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.
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, 20-1000oC, least dense rock, 2000oC to 3000oC.
Layer
/ ConditionsCrust
/ 5-90km thick, least dense rockMantle / 20-1000oC, less dense than the core,
Outer Core / 2000oC to 3000oC, red hot liquid rock
Inner Core / about 5000oC, mostly iron, very dense solid
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: igneous ,
sedimentary , or metamorphic .
When a hot liquid rock cools and solidifies it forms an igneous rock.
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
basaltgranite /
chalk
limestonemudstone
sandstone /