Plate Movements

Part 2

Earth’s Layers & Mantle Convection

How the Plates Move

Mantle Convection

• Rigid lithospheric plates lie on top of

the semi-fluid asthenosphere

• Heat from the

Earth’s core

powers mantle

convection

currents

Mantle Convection Currents

a. The core heats the magma in the asthenosphere

b. Hot magma = less dense and risestoward the surface

c. Near the surface, magma cools = more dense

d. Dense magmasinks back towardthe core and repeatsthe cycle

Mantle Convection

• The lithospheric plates “surf” on top of

the asthenosphere as the magma cycles

Ridge Push

• 2 convection currents meet and…

1. Both cells push magma up at the same spot

2. Some magma breaks through the crust and

forms a ridge

3. The ridge of new crust pushes the older

crust/ lithospheric plates away from the

area = RIDGE PUSH!

• Separating plates = DIVERGENT

BOUNDARY

How the Plates Move:

Ridge Push

Oldest Youngest Oldest

How Plates Move

Slab Pull/Gravity Pull

• 2 convection currents meet and…

1. Both cells are pulling magma down

2. The plates floating on top are pulled

together

a. Mountains:

plates with

the same

density

push crust

upward

Slab Pull/Gravity Pull

b. Subduction: with plates of different

densities, the denser plate slides

under the less dense plate

Bottom plate is pulled down into the

mantle = SLAB/GRAVITY PULL

This plate melts, becomes new

magma, and cycles through the

convection current

• Colliding plates =

CONVERGENT

BOUNDARY

Partner Discussion: Is the Earth

getting smaller or larger?

• NEITHER!!! It stays the same size!

• The new rock that forms at the ridge/rift

is really recycled rock from a plate that

was pulled into the mantle