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8.3 Plates move apart
Learning Goals
• Students will:
-identify different plate boundaries.
-explain what happens when plates
move apart.
-determine how to measure the
direction of plates.
3-Minute Warm-Up
•
Write the sentences, matching each
definition with the correct term
a. Pangaea
b. plate tectonics
c. sea-floor spreading d. continental drift
1. Huge landmass in which all continents were
once joined.
2. Hypothesis that Earth’s continents were once
joined in a single landmass and gradually
moved apart.
3. Theory that Earth’s lithosphere is made up of
huge, moving plates that are carried around
the planet by motions in the mantle.
3-Minute Warm-Up
• Match each definition with the correct term
a.Pangaea
b. plate tectonics
c. sea-floor spreading d. continental drift
1. Huge landmass in which all continents were
once joined. Pangaea - a
2. Hypothesis that Earth’s continents were once
joined in a single landmass and gradually
moved apart. Continental drift - d
3. Theory that Earth’s lithosphere is made up of
huge, moving plates that are carried around
the planet by motions in the mantle. Plate
tectonics - b
Tectonic plates have boundaries
• A plate boundary is where the edge of two
plates meet
• divergent boundary—occurs where plates
move apart. Usually found in the ocean
• convergent boundary—occurs where plates
push together
• transform boundary—occurs where plates
scrape past each other.
The sea floor spreads apart at divergent
boundaries
• Mid-ocean ridges mark these sites where the ocean
floor is spreading apart
• rift valley—a deep valley formed as tectonic plates
move apart. Found along a mid-ocean ridge
• Here, hot material (melted rock) rises to build new
crusts
Mid-ocean ridges and rift valleys
• Mid-ocean ridges are the longest
chain of mountains on Earth
• Contains a rift valley in the center
• Largest one is the Mid-Atlantic Ridge
•
http://emvc.geol.ucsb.edu/3_downloads/M3RegTect/aSoAtlantic/SoAtlanticR.mov
Sea-floor rock and magnetic reversals
• magnetic reversal—the switch in the
direction of Earth’s magnetic field
• No one knows why this occurs
• Magnetic reversals are recorded in the
sea-floor rock
• Makes it look like stripes
• This provided a record of sea-floor
spreading
•
http://emvc.geol.ucsb.edu/3_downloads/M1GTect/fSFSMovies/SFS5_Movie+info/SFS5_MagStripes+RevS
cale.mov
Continents split apart at divergent boundaries
• Like the sea floor, land on divergent
boundaries spread apart
• Heat causes the crust to bulge upward
and makes cracks and magma rises and
creates volcanoes
• As the crack stretches, a rift valley forms
and as it continues to stretch, the valley
becomes thinned out.
• Soon it is below sea level and water from
nearby fills the valley
• This is evident at the Red Sea
Hot spots
• hot spots—places where heated rock rises from
the mantle in plumes or thin columns.
• Hot spots stay in one place as the plate above it
moves. It helps scientists track plate movement
• Hawaiian Islands are formed from hot spots.
– Hot spots formed volcanoes and as the
volcanoes erupted, magma rose and the
water cooled it and formed land. As the plates
moved, the hot spot stayed and formed more
land
Questions
• Draw a divergent boundary in the ocean-use color
• What happens when an oceanic plate moves
apart?
• What happens when the continental plate begins
to split?
• Where is the largest mid-ocean ridge?
• What happens when the floor of a rift valley sinks
below sea level and where is this happening?
• How does a hot-spot volcano form? Use Hawaii as
your example.