science world 2 – chapter 4
rocks
INSIDE THE EARTH
Scientists have never directly observed the inside of the Earth.
However, information gathered from earthquake and volcanic
activity has lead them to infer that the Earth is made up of four
layers:
Crust – the thin skin upon which we stand. It is only about 30
km thick.
Mantle – consists of molten (liquid) rock called magma. It is
about 3000 km thick.
Outer Core – consists of molten iron and nickel. It is about
2000 km thick.
Inner Core – consists of solid iron and nickel. It is about 1400
km in diameter.
Mountains are formed by the magma in the mantle exerting
forces on the Earth’s crust.
VOLCANOES
There are sometimes weak spots in the Earth’s crust that may
crack and allow the magma to flow to the surface. When this
happens a volcano forms. When magma reaches the surface it
is called lava. It is usually about 1000C and is red-hot. As it
cools, it turns into a solid rock, often basalt.
Not all volcanic regions in the world are erupting all the time. If
lava or ash is currently erupting, we say the volcano is active. If
there is a long period of calm, the volcano is classed as
dormant. When there has been no sign of activity for many
thousands of years, the volcano may be declared extinct.
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IGNEOUS ROCKS
Rocks that have been formed from the molten rock of the mantle
are called igneous rocks e.g. granite and basalt. These rocks are
made from crystals, and have definite shapes and straight sharp
edges. There are two important groups of igneous rocks:
Volcanic – these are produced from rapidly cooling lava on the
surface of the Earth. As the rock cools quickly it
does not have time to form large crystals e.g. basalt.
Plutonic – these are produced from magma that solidifies below
the surface of the Earth. As the rock cools very
slowly the crystals formed are much larger as they
have had a long time to grow e.g. granite.
All rocks are made of minerals. These are the building blocks
of rocks. Some rocks contain only one mineral, but most rocks
are a mixture of several different minerals. Mining companies
collect these minerals from rocks in order to extract valuable
substances such as metals or gemstones.
PHYSICAL & CHEMICAL WEATHERING
Weathering is the term used to describe the breakdown of rocks
by natural processes. There are two main types of weathering:
Physical weathering is the breaking up of rocks into smaller
and smaller pieces. This can happen by ice wedging between
rocks, tree roots growing into cracks and the general expansion
and contraction of rocks due to extreme daily temperatures.
Chemical weathering is the changing of the chemicals in the
rocks to new chemicals by the action of water and air. The
minerals in the rock often become very soft during this process
or may dissolve away entirely. Chemically weathered rocks are
often very brittle and may have very different colours from
fresh, unweathered rocks.
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EROSION
Weathering is the process by which rocks are broken down.
Erosion is the movement of soil and other weathered material
from one place to another. The major agents of erosion are:
Water – this includes rain hitting the Earth's surface, water
flowing along a river and the sea pounding along the
coast.
Ice – this involves glaciers picking up rocks as they flow out
to sea and these glaciers then scrape against the land
further on.
Wind – this involves grains of dust, soil and sand being carried
by the wind.
The eroded material is then deposited (dumped) somewhere as
sediment. Erosion is a natural process, but human activity often
causes it to occur more quickly.
SEDIMENTARY ROCKS
Streams carry large amounts of weathered material (sediments).
The size of the sediments being carried depends on the flow rate
of the stream. The largest sediments (e.g. gravel) are deposited
first and the smallest ones (e.g. mud) are deposited last.
Over long periods of time the sediments build up and become
compacted and cemented together, turning loose sediment into
sedimentary rock. Different types of sediments produce
different types of sedimentary rocks. For example:
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sandstone forms from sand.
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shale forms from mud.
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conglomerate forms from pebbles and stones.
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limestone forms from the remains of dead animals.
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coal forms from the remains of dead plants.
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METAMORPHIC ROCKS & THE ROCK CYCLE
Metamorphic rocks are sedimentary or igneous rocks that have
been slowly changed, over long periods of time, due to the heat
and pressure exerted on them. For example, shale gets changed
into slate and limestone gets changed into marble.
The Earth’s surface is constantly being changed by extremely
strong forces. Volcanic activity creates igneous rocks. These
rocks are weathered and eroded to form sedimentary rocks.
Heat and pressure then change these rocks into metamorphic
rocks. This whole process is called the rock cycle. (Look at
Figure 27 on p 85)
SPELLING WORDS
No.
EASY
HARD
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crater
soil
crustal
material
ground
expand
mineral
basalt
granite
crystal
earthquake
movement
layers
natural
fossil
limestone
mantle
glacier
exposed
magma
weathering
erosion
transformed
igneous
sedimentary
metamorphic
volcanic
plutonic
dormant
extinct
compressed
compacted
disintegration
dolerite
miniature
vegetation
recrystallised
conglomerate
quartzite
pumice
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