Transcript 8.3: Plates move apart

Chapter 8: Plate Tectonics
8.1: Earth has several layers
8.2: Continents change position over time
8.3: Plates move apart
8.4: Plates converge or scrape past each other
8.3: Plates move apart
 Before, you learned:
 The continents join and break apart
 The sea floor provides evidence that tectonic plates
move
 The theory of plate tectonics helps explain how the
plates move
 Now, you will learn:
 About different plate boundaries
 What happens when plates move apart
 How the direction and speed of plates can be
measured
Tectonic plates have different
boundaries
 Plate boundary: where the edges of two plates meet
 Types of boundaries:
 Divergent boundary: where plates move apart…mostly
found in the ocean
 Convergent boundary: where plates push together
 Transform boundary: where plate scrape past each other
The sea floor spreads apart at
divergent boundaries
 Also called spreading centers
 As the ridges continue to widen,
a gap called a rift valley forms
 Molten material rises to form new
crust
 Mid-Ocean Ridges and Rift
Valleys
 Mid-Ocean Ridges: like a long
chain of mountains. Rift Valley at
center
magnetic reversal animation
Divergent Boundaries – Mid-Ocean
Ridges and Rift Valleys
 World’s longest ridge: Mid-Atlantic Ridge
 Length of the ocean
 The North and South American plates are moving
away from the Eurasian and African plates
 11, 000km (6214 mi) from Iceland to Antarctica
 24 km (15 mi) wide and 9 km (6 mi) deep
Sea-Floor Rock and Magnetic Reversals
 Scientists studied the sea floor rock and were
surprised by a discovery they made about the
Earth’s magnetic field
 Think of Earth as a bar magnet, with a N and
S pole (not the geographic pole)
 Poles switch places: Magnetic Reversal

Caused by changes in the magnetic field
animation
Each magnetic
reversal is recorded
in sea-floor rock
 Magnetic materials in
the new rock line up
with the Earth’s
magnetic field
 The material hardens
and those minerals
are permanently fixed
in the directions
pointing north and
south
 Date the rocks, and
can have further
evidence of plate
movement
 Most recent reversal:
760,000 years ago

Animation

More animations
 http://www.edumediasciences.com/en/a108-earth-smagnetic-field
Continents split apart at divergent boundaries
 Continents also spread apart at
divergent boundaries
 Boundary begins to form when hot
material rises from deep in the
mantle
 Heat causes the crust to bulge upward
 Crust cracks, a rift valley forms
 Magma rises through the cracked,
thinned crust forming volcanoes
 Rift valley grows wider, the continents
begin to split apart
 The thinned valley floor sinks lower and
lower until it is below sea level
 Water may fill the valley
Hot Spots can be used to track
plate movements
 Hot spots: where heated rock rises in
plumes (thin columns) from the
mantle
Volcanoes often develop above the
plume
 Often far from plate boundaries, but
offer a way to measure plate
movement
 Heat from the plume partly melts
some of the rock in the tectonic plate
above it: eventually the rock above
will melt
 A volcano will form at the surface of
the plate in time


Rises above the sea: an island
 Movement: can measure direction and
speed of plate movements
The lithosphere is made up
of many plates

Ex: about ½ the
African Plate lies under
water
1) have the continents
always been where they
are today?
2) if not, how did they move
to their present
positions…
Mystery Solved! - Section