The Changing Earth
Chapter 1: Plate Tectonics
Section 1.1
Earth Has Several Layers
Earth is made up of materials with different densities.
Theory: Earth began as a spinning mass of rocks and dust 5 (16?) billion years ago.
Explosions from comets and asteroids, and pressure of Earth’s gravity, produce enough heat to melt materials inside Earth.
Dense material sank to the Earth’s center, less dense material moved toward the surface, forming Earth’s layers.
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Earth’s layers have different properties.Earth’s Layers
Layer Composition
Temperature
Thickness
Inner Core solid metals 7000-8000°C
2400 km diameter
Outer Core liquid metals 4400-6100 °C
2300 km thick
Mantle heated rock 870-4400 °C 2900 km thick
Crust Cooler rock 0-700 °C 6-70 km thick
Continued
Earth’s crust and the top of the mantle form the lithosphere.
The lithosphere sits on a layer of hotter, softer rock in the upper mantle called the asthenosphere.
The lithosphere is made up of many plates.
Continued
The lithosphere is split into large and small slabs of rocks called tectonic plates, which fit together like a jigsaw puzzle.
Most large plates contain both continental and oceanic crust.
Tectonic plates provide evidence of continental movement that supports the theory of Pangaea.
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Section 1.2
Continents Change Position Over Time.
Continents join together and split apart.
Alfred Wegener proposed the theory of continental drift in early 1900’s All continents were once joined and
gradually moved apart (still moving). Fossils, studies of ancient climates, and
rock formations provide evidence. Africa and Brazil have matching rock
formations indicating they were once joined.
Wegner’s theorized that all the continents
were once a single landmass.
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Wegner used fossil evidence to support his continental drift theory.
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The theory of plate tectonics explains how plates and their
continents move. Wegener’s theory and sea floor
maps were used to develop the theory of plate tectonics.
Along spreading centers in the sea floor, melted rock rises through cracks, cools, and forms new crust that builds up mid-ocean ridges.
Continued: Mid Ocean Ridge at Spreading Center
Continued
Old crust gets pushed aside and the sea floor slowly spreads apart.
Earth DOESN’T get larger because oceanic crust is destroyed along deep-ocean trenches, where the oceanic plates sink into the asthenosphere.
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Section 1.3
Plates Move Apart
Tectonic plates have different boundaries.
Plate boundary: where the edges of two plates meet Divergent Boundary: occurs where plates
move apart, usually found in the ocean Convergent Boundary: occurs when
plates push together. Transform Boundary: occur when plates
scrape past each other.
Divergent Boundary
Convergent Boundary
Transform Boundary
The sea floor spreads apart at divergent boundaries.
Mid-ocean ridges and rift valleys occur at divergent boundaries in the ocean. Mid-ocean ridges form the longest
mountain ranges on earth. Most contain a rift valley along their
center.
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Continued
Earth’s magnetic poles have switched places several times.
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These magnetic reversals are caused by changes in Earth’s magnetic fields. Bands of rock record periods of magnetic
reversals. As molten material cools, magnetic minerals line up with the magnetic field. When it hardens, the minerals act like
tiny compass needles.
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Continents split apart at divergent boundaries.
Divergent boundaries on continents produce rift valleys.
Magma rises through cracks and forms volcanoes.
As rift valleys grow wider, continents split apart. If the valley continues to widen, the thinned
floor sinks below sea level. It may fill with water to form a sea or lake.
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Hot spots can be used to track plate movements.
Hot Spot: an area of volcanic activity that develops above where magma rises in a plume from the mantle. Can be used to measure plate movement
because it generally stays in one place as the tectonic plate above it moves.
Can provide a fixed point for measuring the speed and direction of plate movements.
The Hawaiian islands are located in the middle of the Pacific Plate. The largest island, Hawaii,
is still over the hot spot.
When the plate moves on, it carries the first volcano away from the hot spot. Heat from the mantle plume will then
melt the rock at a new site, forming a new volcano.
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Section 1.4
Plates Converge or
Scrape Past Each
Other.
Tectonic plates push together and form three
types ofconvergent boundaries.
Continental-continental collision: Two continental plates collide, crumpling and folding the rock between them. Mountains could form.
Continued
Oceanic-oceanic subductions: two oceanic plates collide and the older, denser plate sinks beneath the top plate, forming deep-ocean trenches and island arcs.
Continued
Oceanic-continental subductions: an oceanic plate sinks beneath a continental plate, forming a deep-ocean trench and volcanic coastal mountains.
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Tectonic plates scrape past each other at transform
boundaries. Two plates move
past each other in opposite directions.
No crust is formed or destroyed.
Occurs on the sea floor and on land.
The San Andreas Fault is a transform
boundary and moves about 1 inch per year.
Boundaries are formed when
tectonic plates move. The
direction of the movement
determines the type of
boundary.
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Comparing BoundariesDivergent *plates move apart *in ocean and
on land *produce mid-ocean ridges, rift valleys, volcanoes, earthquakes
Transform *plates move past each other in opposite directions *in ocean and on land
Convergent C-C Collision
*crumples and folds crust * produces mountains, earthquakes
Convergent O-O Subduction
*older, denser plate sinks *produce deep-ocean trenches, island arcs
Convergent O-C Subduction
*oceanic plate sinks under continental plate * forms deep-ocean trench, volcanic coastal mountains
The theory of plate tectonics helps geologists today.
The plate tectonics theory enables geologists to understand how Earth’s continents and ocean basins formed.
Helps scientists predict earthquakes and volcanic activity.