Chapter 2: Plate Tectonics

Theory of plate tectonics is centered on

crustal movement, crustal deformation and mountain building

founder of geology

envisioned internal heat engine causing upheaval of earth’s crust

James Hutton

• paleontologist and geologist

• warping of crust makes mountains

James Hall

• geologist

• shrinking earth caused crust to bend and form mountains

JD Dana

• thought earth had been cooling since its formation

• planet contracts → surface wrinkles

• traverse forces push continental crust sideways

Edward Seuss

Who proposed continental drift?

Alfred Wegener

Continental drift theory

continents used to be the supercontinent Pangea

Alexander (AL) Du Toit; there were two primordial continents:

Laurasia (north) and Gondwana (south)

What is the fossil evidence for continental drift

some fossils from completely different continents were very similar to each other

How do rocks support continental drift?

rocks on opposite sides of the ocean match each other

2 pieces of evidence for past glaciation

• glacial deposits

• grooves in bedrock

Where were glacial deposits found in?

South America, Africa, India, Australia

Paleomagnetism

study of magnetic property of ancient rocks

Paleoclimate evidence for continental drift: ancient rocks and fossils don’t match their current climate. What are some examples of this?

• coal in antarctica

• coral reefs in Greenland

• glacial deposits in Sahara Desert

What are the 2 components of magnetism?

• inclination: horizontal swing

• declination: tilt of magnetization

What is declination in a magnet?

compass points to magnetic north

What is inclination in a magnet?

the angle compass makes with earth’s surface, function of latitude

T/F: Magnetic minerals settling through water column become aligned with earth’s magnetic field

true

T/F: Magnetic minerals in volcanic rocks record the direction of Earth’s magnetic field at the time the lava flow crystallized

true

Did magnetic poles wander or did the continents?

the continents

The following characteristics represent what?

• deep trenches

• immense fracture zones

• submerged volcanoes

• submarine ocean ridges (mid-ocean ridges)

bathymetry of seafloors

The following are chracteristics of:

• extend the length of ocean basins

• can be hundreds to thousands of kilometers long

mid ocean ridges

What are mid-ocean ridges?

submarine hot springs where seawater is discharged

Where is the mid-Atlantic ridge exposed above sea level?

Iceland

Ophiolite

slices of oceanic crust “shoved up” on land

Ocean Drilling Project (ODP) did what?

recover cores of oceanic lithosphere from over 2000 sites globally

What is bathymetry?

Measuring the depth of water in oceans, lakes, and other bodies of water

What is a mid-ocean ridge?

A long mountain range on the seafloor created by lava eruptions through earth

How long do mid-ocean ridges extend?

extend entire ocean basins, can be hundreds to thousands of kilometers long

sites of numerous submarine hot springs where heated water is discharged

Mid ocean range (book def)

What is convection?

The transfer of heat from one place to another through heat or gas movement

Why does hot air rise and cool air sink?

hot air has less density than cool air/cool air has more density than hot air

Who proposed that thermal convection was the driving force behind continental drift?

Arthur Holmes

Who proposed that convective motion of the mantle created and transported oceanic lithosphere?

Harry Hess

What do transform boundaries do in ocean basins?

join two segments of mid-ocean ridge

How are mid-ocean ridges broken apart?

by transform faults and fracture zones

What is paleomagnetism?

The direction of Earth’s magnetic field at the site and time of eruption

When lava erupts and cools down, it… (deals with magnetism)

records the direction of the magnetic at the time (normal/reversal)

Where does the youngest oceanic lithosphere occur?

mid-ocean ridges (ridge axis)

Where does the oldest oceanic lithosphere occur?

progressively further away from mid-ocean ridges and towards the margins of the continents

Hot conduits of bouyant mantle rising towards the surface

mantle plume

What can a mantle plume form? What is the most famous example?

hotspots and volcanoes, Hawaiian islands

Is the oldest or youngest island over a hotpot?

youngest (ex: Hawaii island)

What is the Wilson Cycle?

magma rises beneath continental mass —> lifts lithosphere and produces cracks where magma flows outward —> forms rift valley —> rift is flooded by the sea and becomes ocean floor —> ocean floor spreads —> oceanic lithosphere subduct beneath continent’s edge —> ocean closes —> continental masses come together

T/F: Mid-ocean ridges are perpendicular to the direction of seafloor spreading

False (they run parallel)

T/F: Transform faults are oriented parallel to the direction of seafloor spreading

true

T/F: Magnetic stripes are parallel to mid-ocean ridges

True

T/F: In moving farther away from a hot spot, volcanoes become progressively younge

False

T/F: Deep-sea sediment that accumulates on the seafloor is thickest along mid-ocean ridges and becomes thinner when moving farther away from the ridge axis

False

What are the 3 types of plate boundaries?

divergent, convergent, transform

Divergent boundaries are also called ___________ boundaries. They do what?

constructive; move apart from one another (can create subduction zones)

Convergent boundaries are also called ___________ boundaries. They do what?

destructive; collide with each other

Transform boundaries are also called ___________ boundaries. They do what?

lithosphere conserved; slide past one another

Where are active volcanoes and recent earthquakes located?

edge of continents

4 parts of a subduction zone

trench, accrectionary wedge, forearc basin, volcanic arc

3 types of convergent plate boundaries

ocean-ocean, ocean-continent, continent-continent

ocean-ocean convergence

ocean lithosphere subducts beneath another oceanic plate

ocean-continent convergence

ocean lithosphere subducts beaneath continental lithosphere

continent-continent convergence

two continental plates collide

what is ocean-ocean convergence chracterized by?

linear chain of volcanoes called island arcs (Aleutian Islands)

Example of ocean-continent convergence?

Andes Mountains

Example of continental collision?

India (40 million years ago)

Assemblage of micro-continents, volcanic arcs, and crustal fragments

Collage tectonics

Blocks of crust added to a land mass at a convergent plate boundary

accretion/accreted terranes

4 driving forces for plate tectonics

slab pull, ridge, push, whole mantle convention, layer-cake convection

cold, dense tectonic plate sinks into mantle at subduction zone

slab pull

ridge push

a gravity driven force that results from elevated position of the ridge

whole mantle convection

cold oceanic lithosphere sinks, hot mantle plumes rise

Two disconnected convective layers; upper layer driven by descending cold lithosphere and ascending hot lower layer

layer cake model