Earth

Geology

Earth

Earth

Earth

topography

red=mountainblue=depression

Earth’s surface looks different from other planets.

It seems to have defined ‘continents’.

Why is this?

The continents fit together!

After rearrangement, rocks

and mountain belts match.

After rearrangement, past climates seem to match

(for example, by looking at geological evidence of glaciers, or

swamps).

Glacial grooves

After rearrangement, fossils match.

The theory of continential drift was proposed by Alfred Wegener in the early 1900s to explain these observations.

The theory of continential drift was proposed by Alfred Wegener in the early 1900s to explain these observations.

However, his ideas weren’t popular. People said:

•How are you going to move entire continents?

•What force is strong enough to move continents?

•How can you move rocks across rocks?

Continental Drift Revived, 1940s & 1950s:

Supported by Magnetism of rocks

• The Earth has a magnetic field.

• When molten rocks cool, they lock in their current magnetic field direction.

• If they are moved from their point of origin, their internal magnetic signal shows it

• The continents DEFINITELY moved!

A new theory was postulated to explain continental drift:“Plate Tectonics”

•Earth’s surface divided into many plates that move slowly over the surface and interact with each other

•Their movements are driven from below - internal heat and convection in the Earth (What is convection?)

•This was a REVOLUTIONARY idea

Let’s make sure we understand this.

Crust = 7 - 50 km

Mantle = 2,900 km

Core = 3,470 km

Earth layers

Plates move by “sliding” along soft upper mantle material.

Mantle convectionUpwelling beneath ridges•Hot, buoyant material causes ridges to sit “high”

Downwelling at subduction zones•Cooler, denser, material sinks

Sea Floor spreading (Hess, 1962)

New crust created at ridges (divergence)Old crust consumed at subduction zones (convergence)

Predicts sea floor is young near ridges and ages away from the ridges

Tectonic Plates

Plates are rigid, pushed along by sea floor spreading.

Plate Boundaries

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Once Plate Tectonics was described, geologists

discovered a ton of evidence to support it.

Age of Sea Floor

Cascade Mountains - Famous volcanoes

Hot spots

Shield (hot spot) Volcanoes are different from other volcanoes...

Hot spots - Hawaii

Plate Tectonics: Take-home messages

• Earth’s geology is dominated by the processes of plate tectonics. This is UNIQUE to the Earth.

• Earth’s crust and upper mantle are divided into plates. The plates slide around.

• Plate movement is driven by convection in Earth’s mantle.

• Crust is continuously being re-generated and removed.

• Where plates interact, they form earthquakes, volcanoes and mountains.

Why does the Earth have plate tectonics and not other planets?

Is plate tectonics necessary for life?