Download 8.1 Earth has several layers

8.4 Plates converge or
scrape past each other
Learning Goals
• Students will:
-describe what happens when two
continental plates converge.
-identify what happens when an
oceanic plate converges with another
plate.
-identify what happens when one
plate scrapes past another.
3-Minute Warm-Up
• Write all the statements. If they are
not true, correct them.
1. At a divergent boundary, tectonic
plates collide.
2. Rift valleys form at divergent
boundaries in the ocean and on
land.
3. Divergent boundaries in the ocean
are also called spreading centers.
3-Minute Warm-Up
• Decide if these statements are true. If
not, correct them.
1.At a divergent boundary, tectonic
plates collide. False-At a divergent
boundary, tectonic plates move apart.
2.Rift valleys form at divergent boundaries
in the ocean and on land. true
3.Divergent boundaries in the ocean are
also called spreading centers. true
Tectonic plates push together at
convergent boundaries
• At convergent boundaries crust is either
folded or destroyed
• subduction—the process by which an
oceanic tectonic plate sinks under another
plate into Earth’s mantle
• 3 types of convergent boundaries:
– Continental-continental collision
– Oceanic-oceanic subduction
– Oceanic-continental subduction
Continental-continental collision
(land and land)
• continental-continental collision—occurs where two
plates carrying continental crust push together
• Because both crusts are the same density, neither
plate can sink beneath the other
• Their edges crumple and fold
• Sometimes the folds can be pushed up and form
mountains
http://emvc.geol.ucsb.edu/3_downloads/
M1GTect/dContCollisionMovies/IndiaAsia
Collision.mov
Oceanic-oceanic subduction
(ocean and ocean)
• oceanic-oceanic subduction—occurs where one
plate with oceanic crust sinks under another plate
with oceanic crust
• When oceanic crusts meet, the older crust sinks
because it is colder and denser than the younger
crust
• The older crust sinks into the asthenosphere
• Two landforms that occur:
– Deep-ocean trenches—deep canyons that form
in the ocean floor as a plate sinks
– Island arcs—chains of volcanic islands that form
on the top plate, parallel to the deep-ocean
trench (ex: Japan and Philippine Islands)
Oceanic-continental subduction
(ocean and land)
• oceanic-continental subduction—occurs when
oceanic crust sinks under continental crust
• Oceanic crust sinks because it is denser
• Two landforms occur:
– Deep-ocean trenches-deep canyons. As the
oceanic crust moves, it often causes
underwater earthquakes
– Coastal mountains—they form parallel to the
coast and sometimes these mountains are
volcanoes
http://emvc.geol.ucsb.edu/3_dow
nloads/M1GTect/cSubductionMo
vie/Subduction.mov
Tectonic plates scrape past each other at
transform boundaries
• At a transform boundary, crust is neither
formed or destroyed. They move past
each other in opposite directions.
• In California, where we live, there is part
of a transform boundary called the San
Andreas fault
The theory of plate tectonics
helps geologists today
• By studying the rocks,
geologists can uncover the
history of any region
• This helps scientists predict
what will happen along
plate boundaries
– Earthquakes along transform
boundaries
– Volcanic activity near
subduction areas
Questions
• Draw and label the 3 types of convergent
boundaries. (6 pts.)
• What are deep-ocean trenches? (1 pt.)
• What are island arcs? (1 pt.)
• Explain how coastal mountains are formed (2 pts.)
• What makes the San Andreas Fault a transform
boundary? (1 pt.)
• Why is the theory of plate tectonics so important to
geologists? (1 pt.)