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.
Earth’s layers have different
Layer Composition Temperature Thickness
Inner Core solid metals 7000-8000°C 2400 km
Outer Core liquid metals 4400-6100 °C 2300 km
Mantle heated rock 870-4400 °C 2900 km
Crust Cooler rock 0-700 °C 6-70 km thick
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 split into large and
small slabs of rocks called tectonic
plates, which fit together like a jigsaw
Most large plates contain both continental
and oceanic crust.
Tectonic plates provide evidence of
continental movement that supports the
theory of Pangaea.
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.
See Page 15
The theory of plate tectonics
explains how plates and their
Wegener’s theory and sea floor maps
were used to develop the theory of
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
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
Tectonic plates have different
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
Transform Boundary: occur when plates
scrape past each other.
The sea floor spreads apart at
Mid-ocean ridges and rift valleys
occur at divergent boundaries in the
Mid-ocean ridges form the longest
mountain ranges on earth.
Most contain a rift valley along their
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 on continents
produce rift valleys.
Magma rises through cracks and forms
As rift valleys grow wider, continents split
If the valley continues to widen, the thinned
floor sinks below sea level.
It may fill with water to form a sea or lake.
Hot spots can be used to track
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
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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Tectonic plates push together
and form three types of
folding the rock
Oceanic-oceanic subductions: two oceanic
plates collide and the older, denser plate sinks
beneath the top plate, forming deep-ocean
trenches and island arcs.
Oceanic-continental subductions: an oceanic
plate sinks beneath a continental plate, forming
a deep-ocean trench and volcanic coastal
Tectonic plates scrape past each
other at transform boundaries.
Two plates move
past each other in
No crust is formed
Occurs on the sea
floor and on land.
The San Andreas Fault is a transform
boundary and moves about 1 inch per year.
direction of the
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Divergent *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
*crumples and folds crust * produces
*older, denser plate sinks *produce
deep-ocean trenches, island arcs
*oceanic plate sinks under
continental plate * forms deep-ocean
trench, volcanic coastal mountains
The theory of plate tectonics
helps geologists today.
The plate tectonics
Earth’s continents and
ocean basins formed.
and volcanic activity.