Chapter 8: Plate Chapter 8: Plate TectonicsTectonics

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 apart8.3: 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 Tectonic plates have different boundariesboundaries

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 The sea floor spreads apart at divergent boundariesat 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-Divergent Boundaries – Mid-Ocean Ridges and Rift ValleysOcean 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 Sea-Floor Rock and Magnetic ReversalsReversals 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 Each magnetic reversal is reversal is

recorded in sea-recorded in sea-floor rockfloor 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.edumedia-sciences.com/en/a108-earth-s-magnetic-field

Continents split apart at divergent Continents split apart at divergent boundariesboundaries

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 Hot Spots can be used to track plate movementstrack 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 The lithosphere is made up of many made up of many

platesplates

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 1.2