12.2 – Features of Plate Tectonics
A Cross-Section of Earth
Layer
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Outermost layer made of solid, brittle rock.
__________________ crust can be up to 70 km thick and is made
mostly of a lighter type of rock called granite.
__________________ crust is only about 10 km thick and is made of
dark, heavy rock called basalt.
Thickest layer; about 2900 km thick.
_____________ mantle contains partly molten rock made up of iron
and magnesium-containing rocks.
_____________ mantle flows like thick toothpaste.
Liquid layer.
About 2300 km thick and made up of a mixture of iron and nickel.
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Sphere at the centre of the Earth with a radius of about 1200 km.
Temperature of 5000-6000 oC.
Immense pressure keeps it solid, even at high temperatures.
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Tectonic Plates
Earth’s outer layer is composed of several large, rigid, tectonic plates that are able to
move around. Made up of the crust and the top of the mantle, the plates make up the
____________________, which is 65-100 km thick. There are about ______ major
tectonic plates and many small ones. There are two types of plates – oceanic (made of
basalt) and continental (made of granite).
Below the lithosphere is the ________________________, a partly molten layer in the
upper mantle. Geologists think radioactive elements such as _______________ cause the
asthenosphere to have different temperatures in different areas. Convection currents in
the asthenosphere cause it to move around, carrying the tectonic plates with it. This is
called mantle convection.
_____________ reaches Earth’s surface at spreading centres. If a spreading centre
happens in the ocean, it is called a spreading ridge or oceanic ridge. If it happens on a
continent, it is called a _________________. Magma cools when it reaches the surface
and becomes new rock, pushing the tectonic plates apart. This process is called
_______________.
As the plates are pushed outward in the oceans, it disappears underneath continental plate
at the same time. This is called ______________________. Subduction is one plate
pushing under another one. Areas of subduction, called __________________________,
are often areas where volcanic eruptions and large earthquakes happen. When a tectonic
plate goes underneath another one, and pulls the rest of the plate with it, it is called
_________________.
A place where two tectonic plates are in contact is called a ________________________.
There are 3 main ways plates can interact with each other:
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Divergence (spreading apart)
Convergence (moving together)
Transform (sliding by)
Type of Plate
Boundary
Divergent
Convergent
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Plates are moving apart
In the ocean, sea floor spreading causes plates to move apart.
On land, divergent plates cause rift valleys. (Ex. East African Rift
Valley)
Ocean meeting Continent
 When an oceanic plate slides under a continent, a ___________
forms where the plates make contact.
 Melting oceanic plate comes to the surface and forms __________
along the coast of the continent. This is called a ______________.
 Mountain ranges may also form when continental crust crumples
as it is crushed by oceanic plate. (Ex. Cascade Mountains in BC)
 Many earthquakes occur in coastal areas where there are
subduction zones.
Ocean meeting Ocean
 Cooling causes one plate to be ____________ than the other. The
denser plate slides under the less-dense plate.
 ___________________________ may be formed because of the
subduction zone as magma rises to the surface. (Ex. Japan,
Hawaii)
Continent meeting Continent
 ________________ does not occur because the plates have similar
densities.
 Edges of the plates fold and crumple as they crash into each other,
forming mountain ranges. (Ex. Himalayan mountains)
Transform
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Plates slide past each other.
No mountains or volcanoes form, but __________________ and
____________ (breaks in rock layers due to movement of the
plates) do happen. Faults that happen at transform plate
boundaries are called __________________. (Ex. San Andreas
Fault)
Earthquakes
________________ between moving tectonic plates can produce stress, or the build-up of
pressure. When the plates can no longer resist the stress, there is a massive release of
energy that shakes the crust – an earthquake. _____% of all earthquakes occur at tectonic
plate boundaries.
The _____________ of an earthquake is the location inside Earth where the earthquake
starts. The __________________ is the point on Earth’s surface directly above the focus.
Earthquakes can happen at various depths, depending on the type of tectonic plates
involved. The amount of damage done by an earthquake depends on its _________.
Shallow earthquakes do more damage than ones that happen deep within the Earth.
Type of Seismic
Wave
Primary (P) Wave
Secondary (S) Wave
Surface (L) Wave
Description
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First to arrive (fastest)
Ground squeezes and stretches
Travels through solids, liquids and gases
Second to arrive (slower)
Ground moves in a wave-like up-and-down motion
Travels through solids but not liquids
Travels along Earth’s surface
Last to arrive (slowest)
Ground moves in a rippling motion, like waves on a pond
Earthquakes are measured using ____________________, also called seismographs, to
measure the amount of ground motion during an earthquake. The record produced by a
seismograph is called a _______________________. They provide information about
the time of the earthquake, how long it lasted, and how much shaking there was.
_________________ is a number that rates the strength or energy of an earthquake on the
Richter scale. Each 1-point increase on the scale is equal to a 10x increase in strength.
Volcanoes
There are 3 main types of volcanoes:
Type of
Volcano
Composite
Shield
Rift
Description
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Cone-shaped
Form near subduction zones in volcanic belts
Explosive eruptions due to gases, with thick magma
Largest volcanoes on Earth
Flatter and more spread-out than composite volcanoes
Form over hot spots to make volcanic islands
Less explosive with runny lava
Magma erupts through cracks in Earth’s crust.
Usually not very explosive, but can release large amounts of
lava
Can happen in the ocean or on land