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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, finegrained 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 rocks
2. 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