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Plate Tectonics
Chapter 6
Earth is made up of materials with
different densities.
• Scientists theorize that Earth began as a
spinning mass of rocks and dust about 5
billion years ago.
• Explosions from comets and asteroids
crashing into its surface, along with the
pressure of Earth’s gravity, produced
enough heat to melt materials inside
Earth.
• Over time, dense material sank to the
center of the Earth and less dense
material moved toward the surface,
forming Earth’s layers
Composition of the Earth
Earth’s Layers
Crust
Continental
Mantle
Core
Oceanic
Outer
Asthenosphere
Mesosphere
Tree Map
Inner
Crust
• Earth’s crust and the top of the mantle form
the Lithosphere (rock sphere)
• outermost layer of the Earth
• Cooler rock
Two Types of Crust
• Continental
• Oceanic
• solid outer layer
• thicker than oceanic crust
(average 30km)
• less dense than oceanic
crust
• contains older rocks than
oceanic
• solid outer layer
• thinner than continental
crust (average 5-8km)
• more dense than
continental crust
• recycles its oldest rocks at
subduction zones
Mantle
• Extremely thick and contains most of the
Earth’s mass.
• Scientist look on the ocean floor where
molten rock from the mantle flows out of
active volcanoes
• Asthenosphere- a layer of hotter, softer
rock in the upper mantle.
Core
• OUTER
– melted iron
• INNER
– solid iron
– more dense
The lithosphere is made up of
many plates.
• The lithosphere is split into large and
small slabs of rocks called tectonic plates,
which fit together like a jigsaw puzzle.
• Most of the large tectonic plates contain
both continental and oceanic crust.
• The discovery of Earth’s layers and
tectonic plates helped scientists answer
the mystery of how the continents moved
to their present positions.
Continental drift-is the theory that continents can
drift apart from one another and have done so in the
past.
Alfred Wegner (German scientist) proposed that all the
separate continents of today were once joined in a single
landmass that he called Pangaea.
Evidence of continental drift
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•
•
•
•
Puzzle-like fit of the continents
fossils
similar types of rocks
same ancient climatic conditions
matching glacial grooves
The Theory of Plate Tectonics
explains how plates and their
continents move.
Scientist combined Wegener’s continental drift
hypothesis with information they gained from
mapping the sea floor 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.
• Old crust gets pushed aside, and the sea
floor slowly spreads apart.
• Earth doesn’t get larger, however,
because oceanic crust is destroyed along
deep-ocean trenches, where the oceanic
plates sink into the asthenosphere.
Causes of Tectonic Plate Motion
• Convection-heating and cooling of the
magma in the mantle
• Ridge Push-process when oceanic plate
slides down the lithosphere and
asthenosphere boundary
• Slab pull-denser oceanic crust subducts
under the other crust and melts in the
mantle
Tectonic Plate Boundaries
Tree Map
3 types of boundaries
convergent
continental/continental
collisions
Collision
divergent
oceanic/oceanic
collisions
Subduction
continental/oceanic
collisions
Subduction
transform
Convergent boundary
When two tectonic plates push into one another
• Continental/Continental collisions: plates
buckle and thicken creating mountains
• Example: Himalaya mountain range
• Continental/Oceanic collisions: oceanic plates sink under
the continental plate creating a subduction zone
– Forms deep-ocean trench, volcanic coastal mountains
• Oceanic/Oceanic collisions: one plate slides under the
other one creating a subduction zone
– Produces deep-ocean trench, island arcs
Divergent Boundaries
•When two tectonic plates move
away from one another
•Produces mid-ocean ridges, rift
valleys, volcanoes, earthquakes
•Example:
–mid-ocean ridge
Transform Boundaries
• When two tectonic plates slide past each
other in opposite directions.
• No crust is formed or
destroyed.
• Example:
– San Andreas Fault in California
Hotspots can be used to track plate
movements.
• A hot spot is an area of volcanic activity that
develops above where magma rises in a plume
from the mantle.
• A hot spot can be used to measure plate movement
because it generally stays in one place while the
tectonic plate above it keeps
moving.
• Example: Hawaii islands
• The plate tectonics theory enables geologist
to understand how Earth’s continents and
ocean basins formed. It also helps them to
predict earthquakes and volcanic activity.