the continents were once a single landmass surrounded by on large ocean

Theory of Continental Drift

The continents were once a single landmass surrounded by on large ocean.

Theory of Continental DriftRocks have different

density, so a less dense rock will ‘float’ on a more dense rock.

The continental shelf (part of the lithosphere) is lighter than the asthenosphere.

Theory of Continental DriftThis process is called

buoyancy.

The lighter and heavier materials will form a balance called an ‘isostatic equilibrium’.

Theory of Continental DriftThe continents ‘float’

on the asthenosphere like a boat floats on the ocean.

All they need is a ‘current’ to move them.

Theory of Continental DriftOriginally, there was

one huge continent called Pangaea.

It was surrounded by a single ocean called Panthalassa

Theory of Continental DriftAlfred Wegener

developed his theory in 1912 based on the fossil record.

No one believed him. He was a meteorologist and couldn’t explain how it could happen

Theory of Continental DriftIn the 40’s, new

technology revealed physical characteristics on the sea floor that showed how the continents might ‘drift.’

Theory of Continental Drift

They revealed mid-ocean ridges, trenches and rift valleys.

How the Sea Floor Spreads

How the Sea Floor SpreadsHarry Hess and Robert Dietz suggested an

explanation of both how the sea floor could expand and ‘push’ the continents over the face of the Earth.

How the Sea Floor SpreadsThe denser

(heavier) ocean crust slides under the less dense (lighter) continental crust forcing the continents further apart.

How the Sea Floor Spreads

When the spreading seafloor reaches a continent, it is pushed below the continent in a subduction zone.

How the Sea Floor Spreads

This brings the sea floor with it’s minerals and water below the continent.

How the Sea Floor Spreads

Friction causes the sea floor to melt. The lighter magma flows upwards through cracks, forming volcanoes.

The Ring of Fire

The Ring of FireThis can be seen in

the ‘ring’ of active volcanoes surrounding the Pacific Ocean.

The Ring of FireThese are the plates

that form these subduction zones

The Ring of Fire