to completely cease.

• Splits landmasses into two or more smaller segments along a continental rift

• Break-up of Pangea

Continental Rifting

• As the mantle convects magma rises as a plume from the mantle causing the continental lithosphere to expand and dome upward.

Animations\Upwarping.mov

• The uplifted area splits into three radiating fractures, forming a triple junction.

• Present-day example includes the East African Rift valleys (Afar Triangle).

Animations\RiftValley.mov

• Rifting occurs, and the three plates move outward away from the thermal plume

• Often times one of the arms of the triple junction fails (failed arm) and becomes a fault bounded trough that fills with sediment.

Animations\SeaFillsRift.mov

Animations\DivergingPlates.mov

Divergent Boundaries – Continental Rifting

Insert Divergent Boundary Animation #20

Divergent Boundaries – Continental Rifting

Insert Tectonic Settings and Volcanic Activity Animation #86

Convergent Plate Boundaries• Plates move toward one another (converge) • Destructive Plate Margins• Destroys crust / crust recycled• Compressional stress • Continental collision can occur creating mountain belts

with suture zone and accretionary wedge• Subduction can occur creating deep ocean trenches• Wet melting of the mantle• Create island and continental volcanic arcs• Stratovolcanoes with basaltic – andesitic – rhyolitic

volcanism.• Reverse and thrust faulting • Variable magnitude earthquakes at variable depths • Shortening and thickening of the crust• Extensive folding, deformation, metamorphism, and

igneous intrusions

Convergent Plate Boundaries• Oceanic-Continental Convergence:

– Oceanic plates are returned to the mantle in these destructive plate margins

– Surface expression of the descending oceanic plate is a deep-ocean trench

– Also called subduction zones

– Average angle of subduction = 45

– Angle varies depending on density (age and temperature) of oceanic plate

• Oceanic lithosphere is more dense and therefore subducts beneath less dense continental lithosphere into the asthenosphere.

• Along the descending plate (~100 km depth) partial melting (wet melting) of mantle rock (wedge) above it generates magma.

• Partial melting is triggered by the squeezing water and volatiles from the subducting slab (lowers the melting point of the mantle).

• Resulting volcanic mountain chain is called a continental volcanic arc (Andes and Cascades).

• However, much of this molten rock never reaches the surface – solidifies as igneous rocks at depth and acts to thicken the crust.

Animations\ConvergentPlateMargins.mov

Convergent Boundaries – Oceanic/Continental Crust

Insert Mantle Melting and Plate Tectonics: Wet Melting Animation #52

Convergent Boundaries – Oceanic/Continental Crust

Insert Mantle Melting and Pressure-Temperature Graph Animation #53

Divergent Boundaries – Seafloor Spreading

Insert Magma Rising from Mantle Animation #50

Convergent Boundaries – Oceanic/Continental Crust

Insert Tectonic Settings and Volcanic Activity Animation #86

• The ocean-to-The ocean-to-continent continent subduction zone subduction zone includes:includes:

• Accretionary Prism or Accretionary Wedge – Highly contorted and metamorphosed sediments that are scraped off the descending plate and accreted onto the continental margin.

Convergent Plate Boundaries• The Accretionary

Wedge may contain:1. Mélange – A complexly folded

jumble of deformed and transported rocks.

2. Blueschists – Distinctive metamorphic minerals (glaucophane and lawsonite) indicating high pressures but low temperatures. Forms in the unusual temperature/pressure regime of the subduction zone, where the relatively cool oceanic plate plunges rapidly into deep zones of high pressure.

Pillow Basalts – Part of the

Ophiolite Suite at Olympic

National Park, Washington

• The Accretionary Wedge may contain:3. Ophiolite Suite – Splinters of descending oceanic plate that was scraped off

and incorporated into the accretionary wedge. Mark the zone of contact between colliding continental and oceanic plates. Contain:» Deep-sea sediments (containg marine microfossils)» Submarine basalts (pillow lavas) » Metamorphosed mantle rocks (serpentinized peridotite)

Convergent Plate Boundaries• Oceanic-Oceanic Convergence:

– When two oceanic slabs converge, one descends beneath the other.

– Often forms volcanoes on the ocean floor due to the same mechanisms as oceanic-continental convergence.

– If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands).

Convergent Boundaries – Oceanic/Oceanic Crust

Insert Motion at Plate Boundaries Animation #57

Convergent Boundaries – Oceanic/Oceanic Crust

Insert Tectonic Settings and Volcanic Activity Animation #86

Convergent Plate Boundaries

• Continental-Continental Convergence: – Continued subduction can bring two continents together.

– Less dense, buoyant continental lithosphere does not subduct.

– Slabs of continental crust may override one another.

– Suture Zone = zone of convergence between two continental plates.

– Resulting collision between two continental blocks produces mountain belts (Himalayas, Alps, Appalachians) with:

» Folded Sedimentary Rocks

» Faulting

» Metamorphism

» Igneous Intrusions

Animations\CollisionMountainRange.mov

• Continental-Continental Convergence: – Continued subduction can bring two continents together.

– Less dense, buoyant continental lithosphere does not subduct.

– Slabs of continental crust may override one another.

– Suture Zone = zone of convergence between two continental plates.

– Resulting collision between two continental blocks produces mountain belts (Himalayas, Alps, Appalachians) with:

» Folding

» Faulting

» Metamorphism

» Igneous Intrusions

Continental-Continental Convergence

Convergent Boundaries – Continental/Continental Crust

Insert Convergent Margins – India-Asia Collision 2 Animation #15

Transform Plate Boundaries• Plates slide past one another • Conservative Plate Margins• Lithosphere is neither created nor destroyed

(neither constructive nor destructive plate margins)

• Shear stress • Transform fault zones consisting of strike-slip

faults• Offset of axes of mid-oceananic ridges• Variable magnitude earthquakes at variable

depths• Crust sheared and brecciated

Transform Boundaries

Insert Motions at Plate Boundaries Animation #57

Transform Fault Boundaries

Most join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zones

Transform Boundaries

Insert Transform Faults Animation #90

Transform Fault Boundaries A few (the San Andreas fault and the Alpine fault

of New Zealand) cut through continental crust

Importance of Plate Tectonics

Wilson Cycles • Plate tectonics has been in operation since

Archean times.• Over this immense interval, there have been

multiple openings and closings of ocean basins.

• Plate tectonic model for opening and closing of an ocean basin over time:

1. Opening of new ocean basin at divergent plate boundary.

2. Seafloor spreading continues and subduction begins.

3. Ultimate closure of the ocean basin during the final stage as plates converge during continental collision.

The Wilson

Cycle

Wilson Cycle Animation

Testing the Plate Tectonics Model• Evidence from:

– Ocean Drilling– Mantle Plumes and Hot Spots– Present-Day Rifting (oceanic and continental)– Using Paleomagnetism and Plate Motions to

calculate Rates of Seafloor Spreading– Measuring Plate Velocities and Directions

from Space– Seismic Zones at Subduction Boundaries

(Benioff Zones)– Exotic Terranes

Evidence from Ocean Drilling

• Evidence confirming seafloor spreading from drilling directly into ocean-floor sediment.

• Age of deepest sediments: – Ages increase with increasing distance

from the ridge.

• Thickness of ocean-floor sediments verifies seafloor spreading:

– Sediments nearly absent at ridge and increase in thickness with increasing distance from the ridge.

Spreading Rates from K-Ar Biochronology• Biostratigraphic evidence can be used to

test sea-floor spreading. • If sea-floor spreading has occurred, then

the biostratigraphic age of the sediments immediately overlying the oceanic crust should increase with increasing distance from spreading ridge axes.

• If on the other hand the ocean basins were created rapidly and 'all at once,' we would not expect any relationship between distance from the ridge axis and biostratigraphic age.

• One of the classic early papers demonstrating that biostratigraphic age of basal sediments does in fact increase with distance from ridge axes in a manner consistent with sea-floor spreading was that of Maxwell et al. (1970).

• These authors examined 7 complete sediment cores taken along a transect across the south Atlantic, 5 on the west of the Mid Atlantic Ridge, and 2 on the east of the MAR.

• Their biostratigraphic analyses showed clearly that the "age of the sediments above the basalt bears a direct relation to the distance from the ridge axis," such that "older sediments were found further from the axis" (p. 1049).

K-Ar paleontologic ages of basal sediments in south Atlantic cores vs. distance from

Mid-Atlantic Ridge (MAR).From Maxwell et al. (1970). http://www.geocities.com/earthhistory/CT.htm

Compare Ocean Sediment Thicknesses to Location of Mid-Oceanic Ridge, Deep-Sea

Trenches, and other Features of the Ocean Floors

Evidence from Mantle Plumes and Hot Spots• Mantle Plumes

– Concentrated areas of heat rising from great depths, perhaps at the mantle-core boundary.

– Long-lived structures.

• Mantle Plumes and Hot Spots– Hot spots are present on the Earth's surface above a mantle

plume. – Volcanoes can form over

them (hot-spot volcanism – shield volcanoes – Hawaiian Island Chain)

– As the plate moves over the hot spot new volcanoes form creating a volcano/seamount chain (Hawaiian Islands – Emperor Seamount Chain)

Interplate Tectonics – Hot Spots

Insert Hot Spot Volcano Tracks Animation #43

The Hawaiian Islands

Animations\HawaiianEmperorSeamount.mov

Global distribution of flood basalt provinces (black) and associated hot-spots (red dots).

Red dashed lines are hot spot tracks, which appear as lines of volcanic structures on the ocean floor.

Hot Spots – InterPlate Tectonics

• Intra-Plate Tectonics

• Plate moves over the hot spot

• Create a volcano/seamount chains

• Volcanism – calderas/flood basalts

• Crust is thickened

• Decompression melting

• Shallow, low magnitude earthquakes

Evidence from Present-Day Rifting: Continental

Plumes of magma from the mantle drive rifting of present-day Rift Valleys of East Africa indicating continent breaking up.

Image from U.S. Geological Survey

• Present-day sites of sea floor spreading have the following characteristics:  

• High heat flow • Seismic wave velocity decreases at the

ridges, due to high temperatures • A valley is present along the center of

ridge• Volcanoes are present along the ridge• Earthquakes occur along the ridge

Evidence from Present-Day Rifting: Mid-Ocean Ridges

Evidence from Measuring Plate Motion• Paleomagnetism and Plate Motions

• Paleomagnetism stored in rocks on the ocean floor provides a method for determining plate motions.

• Both the direction and rate of seafloor spreading can be established.

• Calculating Rates of Seafloor Spreading• Width of magnetic stripes on sea floor is related to time.

• Wide stripes = long time

• Narrow stripes = short time

• Knowing the age of individual magnetic stripes using the magnetic polarity time scale, it is possible to calculate rates of seafloor spreading and former positions of continents.

Calculating Rates of Seafloor Spreading and Ages of the Seafloor

http://www.geocities.com/earthhistory/CT.htm

Animations\SeafloorAge.mov

Reconstructing Historical Positions of the Continents

• Measuring Plate Velocities from Space– Accomplished by establishing exact locations

on opposite sides of a plate boundary and measuring relative motions.

– Methods include the use of:• Laser beams from Earth reflected off moon-

based mirrors.

• Man-made satellites in orbit around Earth – Global Positioning System (GPS).

• By measuring distances between specific points on adjacent tectonic plates over time, rates of plate movement can be determined.

Evidence from Measuring Plate Motion

Evidence from Measuring Plate Motion

Seismic Evidence for Plate Motion• Inclined zones of earthquake foci dip at about a 45o angle, near a deep-sea trench.

Benioff Zones, (or Wadati-Benioff Zones).

• Foci depths increase with increasing distances from deep-ocean trenches.

• The zone of earthquake foci marks the movement of the subducting plate as it slides into the mantle.

• The Benioff Zone provides evidence for subduction where one plate is sliding beneath another causing earthquakes at progressively deeper levels.

Evidence from Exotic Terranes• Exotic Terranes: Small pieces of continental crust

surrounded by oceanic crust are called microcontinents.

• Examples: Greenland, Madagascar, the Seychelles Bank in the Indian Ocean, Crete, New Zealand, New Guinea.

• Microcontinents are moved by seafloor spreading, and may eventually arrive at a subduction zone.

• They are too low in density and too buoyant to be subducted into the mantle, so they collide with (and become incorporated into the margin of) a larger continent as an exotic terrane.

• Exotic terranes are present along the margins of every continent.

• They are fault-bounded areas with different structure, age, fossils, and rock type, compared with the surrounding rocks.

Exotic Terranes• Green terranes probably

originated as parts of other continents.

• PinkPink terranes may be displaced parts of North America.

• The terranes are composed of Paleozoic or older rocks accreted during the Mesozoic and Cenozoic Eras.

Animations\AccretionOfFragments.mov

What Drives Plate Tectonics?• Researchers agree that convective flow

in the mantle is the basic driving force of plate tectonics.

• Convective flow occurs via huge thermal convection cells that move very slowly, taking millions of years to complete a cycle.

• Convection cells transfer heat in a circular pattern. Hot material rises; cool material sinks.

• Convective flow in the mantle is driven by the Earth’s “internal heat engine.”

Animations\ConvectionCells.mov

Mantle Convection

Insert Lava Lamp ConvectionAnimation #49

Convection Cells in the Mantle

• Rising part of convection cell = rifting (mid-ocean ridge)• Descending part of convection cell = subduction (deep sea trench)

Animations\ConvectionBoilingSoup.mov

Earth’s Internal Heat Engine• Major processes that have contributed to

Earth’s internal heat:

1. Heat remaining from the formation of the planet (high-velocity impact from colliding nebular debris).

2. Heat released as iron crystallized to form the solid inner core.

3. Heat continuously generated by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K) powers the internal processes that produce volcanoes, earthquakes, and mountains.

Models of Mantle-Plume

Convection

1. Layering at 660 Kilometers

2. Whole-Mantle Convection

3. Deep-Layer Model

Forces that Encourage Plate Movement• Slab-Pull – As slabs descend into the the asthenosphere, the trailing

plate is “pulled” along.• Ridge-Push – A gravity-driven mechanism – elevated oceanic ridge

causes slabs to “slide” down the ridge flanks. • Slab Suction – Drag of subducting slab on adjacent mantle induces

mantle circulation that pulls both plates toward the trench.

Forces that Counteract Plate Movement• Plate Resistance – Friction that occurs as the

subducting plate scrapes against the overridding plate.• Mantle Drag – Convection in the mantle that is

opposite plate motions.

• Unifying Theory that Helps Explain: – Surface Features– Surface Processes– Formation and

Distribution of: • Continents • Ocean Basins • Earthquakes • Volcanic Eruptions• Mountain Belts• Geologic Structures• Ancient Organisms• Surface Environments• Rock Types• Mineral Deposits

• It Influences:– Atmospheric and

Oceanic Circulation – Climate– Geographic Distribution – Evolution and Extinction

of Organisms– Formation and

Distribution of Resources

Importance of the Plate Tectonic Theory

Importance of the Plate Tectonic Theory

Insert Plate Tectonic Boundary Features Animation #66