Plate Tectonics and Continental Drift - Montana State University

Plate Tectonics and Continental Drift

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Questions and Topics

1. What are the theories of Plate Tectonics and Continental Drift?

2. What is the evidence that Continents move?

3. What are the forces that drive plate tectonics?

4. What happens at the boundaries between plates?

5. How do the different types of plate boundaries impact the regional geology and geomorphology?

6. How has continental drift affected the positions of the continents over time?

4

Plate Tectonics• Tectonics

– Movement of

Earth’s crust

• Plate tectonics

– Movement of

discrete

segments of

Earth’s crust in

relation to one

another

Continental Drift

• Alfred Wegener (1880-1930)

– Proposed that all of the continents were once part of a large supercontinent -Pangaea

– Based on:

• Similarities in shorelines

• Distinctive rock and fossil groups found in Africa & South America

Continental drift maps by Wegner (1915)

Continental drift maps by Wegner (1915)

Continental Drift

• Wegner proposed a mechanism for

drift

– Less dense silicic rocks plowed through

more dense ocean floor

– Earth’s rotation was driving force

• Although supporting evidence

existed, the theory was not widely

accepted

Evidence for Continental Drift

• Paleontological

– Similarity of fossils on opposite sides of

the Atlantic Ocean

• Plants and land dwelling animals

• No mechanism to transport across ocean

• Glossopteris on all southern continents

• Divergence of species following break-up

Paleontological evidence

11

Continental Drift

• DuToit’s evidence

– Expanded

Wegener’s ideas

– Mesosaurus fossils

• Found on

Gondwana

continents

• Freshwater

– Could not swim

across Atlantic

12

Continental Drift

• Glossopteris flora

present only in

southern

hemisphere

continents

13

Additional Evidence

• Lystrosaurus

– Heavyset

herbivore

– Found in

Antarctica

• 1969


• Rock type & structures

– Distinct rock type and geologic

structures on both sides of the Atlantic

Ocean

• Cape fold belt and equivalent – S.Africa &

Argentina

• Appalachian Mtns and equivalent – U.S.,

Canada, Scotland & Norway

• Only occur in rocks > 145 mya

15

Continental Drift• Geologic

Similarities

– Brazil and South

Africa have nearly

identical geologic

sequences

• Similar in

Antarctica and

India

– Glacial sediments

– Coal

Rock type & structure evidence


• Glaciation

– Late Paleozoic glaciation

• Covered large portions of the southern continents

• Distinct glacial deposit

• Glacial striations indicate direction of movement

• No evidence for glaciation on northern continents at this time

Reconstruction from glacial deposits


• Paleoclimate

– Evidence of extreme changes in climate

as compared to the present

• Coal deposits in Antarctica

• Evidence from evaporite deposits, eolian

deposits & coral reefs

• Paleoclimate reconstruction shows strange

patterns unless continents are moved

Fig. 17.6. Paleoclimate evidence

23

Paleomagnetism• Apparent Polar

Wander

– First studies

indicated poles

had moved

– Instead, plates

had moved

– North American

and European

paths met

Modern Plate Tectonic Theory

• Original evidence for continental drift was from continental rocks

• Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor

• Geophysics & paleomagnetism provided new data

Geology of the Ocean Floor

• Topography of the ocean basins

– Basins are divided by a large ridge

system

– Ridge system is continuous around the

entire globe

– Central rift valley within the ridge


• Physical properties

– Composed of basalt

– Younger in age than most continental

rocks

– Oceanic crust is thinner than continental

– No evidence of crustal deformation –

folded mountains

Crustal Properties

Crust Density Composition Thickness Age

continental ~2.8 g/cm3 FelsicThick:

20-70 km

Old:up to

4 Byrs

oceanic ~3.2 g/cm3 MaficThin:

2-10 km

Young:

<200 Mys


• Seafloor spreading proposed by Hess (1960)

– Considered new data on ocean floor

– Proposed mechanisms of:

• Mantle convection

• Rifting and volcanism along ridge system

• Continents pushed along w/ spreading seafloor

• Recycling of oceanic crust by subduction

30

Paleomagnetism• Magnetization of

ancient rocks at the

time of their formation

• Declination

– Angle that a compass

needle makes with the

line running to the

geographic north pole

• Rocks lock in this

orientation at formation


• Paleomagnetism

– Fe rich rocks are weakly magnetized by

the Earth’s magnetic field as minerals

form

– Orientation of magnetic field is

preserved

– Magnetic field orientation varies with

position on Earth’s surface


• Polar wandering

– Earth’s north magnetic pole was shown

to have moved through time

• Systematic change in position

– Polar wandering paths varied by

continent

– Multiple magnetic poles are not possible

Reconstruction from paleomagnetic data


• Magnetic polarity stripes in ocean

crust parallel ridges

– Symmetrical on either side of the ridge

– Polarity chrons give age of seafloor

• Increases away from ridge

• Rates of plate motion may be calculated

Fig. 17.10. Patterns of magnetic reversals


• Vine & Matthews (1963) tested

Hess’s hypothesis using

magnetism

– Magnetic polarity reversals recorded

in ocean floor basalt

• Magma cools forming new crust

• Polarity at time of cooling preserved

• Old crust pushed aside

The Mid Atlantic

Ridge

Age of the sea floor


• Seafloor sediments support plate

tectonic theory

– Youngest sediments resting directly on

basalt near the ridge

– Sediment just above the basalt gets

older moving away from the ridge

– Accumulation rates of ~3 mm/1000 yr

Plate Geography

• Lithosphere is divided into individual

plates

– Boundaries based on structural

features, not land and ocean

– Plates are outlined by ridges, trenches

and young mountain belts

– Plates are not permanent features

Major tectonic boundaries

Divergent Plate Margins

• Oceanic-Oceanic Crust

• Mid-oceanic ridge with central rift

valley

• Shallow earthquakes, less than

100km

• Basaltic lavas

Fig. 17.15. Divergent plate margins

Passive Continental Margin

Size comparison of various volcanic features

Divergent Plate Margins

• Continental-Continental Crust

– Rift Valley

– Shallow earthquakes, less than 100km

– Basaltic and Rhyolitic volcanism

• New material rising from the mantle produces basaltic lavas

• Thinning continental crust melts to produce rhyolitic lavas & instrusions

• East African Rift Valley

Convergent Plate Margins

• Oceanic-Oceanic

– Seafloor Trench

– Shallow and deep earthquakes, 0-700

km deep

– Andesitic volcanoes in an island arc

– Japan

The Aleutian Island Chain

Seismic activity in the Aleutian Islands

Oceanic-Oceanic and Oceanic-Continental Subduction

Zones


• Oceanic-Continental

– Subduction Zone

– Shallow and deep earthquakes, 0-700

km deep

– Andesitic volcanoes in a continental arc

– Cascade range


• Continental-Continental

– Intensely folded and thrust faulted mountain belts

– Metamorphic rocks dominate

• Sediments accumulated along continental margin are squeezed

– Igneous rocks commonly included

• Granitic magmas

Convergent plate boundaries

Transform Fault Margins

• Transform faults are large vertical fractures or faults in the crust

– Movement along faults is side to side

– May extend for long distances

– In oceanic crust, deep valleys are formed

– Transform faults may extend onto continents

– San Andreas fault

Juan de

Fuca plate

Rates of Seafloor Spreading

FAST

(East Pacific Rise)

~10-20 cm/year

SLOW

(Mid Atlantic Ridge)

~1-2 cm/year

Life of a person 100 years

Civilization 10,000 years

Stone tools 1,000,000 years

Modern Humans 100,000 years

10 meters

1 km

10 km

100 km

1-2 meters

100-200 m

1-2 km

10-20 km

Width of the Pacific Ocean ~ on the order of 10,000 km (16,000 miles) wide.How long would it take to create this much ocean crust.

Rates of Plate Motion

• Two ways to look at plate motion

– Relative velocity – the movement of

one plate relative to another

• Age of seafloor / distance from ridge

– Absolute velocity – compares plate

movement to a fixed position

• Use hotspots as fixed points of

reference

• Rates vary from 1 to 20 cm/yr

Fig. 17.20. Rates of plate motion around the world

Where do we see deep earthquakes? What is happening there?

Tectonic Mechanisms

• Convection of heat from the core and

mantle drives tectonics

– Convection cells bring new material to

the surface

– Old crust is pushed away from ridges

– Subduction carries cool crust back into

the mantle

Fig. 17.21. Models of plate tectonic motion

Tectonic Mechanisms

• Plates are active participants in the convection process

– Slab pull – dense ocean crust descends under its own weight

– Ridge push – gravity pulls lithosphere down & away from ridge

– Friction – resistance to movement from various sources

Mantle Plumes and Hot Spots

• Mantle plumes may form “hot spots”

of active volcanism at Earth’s surface

– Approximately 45 known hotspots

• Hot spots in the interior of a plate

produce volcanic chains

– Orientation of the volcanic chain shows

direction of plate motion over time

– Age of volcanic rocks can be used to

determine rate of plate movement

– Hawaiian islands are a good example

More evidence….

The World’s Hot Spots

81

Plate Motion• GPS

– Global Positioning

System

– Earth-orbiting

satellites identify

motion

• Transmitter on

satellite

• Ground-based

receiver

• Average rate

– 5 cm/year

Basa

lts

(Oph

iolite

s)

Marine

sedim

ent

s (c

hert

s, lim

est

ones,

re

d c

lays)

Tur

bidites,

clays,

silts,

sand

s

Gra

nite

s and

Rhyolite

Lava

s and

pyro

clast

ics

Ro

ck

/se

dim

en

t ty

pe

Te

cto

nic

se

ttin

gMafic Felsic

Composition of the Ocean Crust

• Seismic surveys suggest oceanic crust is ~7

km thick and comprised of three layers

– First layer is marine sediment of various

composition and thickness (extensively sampled)

– Second layer is pillow basalt overlying basaltic

dikes (extensively sampled)

– Third layer is thought to be composed of sill-like

gabbro intrusions (not directly sampled)

• Ophiolites are rock sequences in mountain

chains on land that are thought to represent

slivers of ocean crust and uppermost mantle

– Composed of layers 1-3 overlying ultramafic rock

JuandeFucaSmoker.av i

Smoker.mov

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Plate Tectonics and Continental Drift - Montana State University