Three different types of rocks

Igneous, Sedimentary and Metamorphic rocks.

Rocks are either a single mineral or a group of minerals that has to go through a process to form. The rock forming process is continually changing all rocks over time. In a very small way the Earth will be different tomorrow, sometimes its a larger change due to earthquakes, plates colliding and volcanic eruptions.

Igneous rocks

Igneous means "of fire". They start off deep within the Earth's magma (molten rock).
They form where the magma rises towards the surface where it may erupt as lava from a volcano or cool and go solid within the Earth's crust.

They are a hard rock.

A natural element such as iron is determined by the number of protons in the atoms nucleus. A mineral can be made up by one type of atoms such as in diamonds or a combination of different atoms such as in quartz. Atoms are arranged in a regular repeating pattern. Each mineral has its own unique crystal structure/pattern and chemical composition.

Igneous rocks are a mass/group of one or more types of interlocking crystals/minerals, which makes them very strong.

Basalt; Most volcanic rock is basalt. This is basalt from Bunbury in the south-west of Western Australia. It is approx 128 million years old.

Photographed at the Western Australian Museum.

Photographed from the Geological Museum

This is an igneous rock that was found in the Kimberley region of Western Australia. It is a Lamproite rock.

It is a fine-grained rock, like basalt. This rock would have come up from a volcano from the Earth's mantle over 150km down.

See the Lamproite page for more information on the secrets of this rock.

Magnetite is an iron ore found in igneous rocks and sedimentary rocks that have metamorphosed.
It is a blackish ore and can be picked up by a magnet. Magnetite is found and mined in the Gascoyne WA.
It has a hardness 5.5-6.

Photographed at the Rockingham Environment Centre

Magnetite is usually black but can sometimes be brown.

Photographed at the Western Australian Museum

Weathering of Rocks

Igneous rocks will weather breaking them down to form sediment.
One of things that controls the weathering of minerals is the temperature at which the minerals originally formed when they crystallized from magma or lava. Minerals which formed at high temperatures (deep in the Earth) and pressures are least stable, and weather most quickly. The temperature on the Earth's surface is much much cooler.
Weathered rock is called saprolite.
Weathering is different to erosion. (scroll down to see erosion).
Minerals in rocks react with their new environment to produce new minerals and each mineral has a different susceptibility to weathering.
Some rocks may only be weathered on the outer layer of the rock as the weathering hasn't affected the whole rock yet.
Warm humid climates generally have more highly weathered rocks than in cold dry climates.

Why do rocks weather?

Because the temperatures and pressures where the igneous rocks formed deep in the Earth is different to the temperatures and pressures they now have due to being closer to or on the Earth's surface which allows them to be more easily weathered.

There are two/three different ways a rock may be weathered;

Physical	This may happen from within or externally to the rock. Repeated heating (fire) and cooling (frost/ice at -5 degrees) may make crystals grow or expand and break the rock; salt can also increase the crystals volume. Plant roots, fungi, lichens, micro organisms; and animals can break the rock. Some people list this as a third group called Biological weathering.
Chemical	Rain water has acids and will dissolve some minerals; Oxygen combined with iron bearing silicate will change the rock to a rusty red/orange. Leaching; dehydration; dissolving of the mineral all are causes of weathering.

What do rocks look like after they have weathered?

Rocks will look very different depending on how they have weathered.
As a result of weathering some rocks will loose their atoms or compounds, some rocks will gain new atoms or compounds and some wont change their atoms or compounds but will change their mass i.e. break up into smaller masses.
A flat area of rock may weather faster than a steeply sloping area of rock where the water runs off.
Once the rock has been completely weathered it will be taken away or eroded, released as a gas into the atmosphere or absorbed by a plants roots where the nutrients feed the plant. The other rocks that don't completely become weathered will become new sedimentary or metamorphic rocks.

In these photos below you can see the same rock but weathered. Photographed at the Geological Museum Perth.

Sedimentary rocks

Sedimentary rocks are either made up of particles of weathered rock, minerals or the shells and bones of dead sea creatures near the Earth's surface; made from pre-existing rocks such as quartz; or made by natural chemicals e.g. limestone.
These particles are often called sediments. They are carried away by erosion i.e. the wind, ice or rivers, which are then dumped in a basin for example.
The previous igneous rock sediments settle in layers, the water is squeezed out and then harden and cement into a new rock over thousands of years.
They are a soft rock.

Larger particles like pebbles that are cemented together with smaller particles are called conglomerate rocks.

Sandstone is made up of tiny grains of quartz joined together by a natural cement such as silica. The Stirling Range in Western Australia is formed mostly of sandstone, slate and phyllite. Sandstone can be different colours like yellow or red.
Another place in Western Australia that is mainly made up of sandstone is the Kennedy Ranges approx. 1000km north of Perth. This area about 250 million years ago, was a basin filled with ocean water. This meant that the particles of dead marine animals were dumped here and later got squished down to form layers of sandstone and shale. Later movements in the Earth's crust pushed up this sandstone to 100 meters above the Lyons river valley below. Over time a lot of this range has been weathered and eroded away. So for these rocks its there second weathering from when they were igneous rocks.

Very fine sized particles of feldspar from granite that has weathered makes clay. They are too fine to see even with a microscope.

Feldspar Photographed at the Western Australian Museum

Clays then also make mudstone and shale. Clays make up nearly half of the sedimentary rocks on Earth.

Limestone is a common West Australian sedimentary rock and is made from calcite. It forms in water. The acid in rain water will dissolve it leaving behind holes and caves. The pinnacles are made from limestone and most of the coast of the south-west of Western Australia is limestone called Tamala limestone.

Photographed at the Rockingham Environment Centre.

Chalk is made from the skeletons of millions of tiny sea creatures. This chalk pictured below has an ammonite in it and this was found in Gin Gin northeast of Perth WA.

Photographed at the Western Australian Museum

Siltstone; Siltstone is a layered sedimentary rock courser than clay but finer than sand. It is similar to mudstone but slightly courser. This siltstone is from Minilya River north of Carnavon and is approx 260 million years old. It was photographed at the Western Australian Museum.
Shale is a sedimentary rock made-up of fine-grained clay-sized weathering debris. Shale is smooth. This was photographed at the Geological Museum

Coal is listed by most geologists as a sedimentary rock see our page on coal

Metamorphic rocks

Metamorphic means to "change". Metamorphic rocks form when igneous or sedimentary rocks are subjected or put through high temperatures and have been buried down deep in the earth. Or they may be crushed by huge pressures underground. This is so powerful that it changes the properties of what the rock is made from and what the rock looks like e.g. take a sedimentary rock like limestone which has been put under strong pressures, it becomes marble, which will now have a different texture and new minerals that are not found in the original limestone.

There are two types of metamorphoses (changes). One is called contact metamorphism and this is when hot magma heats the surrounding rocks and changes them. Its so hot that it bakes the rock and changes the shape and what it's made of. Its so hot that the original rock nearly melts, making the rock go softer and plastic-like, and this is when it will now change shape and cool, into its new shape.

The other type of metamorphism is called regional and this is where deeper rocks under ground are changed when sections of the Earth's crust collide. Because of the intense heat and pressure of these collisions the rocks start to melt in some places, new minerals appear, and the layers are pushed into all different shapes.

Slate is formed from shale which was a sedimentary rock. Slate forms under high pressure, but at a fairly low temperature. This means that fossils from the original shale often survive but may be squashed out of shape by the pressure.

Gneiss (pronounced "nice"). Gneiss is a banded or foliated metamorphic rock with course grains formed under very high temperatures and pressure. Many igneous and sedimentary rocks can become gneiss rocks. Gneiss is made up of layers of minerals. In some gneiss rocks each layer is a different mineral and in others, they are different-sized crystals of the same mineral. Some gneiss is buried beneath high mountains, then millions of years later when the mountains have weathered and eroded away the gneiss can be seen.

Gneiss Photographed at the Geological Museum Perth

Photographed at the Geological Museum Perth

This is a gneiss rock that has garnet crystals in it. It was formed by regional metamorphism (when deeper rocks collide). It has been heated as well as put under pressure and it has changed so much its impossible to tell what the original rock was. Go to our page on Garnet Gneiss

Often times gneisses have had more than one period of metamorphism.

When a common limestone sedimentary rock is put under heat and pressure sometimes marble forms. Impurities in the limestone give marble its many different colours and they can be in veins (like crooked rows) or in patterns. Marble is a valuable mineral.

Dolomitic marble from Wyloo near the coast in Western Australia. Photographed at the Western Australian Museum »»»»»»»

How do we get oil?

Oil is a mixture of the element carbon and the gas hydrogen, which is believed to have originally come from small sea creatures known as plankton????. When the plankton die, their remains accumulate on the seabed. As they become buried in mud they rot and break down and the carbon makes droplets of oil.
Over time (millions of years) the mud is buried under very thick layers of sediment and became rock. Sometimes the droplets of oil join together to make big underground pools. These get bigger and bigger until they form an oil reservoir which is pushed up as the earth folds.
This happened in the Carboniferous Period about 300 million years ago.

How do we know how old a rock or fossil is?

There are two ways of dating fossils (first you ring the fossil up and whisper sweet nothings in its ear.....)

One way to tell the age of a fossil is called "Relative dating". This is where you will be able to know what geological period the rock or fossil came from but you wont know the year.
This done by comparing the layers of sedimentary rock in the sample with other samples already dated. Different layers build up differently in different geological periods.

The other way is called "Atomic dating" and by this method you can work out what year a rock or fossil is from.
Inside the sedimentary rocks are different older rocks so if scientists can know how old the other rocks are around the fossil it gives them a range of how old the fossil is but only down to millions or perhaps to thousands of years. To do this the scientist will measure how much radioactive material is left in the rock, as the rock ages it loses this radioactive material.