Plate Tectonics

What is plate tectonics?

Earth’s lithosphere is broken into plates that move on the asthenosphere.

The movement of these plates is ‘Plate Tectonics’.

The theory of plate tectonics . . .

The lithosphere is broken into plates that move on the asthenosphere.

Volcanoes and earthquakes occur in specific belts along these plate boundaries.

The age of rocks on the ocean floor give us clues to tectonic movement.

Continental Drift theory . . .

Alfred Wegner, a German scientist hypothosized that the continents have drifted over time.

Evidence suggesting this theory is the shape of coastlines, fossil distribution, and distinctive rock formations.

Tectonics occurs as the lithosphere of the crust floats on the plastic asthenosphere at the surface of the mantle.

How do the plates move?

There are three types of plate boundaries: Divergent boundaries -- where new crust is

generated as the plates pull away from each other.

Convergent boundaries -- where crust is destroyed as one plate dives under another.

Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.

Divergent Boundaries

Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle.

Convergent Boundaries The size of the earth has not changed significantly during the past 600 million years, and very likely not since shortly after its formation 4.6 billion years ago. The earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created.

Two types of converging boundaries:

Subduction Boundary:

When an ocean plate subducts [goes under] either another oceanic plate or a continental plate it forms a subduction boundary.

Collision Boundary: Form when the two

converging plates are both continental.

The collision causes the crust to be lifted into mountain ranges, such as Mt. Everest in India’s Himalayan Mountains.

Transform Boundaries:

The zone between two plates sliding horizontally past one another is called a transform boundary.

Most transform faults are found on the ocean floor.

Plates move apart at divergent boundaries, toward each other at convergent boundaries, and past each other at transform boundaries.

Convergent boundaries can be either collision [between continents] or subduction [between oceans or between oceans and continents].

Review . . .

Magnetism helps reveal age of ocean floor:

Evidence for tectonic activity is demon-strated by. . .

Magnetic reversals in rocks on either side of mid-ocean ridges

High heat flow along ridges

Youngest crust is near ridge center

We know plates move, but the question is; HOW do they move?

Well, I’m going to tell you . . .

Three hypotheses describe how plates are moved:

1. Mantle Convection:

The transfer of heat from Earth’s inner core to its outer core.

These convection currents of heat may drag the plate away from its divergent boundary.

2. Ridge Push

Ridge push - Gravity acting on the topography of ridges tries to spread the ridge outward.

This pushes the rest of the plate away from the ridge.

3. Slab Pull

Slab pull – The edge of the subducting plate is cold and dense; therefore, as it sinks into the mantle, it carries the rest of the plate with it.

So, just to review . . .

The three hypotheses for plate movement are: 1. Mantle Convection: Convection currents

move the plate. 2. Ridge Push: Gravity acts on the newly

formed rock along ocean ridges forcing them to slide down [divergent]

3. Slab Pull: As the plate subducts, it pulls the rest of the plate down with it. [convergent]

Plate Movement and Continental Growth

Plate movements have caused Earth’s continents to change their positions on the globe over time.

We started with Pangea, [from the Greek, pan = all, gaia = Earth]

And the continents grow . . .

New material continues to be added to the continents over time, constantly changing both shape and position.

Before: After:

Tectonic activity is constant . . .

The breakup of Pangea continues today as the plates continue to move

But, what was here first?

Craton: At the core of each continent is an ancient rock called the craton

The North American Craton, containing the oldest rocks on the continent, date back about 1 billion years.

The Canadian Shield and platform make up the North American craton. The shield is exposed rock, while the platform is buried rock.

A little vocab review . . .

A terrane is a large block of lithospheric plate that has been moved, often a great distance, and attached to the edge of a continent.

Terranes lead to continental growth.

When two continental plates collide, they can weld to each other and fuse, adding to the size of the original continent.

Just a little more on terranes . . .

There are three characteristics used to identify a terrane:

1. Faults 2. Fossils 3. Magnetic

polarity

Last, but not least, a terrane. . .

…. is completely bounded by major faults.

…has fossils different than those in surrounding areas.

…has a different magnetic record than that of the surrounding rock.

So, now we know . . .

The lithosphere is broken into plates that move on the asthenosphere.

There are divergent, transform and convergent boundaries between those plates.

Plate movements are thought to be caused by mantle convection, ridge push and/or slab pull.

The position and shape of Earth’s continents has changed over time and is still changing.

And Plate Tectonics is responsible for . . .

Volcanoes and the formation of igneous rocks

Earthquakes and fissures at plate boundaries

The formation of metamorphic rocks at convergent plate boundaries, igneous dikes and igneous sills.

Dike

Sill