ROCKS AND THEIR FORMATION

Uniformitarianism

• Early geologists thought that the physical features of the earth had been formed by sudden catastrophic events

James Hutton concluded that this was not the case:

1. The geological processes active now, were also active in the past

2. The present physical features were formed by these same processes, over long periods of time

What are Rocks?

Rocks are a group of minerals bound together

There are three types of rocks:

1. Igneous

2. Sedimentary

3. Metamorphic

IGNEOUS ROCKS

Igneous rocks are formed by the cooling and hardening of hot molten rock from inside the earth.

• Plutonic (intrusive) rock is formed underground and intrude into other rock masses

• Volcanic (extrusive) rock is formed when lava cools on the surface, or out of volcanic dust or ash.

MAGMA

There are two main kinds of magma which differ in chemical composition

FELSIC: high silica content (little Ca, Fe, Mg), thick and slow-flowing, and forms light coloured rocks (eg. Granite)

MAFIC: lower silica content (high Ca, Fe, Mg), hotter, thinner and more fluid, and forms darker coloured rocks (eg. Basalt)

TEXTURES OF IGNEOUS ROCK

A rock’s texture can be classified as glassy, fine-grained or coarse-grained. Texture is affected by:

• Crystal size (which depends on how fast the magma hardens)

• Amount of gas dissolved in the magma (crystals grow faster because it helps ions move around)

Magma trapped deep within the crust hardens slowly; magma that reaches the surface as lava hardens very quickly

SEDIMENTARY ROCKS

Sedimentary rocks form when sediments harden into rocks

Three Types:1. Clastic: formed from

fragments of other rocks2. Chemical: formed from

mineral grains that come out of solution

3. Organic: formed from the remains of plants/animals

Clastic Rocks

• Fragments come from weathering of existing rocks

• Picked up by wind, waves, glaciers and running water

• Sediments are deposited where water slows down

• Natural cements (silica, calcite, iron oxide) stick the sediments together into firm, cemented rock

Sediment Sorting

Sorting of sediments produces separation of the particle sizes

• Pebbles/gravels are dropped first (form conglomerate)

• Sands (form sandstone)• Silts, clays are dropped last

(form shale)

Chemical Origin

• Water contains dissolved minerals, which can fall out of solution (precipitate) due to evaporation or chemical action

• Limestone can be formed from tiny grains of calcite deposited from sea or lake waters

• Other examples are rock salt (halite) and rock gypsum

Organic Origin

• Calcite is dissolved out of rocks on land, carried to an ocean or lake, and taken out of the water by shell-producing organisms (eg. Clams, oysters, sea snails)

• When these organisms die, their shells pile up and are broken down into fragments, and can form organic limestone

METAMORPHIC ROCKS

• Metamorphic rocks are not formed from magma or sediment

• Metamorphic rocks “morph” (change) from existing rock, due to heat, pressure and chemicals

• Pressure squeezes grains closer together (more dense, less porous)

• Heat and chemicals may rearrange the particles (new minerals may be formed)

Types of Metamorphism

Regional Metamorphism: large areas of rock are under intense heat/pressure, which occurs during mountain-building

• Heat comes from friction of rocks, and pressure from overlying weight and the squeezing of moving rocks

Contact Metamorphism: when hot magma forces itself into overlying rock (intrusions) and bakes the rock

FOLIATION

• Occurs when the pressure on the rocks squeezes minerals into parallel layers

• The rocks tend to split easily along these layers

THE ROCK CYCLE

The three types of rocks are very closely related:• Igneous rocks are attacked by erosion and

weathering to form sediments• Crustal movements can cause deep rocks to

reach high temperatures and to melt back into magma

• Igneous and sedimentary rocks exposed to intense heat and pressure can become metamorphic