Physical ScienceEarth and SpaceYear 8 Science

The rock cycle, fossils, soil and using Earth materials

What this lesson covers

Igneous, sedimentary and metamorphic rock
The rock cycle and the processes that drive it
How fossils form in sediment
What soil is made of
Choosing Earth materials for a job by their properties

Key idea

Three main types of rock

Geologists group rocks by how they formed, not by how they look.
Igneous rock forms when molten rock (magma underground, lava at the surface) cools and hardens.
Sedimentary rock forms from bits of older rock, shells or plant remains (sediment) that settle in layers, get buried, and are squeezed and cemented together.
Metamorphic rock forms when existing rock is changed by great heat and pressure deep underground, without melting.

Type	How it forms	New Zealand examples
Igneous	Molten rock cools	Basalt of Auckland's volcanic cones, pumice from Taupō
Sedimentary	Layers of sediment buried and cemented	Greywacke of the main ranges, Ōamaru limestone
Metamorphic	Heat and pressure change a rock	Otago schist, used for stone walls in Central Otago

Key idea

Crystal size tells you how fast it cooled

As molten rock cools, mineral crystals grow in it.
Magma that cools slowly, deep underground, gives crystals time to grow large, like the big crystals in granite.
Lava that cools quickly at the surface gives only tiny crystals, like those in basalt.
Lava that cools almost instantly can form glassy obsidian with no crystals at all, and lava full of gas bubbles cools into light, frothy pumice that floats.

Key idea

Try it: igneous, sedimentary or metamorphic?

Key idea

The rock cycle

Rocks are not permanent. Over thousands to millions of years they are broken down, moved, buried, heated and melted, so one type of rock becomes another.
Weathering breaks rock at the surface into pieces; erosion carries the pieces away by water, wind or ice; deposition drops them in layers.
Burial, compaction and cementation turn those layers into sedimentary rock.
Heat and pressure turn any rock into metamorphic rock; enough heat melts it into magma, which cools into igneous rock.
There is no single route: any type of rock can be weathered, changed by heat and pressure, or melted.

Worked example

Tracing a journey round the cycle

A piece of basalt on Rangitoto ends up, millions of years later, as part of a band of schist deep underground. Describe one possible route.

Basalt is igneouscooled lava
Rain, waves and plant roots break it into grainsweathering
Streams and the sea carry grains off and drop them on the sea floorerosion and deposition
Buried under more layers, the grains are squeezed and cementedbecomes sedimentary rock
Pushed deep where it is hot and squeezed, it turns to schistbecomes metamorphic rock
Igneous, then sediment, then sedimentary, then metamorphic

Key idea

How a fossil forms

An animal or plant dies and is quickly covered by sediment, such as mud or sand on a sea floor or lake bed. Being buried fast stops it being eaten or rotting away completely.
Soft parts usually rot; hard parts such as shells, bones and teeth last longer.
More layers pile on top. Water seeping through the sediment carries dissolved minerals, which slowly fill and replace the material of the hard parts.
Over a very long time the remains become a stony copy inside sedimentary rock. Much later the rock may be lifted and worn away, exposing the fossil.
This is why fossils are found in sedimentary rock and almost never in igneous rock, which forms from melted material.

Four stages of fossil formation: a dead shellfish settles on the sea floor; it is buried by sediment layers; over a long time minerals replace the shell; the rock is lifted and worn away, exposing the fossil

Key idea

What soil is made of

Soil is much more than crumbled rock. It has five main parts.
Rock particles of sand, silt and clay, made by weathering.
Humus: the dark, decaying remains of plants and animals.
Living things: plant roots, fungi, bacteria, worms, insects and many more.
Air and water, held in the spaces between the particles.

Key idea

Properties decide uses

People choose Earth materials by matching their properties to the job.

Material	Key property	Use
Iron (made into steel)	Strong and dense	Bridges, building frames, nails
Aluminium	Strong but much less dense than steel	Aircraft, boats, window frames
Salt	Dissolves in water, stops bacteria growing	Preserving food, flavouring
Mica	Splits into thin sheets that resist heat and do not conduct electricity	Insulating parts in toasters and heaters
Limestone	Neutralises acid, easy to crush	Lime to sweeten farm soils, gravel for paths, cement
Pounamu	Very tough, takes a sharp edge and a polish	Traditional adzes (toki) and taonga
Volcanic hāngī stones	Hold heat and do not crack when heated	Cooking food in a hāngī

Key idea

Try it: match the material to the job

Worked example

Why not any old river stone?

A school is planning a hāngī. Someone suggests using sandstone pebbles from a nearby stream. Explain why the hāngī team will say no.

A hāngī stone must be heated until very hot without breakingthe job
Sandstone is grains cemented together and can soak up waterits properties
Heated, the water in it turns to steam and can split the stone apart violentlythe danger
Use dense volcanic stones chosen for the job, because they hold heat and do not shatter

Common mistakes

Metamorphic rock forms when rock melts and cools again.
Melting and cooling makes igneous rock; metamorphic rock is changed by heat and pressure without melting.
The rock cycle always goes igneous, sedimentary, metamorphic in that order.
Any rock type can turn into any other, so there are many routes round the cycle.
A fossil is the actual bone or shell of the animal.
In most fossils, minerals have replaced the original material, leaving a stony copy.
Soil is just ground-up rock.
Soil also holds humus, living things, air and water, which is why it can grow plants.

Try it at home

Make sediment layers in a jar

You need: a large clear jar with a lid, a cup of garden soil, a cup of sand, a handful of small gravel, water

Put the gravel, sand and soil into the jar.
Fill the jar about three-quarters full with water and screw the lid on tightly.
Shake it hard for 30 seconds, then stand it on a shelf where it will not be bumped.
Check it after 5 minutes, after an hour and the next day, and draw the layers each time.
Look for sorting: the gravel settles first at the bottom, then sand, then fine silt and clay on top, with bits of humus floating; this is how sediment layers build up before they become sedimentary rock.

Safety: Garden soil can carry germs such as tetanus bacteria, so wear gloves or wash your hands well afterwards, and keep the lid tight while shaking.

Your turn

A rock has large interlocking crystals. Is it more likely to have cooled quickly at the surface or slowly underground?

Which type of rock forms when limestone is changed by heat and pressure into marble?

Name the three processes, in order, that move a rock at the surface into a new sediment layer.

Why are fossils rarely found in igneous rock?

Why is aluminium chosen over steel for an aircraft body?

Show answer

1. Slowly underground, which gives crystals time to grow large. 2. Metamorphic rock. 3. Weathering, erosion, then deposition. 4. Igneous rock forms from molten rock, which would destroy any remains. 5. It is strong but much less dense, so the aircraft is lighter.

Summary

Recap

Igneous rock forms from cooled molten rock, sedimentary rock from buried and cemented sediment, and metamorphic rock from heat and pressure.
In the rock cycle, weathering, erosion, deposition, burial, heat, pressure, melting and cooling keep turning one type of rock into another.
Fossils form when remains are buried in sediment and minerals slowly replace them.
Soil is rock particles, humus, living things, air and water.
People choose Earth materials, from steel to pounamu, by matching their properties to the job.

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