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Chapter 5 Plate Tectonics
Section 1: Earth’s Interior
1. Exploring Inside Earth
a. Geologists cannot dig to the center of the Earth
i. The conditions are extreme
ii. More than 6000 km to reach the center
iii. Geologists have used 2 main types of evidence to learn about Earth’s interior
1. Evidence from rock samples
a. Rocks from inside Earth give clues about Earth’s structure
b. Geologists have drilled holes up to 12 km
i. The drills bring up samples of the rock
c. Geologists can make inferences about conditions deep inside Earth
d. Forces inside Earth sometimes blast rock to the surface from more than 100
km deep
2. Evidence from seismic waves
a. Seismic waves: vibrations that travel through Earth carrying the energy
released during an earthquake
b. Geologists study how seismic waves travel through Earth
i. Their speed and paths reveal Earth’s structure
c. Now we know the Earth is made up of layers
2. A journey to the center of Earth
a. The 3 main layers are the crust, the mantle, and the core
i. They vary greatly in size, composition, temperature, and pressure
b. Temperature
i. As we begin to tunnel to the center, we go through cool rock
ii. At about 20 meters down the rock is getting warmer
iii. For every 40 meters from that point, the temperature increases 1°C
iv. After several tens of kilometers, the temperature increase slows.
v. The high temperatures are the result of heat left over from the formation of Earth
vi. Radioactive substances also release energy inside Earth
c. Pressure: results from a force pressing on an area
i. Inside Earth, pressure increases as you go deeper because of the weight of the rock above
3. The Crust: the layer of rock that forms Earth’s outer layer
a. Layer of solid rock that includes both dry land and ocean floor
i. Includes rocks, mountains, soil, and water
b. Much thinner than the layer below it
c. Thickest under high mountains and thinnest beneath the ocean
d. Mostly between 5 and 40 km thick
i. Can be up to 70 km
e. The crust beneath the ocean is called oceanic crust
i. Consists mostly of basalt
1. Basalt: dark rock with a fine texture
f. Continental crust forms the continents
i. Consists mostly of rocks such as granite
1. Granite: a rock that usually is a light color and has a coarse texture
4. The Mantle: a layer of hot rock
a. Made up of rock that is very hot, but solid
b. 3,000 km thick
c. Divided into layers based on the physical characteristics of those layers
i. The Lithosphere
1. Uppermost part of the mantle
2. Very similar to the crust
3. Lithos means “stone”
4. About 100 km thick
ii. The Asthenosphere
1. Below the lithosphere
2. Less rigid than the rock above
3. Somewhat soft like road tar heated by the sun
4. Asthenes means “weak”
5. Solid
iii. The Lower Mantle
1. Beneath the asthenosphere
2. Extends all the way to Earth’s core
iv. The Core
1. Made mostly of the metals iron and nickel
2. Consists of two parts—a liquid outer core and a solid inner core—together 3,486 km
a. Outer Core: a layer of molten metal that surrounds the inner core
i. Liquid
b. Inner Core: a dense ball of solid metal
i. Extreme pressure squeezes the atoms of iron and nickel so much that
they cannot spread out and become liquid
3. The core and Earth’s Magnetic Field
a. The movements in the liquid outer core may create Earth’s magnetic field
b. Earth acts like a giant bar magnet
c. Compasses align with Earth’s magnetic field
Section 2: Convection and the Mantle
1. Types of Heat Transfer
a. Heat always moves from warmer to cooler
b. Cold is the absence of heat
i. Things feel cold because the heat is leaving our hands
c. There are 3 types of heat transfer
i. Radiation: heat transfer of energy through space
1. No direct contact
2. Ex: Sunlight or a flame
ii. Conduction: heat transfer within a material or between materials that are touching
1. Ex: A spoon in a pot of hot soup heats up
a. Particles near the bottom of the spoon vibrate faster, so they bump into
other particles to heat them up too
iii. Convection: heat transfer by the movement of currents within a fluid—liquids and gases
1. Heated particles of fluid begin to flow
2. The flow transfers heat from one part of the fluid to another
3. Caused by the differences of temperature and density within a fluid
a. Density: a measure of how much mass there is in a volume of a substance
4. When a liquid or gas is heated, the particles move faster and spread apart
a. The particles of the heated fluid occupy more space so density decreases
5. When a liquid or gas cools, its particles move more slowly and settle together so
density increases.
2. Convection Current: the flow that transfers heat within a fluid
a. Heating and cooling of the fluid, changes in the fluid’s density and the force of gravity combine to
set convection currents in motion.
b. As the soup at the bottom of a pot gets hot, it expands and therefore becomes less dense. The
warm, less dense soup moves upward and floats over the cooler, denser soup.
c. At the surface, the warm soup cools, becoming denser. Then gravity pulls this cooler, denser soup
back down to the bottom, where it is heated again.
3. Convection Currents in Earth
a. Heat from the core and the mantle itself causes convection currents in the mantle
b. The great heat and pressure in the mantle cause solid mantle rock to flow very slowly
c. Geologists think that plumes of mantle rock rise slowly from the bottom of the mantle toward the
top. The hot rock eventually cools and sinks back through the mantle
d. Similar convection currents occur in the outer core that cause Earth’s magnetic field
Section 3: Drifting Continents
1. Continental Drift: idea that the continents slowly moved over Earth’s surface
a. Alfred Wegener—1910—German Scientist
i. He hypothesized that all the continents were once joined together in a single landmass and
have since drifted apart
ii. Pangaea: the name of the single landmass that broke apart to create the continents
1. Existed about 300 million years ago
2. This was the time when reptiles and winged insects first appeared
3. Tropical forests, which later formed coal deposits, covered large parts of the Earth’s
surface
iii. Wegener gathered evidence about continental drift by studying land features, fossils, and
evidence of climate change
b. Evidence from Land Features
i. Wegener pieced together maps of Africa and South America, he noticed that mountain
ranges on both continents line up
ii. He noticed that European coal fields match up with coal fields in North America
c. Evidence from Fossils
i. Fossil: any trace of an ancient organism that has been preserved in rock
ii. Glossopteris, a fernlike plant, was found in Africa, South America, Australia, India, and
Antarctica
iii. Mesosaurus and Lystrosaurus have both been found in places now separated by oceans
d. Evidence from Climate
i. As continents move toward the equator, the climate becomes warmer
ii. As continents move toward the poles, the climate becomes colder
iii. Fossils of tropical plants were found on an island in the Arctic Ocean
iv. Continental glaciers once covered South Africa
v. According to Wegener, the climates of these places changed because the landmasses had
moved
e. Wegener’s Hypothesis Rejected
i. Wegener could not provide a satisfactory explanation for the force that pushes or pulls the
continents
1. Since he could not prove it, most geologists rejected it
Section 4: Sea-Floor Spreading
1. Mid-Ocean Ridges: an undersea mountain chain where new ocean floor is produced
a. Scientists mapped the mid-ocean ridges using sonar in the 1900’s
i. Sonar: a device that bounces sound waves off underwater objects and then records the
echoes of these sound waves
1. The time it takes for the echo to arrive indicates the distance to the object
ii. Most of the mountains in the mid-ocean ridge system lie hidden under hundreds of meters
of water
iii. In a few places, the ridge pokes above the surface
1. Ex: The island of Iceland is a part of the mid-ocean ridge
2. What is Sea-Floor Spreading?
a. Harry Hess, an American geologist, carefully examined maps of the mid-ocean ridge system
i. He began to think Wegener was right about Pangaea
ii. In 1960, he proposed sea-floor spreading
1. Sea-floor spreading: a process that continually adds new material to the ocean floor
a. The sea floor spreads apart along both sides of a mid-ocean ridge as new
crust is added.
b. The ocean floors move like conveyor belts, carrying the continents along with
them
c. Begins at a mid-ocean ridge, which forms along a crack in the oceanic crust
d. Molten material beneath the surface rises and erupts along the ridge
e. Older rock moves outward on both sides of the ridge
f. As the molten material cools, it forms a strip of solid rock in the center of the
ridge
g. It forms a new strip of rock
3. Evidence for Sea-Floor Spreading
a. Supporting Evidence: eruptions of molten material, magnetic stripes in the rock of the ocean floor,
and the ages of the rocks themselves
i. Evidence from Molten Material
1. Scientists proved that new material is erupting along mid-ocean ridges
2. They took a submarine, Alvin, to a ridge valley where they found pillow shaped
rocks, or like toothpaste squeezed from a tube
a. Rocks like this only form when molten material hardens quickly after
erupting under water
ii. Evidence from Magnetic Stripes
1. While studying the ocean floors, scientists found that the Earth’s magnetic poles
have reversed themselves many times.
a. The last reversal happened 780,000 years ago
2. The rock that makes up the ocean floor lies in a pattern of magnetized “stripes”
3. The rock contains iron
a. As the rock cooled, the iron bits lined up in the direction of Earth’s magnetic
poles
iii. Evidence from Drilling Samples
1. The Glomar Challenger, a drilling ship built in 1968, gathered samples from the
ocean floor
2. This showed scientists that the newest rocks were always in the center of the ridges
a. Made scientists assured that sea-floor spreading really has taken place
4. Subduction at Trenches
a. Deep-ocean trenches: a deep valley along the ocean floor beneath which oceanic crust slowly sinks
toward the mantle
i. The oceanic crust bends downward
ii. Part of the ocean floor sinks back into the mantle at deep-ocean trenches
b. The Process of Subduction
i. Subduction: the process by which ocean floor sinks beneath a deep-ocean trench and back
into the mantle
ii. Crust closer to a mid-ocean ridge moves away from the ridge and toward a deep-ocean
trench
iii. Sea-floor spreading and subduction work together
1. They move as if the ocean floor were a giant conveyor belt
c. Subduction and Earth’s Oceans
i. Subduction and sea-floor spreading can change the size and shape of the oceans
ii. The ocean floor is renewed about every 200 million years
iii. The Pacific Ocean covers almost 1/3 of Earth
1. It is shrinking
2. Sometimes a deep ocean trench swallows more oceanic crust than a mid-ocean
ridge can produce
3. If the ridge does not add new crust fast enough, the width of the ocean will shrink
iv. The Atlantic Ocean is expanding
1. It has only a few short trenches
2. The ocean floor has nowhere to go
a. As the ocean floor spreads, the continents along its edges also move
b. The whole ocean gets wider
Section 5: The Theory of Plate Tectonics
Plates: sections of the lithosphere that carry the continental and oceanic crust
Scientific Theory: a well-tested concept that explains a wide range of observations
1. How Plates Move
a. The theory of plate tectonics explains the formation, movement, and subduction of Earth’s
plates
i. States the pieces of Earth’s lithosphere are in slow, constant motion, drive by
convection currents in the mantle
ii. During subduction, gravity pulls one edge of a plate down into the mantle.
iii. The plates collide, pull apart, or grind past each other causing changes
1. Changes include volcanoes, mountain ranges, and deep-ocean trenches
2. Plate Boundaries
a. The edges of Earth’s plates meet at plate boundaries
b. Faults: breaks in Earth’s crust where rocks have slipped past each other
i. Faults form along plate boundaries
c. There are three kinds of plate boundaries.
i. Divergent Boundaries
1. The place where two plates move apart, or diverge
2. Most occur along the mid-ocean ridges where sea-floor spreading occurs
3. Also occur on land
a. Two of Earth’s plates slid apart and a deep valley, called a rift valley,
forms
i. Ex: Great Rift Valley in East Africa
ii. Convergent Boundaries
1. The place where two plates come together, or converge
2. The result is called a collision
a. The density of the plates determines which one comes out on top
3. Continental crust can collide with oceanic crust
a. Oceanic crust is more dense than continental crust so subduction occurs
and the oceanic plate sinks beneath the continental plate
4. When two continental plates collide, neither are dense enough to sink very far
a. The collision squeezes the crust into mountain ranges
iii. Transform Boundaries
1. A place where two plates slip past each other, moving in opposite directions
2. Earthquakes often occur along transform boundaries, but crust is neither created
nor destroyed
iv. Plate Motions Over Time
1. Geologists have evidence that other supercontinents formed and split apart
before Pangaea