Plate Tectonics and Continental Drift - Montana State ... 2008/Suits/Easc... · Plate Tectonics and...

Plate Tectonics and Continental Drift

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

• 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

Evidence for Continental Drift

• 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

Rock type & structure evidence

Evidence for Continental Drift

• 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

Evidence for Continental Drift

• 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

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

Geology of the Ocean Floor

• 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

Geology of the Ocean Floor

• 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

Geology of the Ocean Floor

• 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

Geology of the Ocean Floor

• 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

Geology of the Ocean Floor

• Magnetic reversals– Earth’s magnetic field polarity has

reversed through time• Normal polarity – Nmagnetic = Ngeographic

• Reversed polarity - Nmagnetic = Sgeographic

• At least 12 reversals in last 4 my

• Magnetic chronology is established by combining polarity chrons with radiometric age dating

Geology of the Ocean Floor

• 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

Geology of the Ocean Floor

• 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

Age of the sea floor

Geology of the Ocean 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

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

Fig. 17.15. Divergent plate margins

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

Convergent Plate Margins

• Oceanic-Continental

– Subduction Zone

– Shallow and deep earthquakes, 0-700 km deep

– Andesitic volcanoes in a continental arc

– Cascade range

Convergent Plate Margins

• 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

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

Intraplate volcanism

Rising mantle plumes can

produce localized hotspots

and volcanoes when they

produce magmas that rise

through oceanic or

continental crust. Hawaii

is an example

Composition?

Diamonds ascend to the

Earth's surface in rare

molten rock, or magma, that

originates at great depths.

Carrying diamonds and

other samples from Earth's

mantle, this magma rises

and erupts in small but

violent volcanoes. Just

beneath such volcanoes is a

carrot-shaped "pipe" filled

with volcanic rock, mantle

fragments, and some

embedded diamonds. The

rock is called kimberlite

after the city of Kimberley,

South Africa,

Ants, erosion and Namibian diamonds

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