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Plate Tectonics
8th Grade
Chapter 5
5.1 Vocabulary Terms
• Continental drift
• Pangaea
• Fossil
Wegener’s Hypothesis (pg 164)
• In 1915, Wegener hypothesized that all the continents were once
joined together in Pangaea about 300 million years ago and have
since drifted apart.
• Evidence shows
• Land features from different continents line up (such as mountains) and
resource deposits (such as coal).
• Fossils found on continents that are now separated by oceans (such as plants
and freshwater reptiles).
• Fossils of tropical plants are found in artic environments.
• Since Wegener could not identify the cause of continental drift, his
idea was rejected.
5.2 Vocabulary Terms
• Mid-ocean ridge
• Sonar
• Deep-ocean trench
• Subduction
Mid-Ocean Ridges (pg 168)
• Scientists used sonar and found mid-ocean ridges that extend into all
of Earth’s oceans.
• Steep-sided valley splits the tops of some mid-ocean ridges.
• These ridges form the longest mountain ranges on Earth.
Sea-Floor Spreading (pg 170)
• Sea-floor spreading begins at a mid-ocean ridge which forms at a
crack in the oceanic crust.
• New molten material rises, erupts, cools, and hardens to form a solid
strip of rock along the ocean floor.
• Sea-floor spreading adds more crust to the ocean floor while older
strips of rock move outward from either side.
• Some mid-ocean ridges have a valley along the center.
Evidence of Sea-floor Spreading
• Ocean Material
• Pillow-shaped rocks from molten material that hardens quickly from erupting
under water
• Magnetic Stripes
• When molten materials cools, magnetic minerals line up based on Earth’s
magnetic poles at that time
• Patterns on one side of ridge mirror on other side & shows the time frame
and rate of the spreading
• Drilling Samples
• Youngest rock samples found at the center of ridges
Deep-Ocean Trenches (pg 172)
• Part of the ocean floor sinks back into the mantle at deep-ocean
trenches.
• Changes in density affect the ocean floor. As new rock cools, it
becomes dense. It moves over time and collides with an edge of a
continent. Gravity pulls it down back into the mantle.
• Crust closer to a mid-ocean ridge moves away from the ridge and
toward a deep-sea trench.
Earth’s Oceans (pg 173)
• The process of subduction & sea-floor spreading work together to
change the size and shape of the oceans.
• This process is determined by how fast new crust is created & old
crust sinks.
• An ocean with few trenches will grow larger, like the Atlantic.
5.3 Vocabulary Terms
• Plate
• Divergent boundary
• Convergent boundary
• Transform boundary
• Fault
• Rift valley
Theory of Plate Tectonics (pg 175)
• In the 1960s, scientists combined sea-floor spreading, Earth’s plates,
and plate motions into the theory of plate tectonics.
• Earth’s plates are in slow, constant motion, driven by convection
currents in the mantle.
Plate Movement
• The plates move because they are on top of large convection currents
in the mantle.
• Gravity pulls denser plate edges downward into the mantle and the
rest of the plate move too.
Motions Over Time (pg 176)
• Plates move 1-12 cm per year and have been for millions of years.
(North America and Europe are moving apart at a rate of 2.5 cm per year. About as fast as your
fingernails grow.)
• Plate movement produces earthquakes, volcanoes, mountain ranges,
and deep ocean trenches.
Divergent Boundaries (pg 177)
• Occur along the mid-ocean ridges, where new crust is added during
sea-floor spreading. In a few places, the ridges rise above sea-level.
• East African rift system is created along a divergent boundary.
Convergent Boundaries (pg 178)
• When plates collide, the denser rock sinks under the less dense plate
and pushes it up.
• Andes Mountains in Chile are made at a convergent boundary.
Transform Boundaries (pg 179)
• Beneath the surface, the sides of the plates are rocky and jagged,
causing the plates to “lock together.” When forces cause them to
unlock, the plates slip along the boundary creating earthquakes.
• Crust is neither created nor destroyed.
• The San Andreas fault is along a transform boundary.
5.4 Vocabulary Terms
• Stress
• Tension
• Compression
• Shearing
• Normal fault
• Hanging wall
• Footwall
• Reverse fault
• Strike-slip fault
• Folds
• Anticline
• Syncline
• Fault-block mountains
• Plateau
Change from Stress (pg 180)
• While some rocks may bend at first, all rocks in Earth’s
crust will break at some point.
• Stress adds energy to rock that is released when the rock
changes shape or breaks.
• Tension, compression, and shearing work over millions of
years to change the shape and volume of rock.
How Faults Form (pg 182)
• Normal faults occur where two plates diverge and the hanging wall
slips downward.
• Reverse faults form when plates converge and the footwall moves
down.
• Strike-slip fault occurs at a transform boundary.
New Landforms (pg 184)
• Rocks stressed by compression may bend without
breaking creating anticlines and synclines. Folding
produces some of the world’s largest mountain ranges.
• Two normal faults can cause valleys to drop on either side
of a block of rock creating fault-block mountains.
• Some plateaus form when forces push up a large, flat
block of rock that is wider than it is tall.
5.5 Vocabulary Terms
• Volcano
• Magma
• Lava
• Hot spot
Volcanoes and Boundaries (pg 189)
• Volcanoes form in a regular pattern along Earth’s plates.
One major belt of volcanoes, the Ring of Fire, is found
along the rim of the Pacific Ocean.
• At divergent boundaries, the crust fractures and magma
pushes to the surface. Found along mid-ocean ridges
and Great Rift Valley in Africa.
• At convergent boundaries, subduction at deep-ocean
trenches brings water under the other plate and lowers
the melting point so that magma is formed and rises up.
• Sometimes creates a string of islands called an island
arc is created along the curve of the trench.
• When oceanic plate sinks beneath continental crust,
volcanic mountains, like Andes, are formed.
Hot Spots (pg 191)
• A volcano forms above a hot spot when magma erupts through the
crust and reaches the surface.
• Hot spots stay in one place for millions of years while the plate moves
over them.
• Yellowstone National Park is over a huge hot spot.
5.6 Vocabulary Terms
• Magma chamber
• Pipe
• Vent
• Lava flow
• Crater
• Silica
• Volcanic ash
• Cinders
• Bombs
• Pyroclastic flow
• Dormant
• Extinct
Volcanic Eruption (pg 194)
• Magma forms in soft layer of hot rock in the upper
mantle, just below a layer of hard rock.
• Gases are dissolved in magma. As magma rises and
pressure decreases, the dissolved gases expand. The
force of expanding gases pushes magma from the
magma chamber through the pipe until it flows out the
vent.
Types (pg 195)
• Quiet eruptions happen when hot, low-silica magma is
thin and runny and flows easily. Gases bubble out gently.
• Pahoehoe forms from fast-moving, hot lava that is thin
and runny. It looks like rope-like coils.
• Aa forms from cooler, thicker, slower-moving lava. It has
jagged surface.
• Quiet eruptions have built up Hawai’i.
• High-silica magma is thick and sticky so it plugs up the volcano’s pipe.
The trapped gases build up pressure until it explodes.
• Explosive eruption throws lava into the air where it breaks into
bombs, cinders, and ash.
Hazards (pg 197)
• In a quiet eruption, lava flows from vents and sets fire to everything in
its path.
• In an explosive eruption, a fast-moving cloud rushes down the sides
and covers everything.
Stages of Activity (pg 198)
• Geologist use terms active, dormant, or extinct to
describe a volcano’s activity.
• Changes in activity can give warning signs.
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Tiltmeters detect elevation and surface changes
Gases escaping
Small earthquakes
Rising temperatures in underground water