Download Chapter 10: Plate Tectonics

Chapter 10: Plate Tectonics
Vocabulary
1. continental drift
11.divergent boundary
21.convection
2. mid-ocean ridge
12.convergent boundary
22.slab pull
3. sea-floor spreading
13.transform boundary
23.rifting
4. paleomagnetism
14.oceanic crust
24.terrane
5. supercontinent
15.continental crust
25.supercontinent cycle
6. rift
16.earthquakes
26.Pangaea
7. magma
17.rift valley
27.Panthalassa
8. plate tectonics
18.subduction zone
28.Laurasia
9. lithosphere
19.island arc
29.Gondwanaland
10.asthenosphere
20.fracture zones
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, 510 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