Chapter 10: Plate Tectonics

Vocabulary

1. continental drift

2. mid-ocean ridge

3. sea-floor spreading

4. paleomagnetism

5. supercontinent

6. rift

7. magma

8. plate tectonics

9. lithosphere

10.asthenosphere

11.divergent boundary

12.convergent boundary

13.transform boundary

14.oceanic crust

15.continental crust

16.earthquakes

17.rift valley

18.subduction zone

19.island arc

20.fracture zones

21.convection

22.slab pull

23.rifting

24.terrane

25.supercontinent cycle

26.Pangaea

27.Panthalassa

28.Laurasia

29.Gondwanaland

I.Continental Drift

A.Definition: hypothesis that a single large

landmass broke up into smaller landmasses

to form the continents, which drifted to their

current locations

1. Proposed by Alfred Wegener, 1912

B.Evidence

1. Fossils of the same species were found on

separate continents, like South America &

Africa -- no evidence of land bridges &

unlikely that organisms traveled across the

Atlantic… therefore, the continents must

have been joined at some point

2. Rocks on separate continents were similar in age

& type

3. Mountain ranges seemed to form continuous

chains when the continents were envisioned

together

4. The same types of glacial debris were found in S.

America & Africa -- so these areas were once

covered by the same climate

C.Mechanism

1. Wegener proposed that the continents plow

through the rock of the ocean floor -- no

evidence, found to be physically impossible

2. Discovery of mid-ocean ridges (underwater

mountain ranges that have narrow, steep

valleys in the center) provided evidence for

continental drift

a. Sediment on sea floor is thinner closer

to a ridge -- suggests that sediment

near ridges is younger/newer than

sediment further away

b. Oceanic rock is very young -- oldest is

about 200 million years old, whereas

continental rock can be up to 4 billion

years old

3. 1950s: the idea of sea-floor

spreading was suggested

a. Valley at the center of the

mid-ocean ridge is a rift

(crack in Earth’s crust)

b. Through the rift, molten

rock (magma) rose to fill the

crack; then, the magma

cooled & solidified to form

new oceanic crust

c. If the ocean floor is moving,

maybe the continents are

too!

D. Paleomagnetism

1. Definition: magnetism of rock due to presence of iron in solidified magma & in

Earth’s core

2. Earth’s magnetic field has reversed several times in Earth’s history – depends on

the flow of iron-rich minerals

3. Rocks with certain magnetic orientations are grouped together chronologically,

which can help determine the age of the ocean floor

4. This supported the idea of sea-floor spreading

II. Theory of Plate Tectonics

A.Background

1. Earth’s interior can be divided into 3 zones based on composition & 5 zones based on

structure

2. Compositional Zones:

a. Crust: thin, solid, outermost layer -- oceanic (dense, rich in iron & magnesium, 5-

10 km thick) & continental (low density, silica-rich, 15-80 km thick)

b. Mantle: denser than crust & 2,900 km (~1,802 mi) thick

c. Core: center sphere made of iron & nickel & 7,000 km diameter

3. Structural Zones

a. Lithosphere: solid, outer layer of Earth that consists of the crust & the rigid upper

part of the mantle; 15 to 300 km thick

b. Asthenosphere: solid, plastic layer of the mantle beneath the lithosphere; made of

mantle rock that flows very slowly, which allows tectonic plates to move on top of

it; about 200 to 250 km thick

c. Mesosphere: strong, lower part of the mantle between the asthenosphere & outer

core; begin right below the asthenosphere, goes to a depth of about 2,900 km

d. Outer Core: liquid

e. Inner Core: solid, begins

at a depth of 5,150 km

B. Plate tectonics: theory that explains how large pieces of the lithosphere,

called tectonic plates, move and change shape

1. Tectonic plates (both oceanic & continental) “ride” on the liquid asthenosphere

2. About 15 major tectonic

plates have been

identified

3. Plates are often

bordered by major

surface features, such as

mountain ranges or

oceanic trenches

C. Types of Plate Boundaries

1. Divergent boundary: two plates

moving away from each other

a.Magma rises to the surface and

cools to form warm, light rock;

this rock sits higher than the

surrounding sea floor because it is

less dense

b.This usually occurs on the sea

floor at mid-ocean ridges; forms a

narrow valley where the plates

separate (rift valley)

2. Convergent boundary: two tectonic plates that are colliding;

can happen in 3 ways

a.Oceanic-oceanic: one plate subducts under the other plate,

and a deep-ocean trench forms; a chain of volcanic islands,

called an island arc, may form (example: Japan)

b.Oceanic-continental: denser oceanic lithosphere subducts,

or sinks, under the less dense continental lithosphere;

region along this plate boundary is called a subduction

zone

c. Continental-continental: colliding edges crumple and

thicken, which cause uplift that forms large mountain

ranges (example: Himalayas)

3. Transform boundary: two tectonic plates

that are sliding past each other

horizontally

a. Plate edges at a transform boundary

scrape against each other in sudden

bursts of motions that are felt as

earthquakes (example: San Andreas

Fault)

b. Can also occur along mid-ocean

ridges; short segments of transform

boundaries there are called fracture

zones

D. Causes of Plate Motion

1. Movement of tectonic plates is due to convection: movement of heated material due to

differences in density that are caused by differences in temperatures

2. Occurs in convection cells: cooler, denser material sinks, and warmer material rises

above it

3. This happens in the

mantle/asthenosphere, and

as the material in the mantle

moves, the tectonic plates move

along with it

4. Ridge Push: force exerted by one tectonic plate on

another, causing the second plate to be pushed

away from the mid-ocean ridge

a.Newly formed rock at a mid-ocean ridge is

warmer and less dense than older rock nearby.

b.As the newer rock cools and becomes denser, it

begins to slide down the slope between the

lithosphere and asthenosphere, causing ridge

push

5. Slab Pull: force exerted by one part of a

tectonic plate on the rest of the plate, causing

the whole plate to be pulled into the

asthenosphere

a.When the lithosphere cools and becomes

dense enough, it begins to subduct into the

asthenosphere.

b.As the leading edge of the plate sinks, it

pulls the rest of the plate along behind it

(slab pull)

III.The Changing Continents

A. Reshaping Earth’s Crust

1.Continents are always changing – gaining material, losing material, and changing shape

over millions of years

2.Rifting: process by which Earth’s crust breaks apart; can occur in oceanic or continental

crust

a. Reason unknown

b. Thick continental crust prevents the heat from the mantle from escaping; maybe as

heat builds up, continental crust weakens & splits

3. Continents can also change by gaining material

a. Terrane: piece of lithosphere (crust) with a unique geologic history different from the

surrounding lithosphere, which become part of a continent at convergent boundaries in

a process called accretion

b. Characteristics:

i. Different fossils

ii.Faults at boundaries

iii.Different magnetic properties

B. Effects of Continental Change

1.Climate Change: changes based on locations & geologic features – mountains

affect air flow, wind patterns, precipitation

2.Biological Change: populations are separated when continents drift – can lead

to natural selection & evolution!

C. Supercontinent Cycle

1.Definition: process by which supercontinents form,

break apart, & reform over millions of years

2.Cycle: plates move toward convergent boundaries &

collide, then heat from the mantle builds up &

causes rifting to break them apart

3.Pangaea: supercontinent formed during

the Paleozoic Era, surrounded by a large

ocean called Panthalassa

a.Broke up during the Mesozoic into

two smaller supercontinents: Laurasia

(north) & Gondwanaland (south)

b. Through rifting Laurasia separated into North America &

Europe, with the area between becoming the North Atlantic

Ocean

c. Gondwanaland separated into South America, Africa, India,

Australia, and Antarctica, also forming the South Atlantic Ocean

d. Fun fact: India was its own continent before it collided with Asia

4. More recent plate movement caused

the Rockies, Andes, Alps, and other

bodies of water to form

5. Future: at current plate movement

rates, South America & Africa will

collide in 150 million years, and

another massive supercontinent will

form in 250 million years