UNIT 1

LANDFORMS AND WATERFORMS

Plate Tectonics

Plate tectonics – the idea that the earth’s surface is covered by tectonic plates that are continually moving.

The main features of plate tectonics are:

Convection currents beneath the plates move the crustal plates in different directions.

The source of heat driving the convection currents is radioactivity deep in the Earths mantle.

Tectonic plates – large pieces of the earth’s crust.

Why do tectonic plates move?

Convection Cells - Hot magma rises,spreads laterally, cools and sinks deeper into the mantle to be reheated.

Plate Movement

As convection cells cause plate movement, distinct types of movement are caused:

1. Compression – plates pushing towards each other.

2. Tension – plates pulling away from each other.

As the plates move, compression and tension occur at different plate boundaries.

Compression in South America between Nazca and South American plate and tension in Iceland between North American and Eurasian plate.

FOLDING As plate movement causes compression, rock layers can actually bend and be thrust upwards causing fold mountains.

Anticline – the upfold in folded rock.

Syncline – the downfold in folded rock.

Most major mountain ranges were formed by the collision of continental plates.

FAULTING

Compression can cause the rock layers to break or fault. Reverse fault – one block of rock is pushed up above the other.

Overthrust Fault – one block of rock is pushed up and over the rock on the other side of the fault.

Tension can cause rock layers to break or fault. Normal Fault – one block of rock drops down lower than the other.

VOLCANOES Volcanoes occur when 2 plates come into contact. Friction causes the rock to melt. As the magma heats up it expands and moves upward through cracks to the surface. A mild eruption will have lava flows. Thick lava tends to solidify, forming a plug. When gases build up behind this a violent explosion may occur.

Molten rock below the surface of the Earth is known as magma.

After it erupts from a volcano it is called lava.

There are 3 main types of volcanoes:

1. Ash and cinder cone – Symmetrical in shape with steep sides and a large crater. Often quite explosive and composed mostly of ash and cinders.

2. Shield cone – mild eruptions of thin liquid lava flows that form broad, flat cones that cover a large area.

3. Composite cone – have layers of ash and cinders intermixed with layers of lava. Non-Violent slow emissions of lava one time and violent ash eruptions next time. Smaller cones may form at weak points on the sides.

PLATE BOUNDARIES Plate boundaries are where compression and tension occur – so it is here that you will find the majority of volcanoes, earthquakes and mountain ranges.

Western North American Volcanoes.