2 The Earth and its Rocks
Summary
Structure
The earth is thought to be made from three main layers, Mantle, Inner
and Outer Core, and Crust. Correctly label the diagram below.
Inner Core
Outer Core
Mantle
Crust
Complete the table giving information about each layer.
Use the following phrases: about 5000oC, less dense than the core,
mostly iron, 5-90km thick, red hot liquid rock, very dense solid, 201000oC, least dense rock, 2000oC to 3000oC.
Layer
Conditions
5-90km thick, least dense rock
Crust
20-1000oC, less dense than the core,
Mantle
2000oC to 3000oC, red hot liquid rock
Outer Core
about 5000oC, mostly iron, very dense solid
Inner Core
Scientists have learned about the structure of the earth from the study of
Seismology which is the study of
earthquakes
.
Volcanoes and earth movements can bring up rocks which formed a great
depths; rocks which have cooled slowly have large
crystals, while
rapid cooling has made the crystals
small
.
Rock Type
Rocks can be of three types:
sedimentary
, or
igneous
metamorphic
When a hot liquid rock cools and solidifies it forms an
rock.
,
.
igneous
When layers of sediment get squashed together and cemented together a
Sedimentary
rock is formed.
Both igneous and sedimentary rocks can be changed into a new type of
rock – a
metamorphic
rock.
Weathering and Erosion
Rocks do not last for ever. They are continually being broken down by
the process called
weathering . This is caused by repeated
heating and cooling, the expansion of freezing water, and the effects of
wind and rain on the surface.
The bits of broken up rock get transported by rivers to the sea where they
form layers and eventually get stuck together to form layers of
sedimentary rock.
The combination of weathering followed by transport gives the overall
process called erosion
.
Limestone is a sedimentary rock because it is laid down in layers, but it is
made, not of previously existing rocks, but by the remains of shells
and skeletons of marine creatures.
Fossils
Some sedimentary rocks contain fossils. Fossils are indications of
prehistoric
life , often they are formed from the remains of
organisms, but they may simply be footprints or trails. By comparing
fossils with similar organisms living today, we can get information as to
what conditions were like when these organisms were
alive .
Building Materials
Some rocks have properties which make them suitable as building
materials – some are waterproof, some are easy to cut and shape, some
are very strong. Clay, which is a soft rock, can be made hard and strong
by heating it strongly in a kiln (in effect, making an artificial
metamorphic rock!).
Clay can also be heated with limestone to make cement, used to make
concrete.
Soil
The top layer of weathered rock usually forms soil.
As well as broken up rock, soil also contains
air , water ,
humus
(dead plant material), and living organisms (worms,
bacteria, beetles etc.).
The type of soil formed depends on the type of
rock from which it
was made.
Different soils have different properties:
Clay soils are tightly packed, have little air, and drain poorly.
Sandy soils have bigger spaces, plenty air and are quick draining.
A loam has a mixture of clay, sand and plenty of humus; this is the best
all round soil for farming.
Rock Types
Put the following examples of rocks into the correct column in the table
Basalt, chalk, granite, limestone, marble, mudstone, sandstone, slate.
Igneous
Sedimentary
Metamorphic
basalt
chalk
Marble
granite
limestone
slate
mudstone
sandstone