ch.11.rocks and soil (part 1)

ROCKS AND SOILCHAPTER 11 (PART 1)

CHEMISTRY

ROCKS- We live on a rocky planet that spins on its

axis once a day and moves in an orbit around a star (the sun) at 29 km/s.

- How did the star, the earth and its movements come about?

BIG BANG EXPLOSION

THE SCIENTISTS BELIEVE THAT THE UNIVERSE CAME INTO EXISTENCE WITH A HUGE EXPLOSION.

- The atoms were drawn together by the force of gravity to form huge gas ‘clouds’.

- In these clouds of gas the gravity brought the atoms even closed together to form hot spheres of gas.

- Inside each sphere the force of gravity drew the atoms closer still until the temperature and pressure became so great that nuclear fusion takes place.

- In this process hydrogen atoms are converted into helium atoms and large amounts of energy is released.

- The energy escaped from the surface of the gas spheres as light and heat.

- The spheres became what we now call stars.

- When the star uses all its supply of hydrogen to make helium, it starts using helium to make other elements by nuclear fusion.

- These elements include carbon, nitrogen, oxygen, sodium, magnesium, aluminium, silicon phosphorus, sulphur, chlorine, potassium, calcium, chromium and iron.

- After the star makes iron, it cannot form further elements as iron is heavy and cannot be compressed.

- The star may then swell up and form a giant red star, then release gas and dust as a nebula around it, and then shrink to form a white dwarf star.

- Very large stars do not release gas and dust. They explode forming a supernova and shoot out gas and dust over a wide region in space.

- The conditions in very large stars allow larger elements to be formed.

THE STRUCTURE OF EARTH

THE CORE• INNER CORE• IS A BALL OF IRON AND NICKEL.• 2740 KM IN DIAMETER.

• RADIOACTIVE ELEMENTS PRESENT (URANIUM). THEY GENERATE HEAT AND KEEP THE CORE AT 5000 ⁰C.

• METALS IN INNER CORE STAY SOLID DUE TO HIGH PRESSURE.

• OUTER CORE• MADE OF IRON AND NICKEL.

• 2000 KM THICK.

• THE METALS ARE IN LIQUID FORM.

• AS THE EARTH TURNS, THE INNER CORE MOVES AT A DIFFERENT SPEED THAN THE OUTER CORE. THIS IS THOUGHT TO GENERATE THE EARTH’S MAGNETIC FIELD.

Outer core

Inner core

THE MANTLE• THE MANTLE IS MADE OF ROCKY MATERIAL.

• 2900 KM THICK.

• COMPOSED MAINLY OF THE ELEMENTS IRON, SILICONE, OXYGEN AND MAGNESIUM.

• THE MAIN COMPOUND IN THE MANTLE ARE CALLED SILICATES (SILICONE AND OXYGEN ATOMS)

• AT A DEPTH OF 2000 KM UNDER EARTH’S SURFACE THE TEMPERATURE IS 1500 ⁰C.

• THE UPPER PART OF THE MANTLE IS COOLER AND UNDER LESS PRESSURE SO THE ROCKY MATERIALS BEHAVES LIKE A THICK LIQUID.

• THE LOWER PART OF THE MANTLE IS HOTTER, BUT DUE TO THE HIGH PRESSURE IT REMAINS SOLID.

THE CRUST• THE EARTH’S CRUST IS MADE FROM

MUCH COOLER ROCKS THAN THE MANTLE.

• MINERS AND CAVERS CAN FEEL THE INCREASE IN TEMPERATURE AS THEY GO DOWN INTO THE EARTH’S CRUST.

TYPES OF ROCK

There are 3 types of rocks:

Igneous rocks

Sedimentary rocks

Metamorphic rocks

1) Igneous rocks( Also called fire rocks - form from magma)

Extrusive igneous

rocks

Intrusive igneous

rocks• EXAMPLE: BASALT

• REACHES THE EARTH’S SURFACE

• EXAMPLE: GRANITE

• FORMS WITHIN THE CRUST

- Both rocks are made from crystals of minerals that stick together.

- Basalt is formed from small crystals (cools quickly)

- Granite is formed from large crystals (cools slowly)

Rapid cooling

- Some rocks cool down so quickly that crystals do not have time to form.

- Example: obsidian.

- Pumice forms from frothy lava

Volcanoes- 1st rocks to be identified as igneous rocks

were those that formed from volcanic eruptions.

- There are 2 types of volcanoes: Shield volcanoes (basaltic volcanoes)

And cone shaped volcanoes (andesitic volcanoes)

2) Sedimentary rocks

Rocks from rocky

fragments

Rocks from minerals

Rocks from parts of

living things

- Rocks are formed from rocky fragments and small particles of sand that are produced by weathering.

- The rocky fragments and sand are carried by rivers and settle out when the water current slows down.

- Over thousands or million of years the layers build up and become squashed together due to the high pressure.

a) Rocks from rocky fragments

- Lime stone

- Silt stone

- Mud stone

Examples

- The shells of ancient living things have formed sediments that have turned to rock.

- Limestone is formed of shell of sea creatures such as molluscs that collected at the bottom of the sea.

- Chalk forms from the tiny shells of Protoctista that lived in the plankton.

b) Rocks from parts of living things

- Seawater contains many chemicals.

- They have been dissolved out of the minerals in the rocks and been washed down rivers into seas and oceans.

- When the seas dried up the chemicals left join to form crystals. These rocks are called evaporites.

- Examples: Rock salt and gypsum

c) Rocks from dried up seas

3) Metamorphic rocksFormed from igneous and sedimentary rocks that have been heated or squashed in the earth’s crust

Rock Metamorphic rock

Limestone MarbleShale Slate

THE ROCK CYCLE

Uses of rocks

Igneous rocks

UsesGranite

When granite is polished it becomes attractive and is used in decorative

stone work at entrances of buildings.Basalt

Used to form protective shielding in nuclear power stations (prevents

escape of radiation)Obsidian

Broken to produce very sharp cutting edges. Used in stone age as knives.

Used today to make scalpels for surgeons.

Pumice

Used to make concrete in roman times. (still used to make lightweight

concrete blocks)Breaks easily when rubbed (used to clean skin and in some toothpastes)

Sedimentary rocks

Uses

Gypsum Used in making cement and plaster.

Rock saltUsed for spreading on snowy roads to melt the snow (by lowering the

freezing point)

Limestone

-Can be used as building stone.-Pyramids of Egypt are made from limestone blocks.-Limestone chippings are used to make roads.-Finely ground limestone is used to make cement and even some toothpastes.

Metamorphic rocks

Uses

MarblesUsed in making statues.Used to make topes of expensive and decorative tables and important buildings.

SlateNon-porous rock that forms

lightweight sheets.In the past it was used as roofing

material for houses.