plates move apart. what happens when plates move apart? think about…
TRANSCRIPT
1.3 Notes
Plates Move Apart
What happens when plates move
apart?
Think About…
A plate boundary is where the
edges of two plates meet.
Tectonic Plates Have Different Boundaries
A divergent boundary occurs
where plates move apart.
Tectonic Plates Have Different Boundaries
Most divergent boundaries are
found in the ocean.
Tectonic Plates Have Different Boundaries
A convergent boundary occurs
where plates push together.
Tectonic Plates Have Different Boundaries
A transform boundary occurs
where plates scrape past each other.
Tectonic Plates Have Different Boundaries
Today, we will focus only on
divergent boundaries-both in the ocean and on land.
Tectonic Plates Have Different Boundaries
In section 1.2, we learned about
seafloor spreading centers. Spreading centers are a type of
divergent boundary.
The Sea Floor Spreads Apart at Divergent
Boundaries
The world’s longest
mid-ocean ridge is the Mid-Atlantic Ridge, which runs down the middle of the Atlantic Ocean.
Mid-Ocean Ridges and Rift Valleys
The Mid-Atlantic
Ridge is almost 11,000 km (6214 mi) long.
Its rift valley is 25 km (15 mi) wide.
It is 9 km (6 mi) deep!
Mid-Ocean Ridges and Rift Valleys
When scientists began studying rocks
on the sea floor, they made interesting discoveries about Earth’s magnetic field.
Sea-Floor Rock and Magnetic Reversals
The Earth can be compared to a
bar magnet, which has a north and south pole.
Sea-Floor Rock and Magnetic Reversals
Earth’s magnetic poles switch
places every so often. The north pole becomes the south
pole and the south pole becomes the north pole.
This is called a magnetic reversal.
Sea-Floor Rock and Magnetic Reversals
No one yet knows why these
magnetic reversals occur. Note: Earth’s geographic poles
never change (the north pole is called the “north” pole regardless of the direction of Earth’s magnetic field).
Sea-Floor Rock and Magnetic Reversals
Earth’s magnetic
reversals have been recorded in the sea floor.
As molten rock cools in the ocean, its magnetic minerals line up with Earth’s magnetic field.
Sea-Floor Rock and Magnetic Reversals
As you can see in the diagram, the
magnetic reversals are apparent in the form of stripes in the sea-floor rock.
By dating the rock, scientists found that the most recent reversal occurred 760,000 years ago.
Sea-Floor Rock and Magnetic Reversals
Divergent boundaries can also
break continents apart.
Continents Split Apart at Divergent Boundaries
As magma 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.
Continents Split Apart at Divergent Boundaries
Magma begins to rise through the
cracked, thinned crust, forming volcanoes.
Eventually, the continent begins to split apart.
Continents Split Apart at Divergent Boundaries
Eventually, the rift
valley may fall below sea level as it sinks lower and lower.
Water from oceans or lakes may fill the area, creating a sea or lake.
Continents Split Apart at Divergent Boundaries
This is how the
Red Sea formed, as the African and Arabian plates have been moving apart for several million years.
Continents Split Apart at Divergent Boundaries
The Great Rift Valley in eastern
Africa is showing that the African plate is splitting apart.
Continents Split Apart at Divergent Boundaries
Hot spots are places where heated
rock rises in plumes from the mantle.
Hot Spots Can Be Used to Track Plate Movements
Hot spots stay in place even while
the tectonic plate above it keeps moving.
Hot Spots Can Be Used to Track Plate Movements
Over time, hot spots form
volcanoes at the surface.
Hot Spots Can Be Used to Track Plate Movements
The volcano can
eventually become high enough to rise above the sea as an island.
The Hawaiian Islands are being built as the Pacific Plate moves slowly over a hot spot.
Hot Spots Can Be Used to Track Plate Movements
After a volcano is formed
and the plate continues moving, it carries the first volcano away from the hot spot.
The hot spot will continue to make new volcanos as the plate moves over it.
Hot Spots Can Be Used to Track Plate Movements
The movement of the Yellowstone hot
spot can be tracked by following the trail of inactive volcanoes in its wake.
Hot Spots Can Be Used to Track Plate Movements
The Yellowstone hot spot tells us
that the North American plate is moving southwest at about 2.3 cm per year.
Hot Spots Can Be Used to Track Plate Movements
1. New crust forms along a(n)A. convergent boundaryB. divergent boundaryC. ocean trenchD. subduction zone
Review
2. Earth’s magnetic reversals are recorded inA. the sea floorB. the mantleC. deep-ocean trenchesD. the asthenosphere
Review
3. At a mid-ocean ridge, the pattern formed by magnetic mineralsA. is the same on either side of the ridgeB. is randomly arranged at the ridgeC. is older along mid-ocean ridges and younger near ocean trenchesD. shows that hot spots in Earth’s mantle cause volcanoes to form.
Review