Download 1-3 Notes: Divergent Boundaries Think About… • What causes

1-3 Notes: Divergent Boundaries
Think About…
 What causes plates to move apart?
Tectonic Plate Boundaries
 A plate boundary is the place where two plates meet.
 A ______________________ boundary is where plates move apart.
 A ______________________ boundary is where plates push
together.
 A ______________________ boundary is where plates move past
each other.
Seafloor Spreading
 In 1-2, we learned about seafloor spreading centers.
 Seafloor spreading centers are a type of _____________________
boundary.
 The world’s longest mid-ocean ridge is the Mid-Atlantic Ridge.
 It is ___________ km long, with a rift valley 25 km wide!
 Its deepest point is 9 km.
Magnetic Reversals
 The Earth can be compared to a bar magnet, which has a North and
South pole.
 Every 200k-300k years, Earth’s magnetic poles switch places.
 This is called a ________________________________________.
 No one knows why magnetic reversals occur.
 Note: the _______________________ North and South poles never
change-just the magnetic field.
 Evidence of Earth’s magnetic reversals can be found on the seafloor.
 As molten rock cools in the ocean, its _______________________ line
up with Earth’s magnetic field.
 When the magnetic field reverses, the _________________________ changes.
 Earth’s past magnetic reversals are apparent in the stripes of magnetic minerals in the seafloor.
 By testing the ages of rocks, scientists have determined that the most recent reversal happened ______________
years ago.
Continents Split Apart at Divergent Boundaries
 Divergent boundaries are found in the ocean at sea-floor
spreading centers.
 The ________________ erupting through the crack creates new
crust and pushes the two plates apart.
 Divergent boundaries can also break continents apart.
 As ___________________ rises toward the surface, it causes the
crust to bulge upward.
 The crust begins to crack as it is stretched, and a rift valley forms.
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Magma begins to erupt through the cracked, thinned crust,
forming __________________________.
Eventually, the continent __________________ apart.
The rift valley may be filled with water from nearby oceans and
____________________.
The African and Arabian plates began ___________________
several million years ago
This is how the ______________________ formed.
The African plate is also beginning to split apart in the Great
Rift _____________________.
Hot Spots
 Hot spot: a place in the lithosphere where a thin column of magma
rises from the ______________________.
 Hot spots stay in place while the plate above it keeps
____________________.
 Over time, hot spots form ____________________.
 The volcano can become high enough to rise above
the sea as an _____________________.
 This is how the Hawaiian Islands were created.
 After an island is formed, the moving plate carries it away from the
_______________________.
 The hot spot continues making new islands as the plate moves over it.
 The Hawaiian islands will continue to build up as the ___________________ Plate moves over the
hot spot.
 The movement of the North American plate is shown by the trail of volcanoes formed by the
hotspot beneath __________________________.
 The trail of inactive volcanoes formed by the hotspot tell us the North American plate is moving
southwest at a rate of about ______ cm per year.
Review
____1. New crust forms along a(n)
A. convergent boundary
B. divergent boundary
C. subduction zone
D. ocean trench
____2. At a mid-ocean ridge, the pattern formed by magnetic minerals
A. is the same on either side of the ridge
B. is randomly arranged at the ridge
C. is older along mid-ocean ridges and younger near ocean trenches
D. Shows that hot spots in Earth’s mantle cause volcanoes to form.