Igneous, Metamorphic and sedimentary rocks

Types of rocks & how they form. ​

• ​Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies.

• The rock melts deep within the Earth's crust at active plate boundaries or hot spots, and rises toward the surface.

• Under the ground the molten rock is called magma - intrusive rocks are formed when magma solidifies below the surface

• When the molten rock erupts it is called lava - extrusive rocks form from lava

​Igneous rocks

• As the magma or lava cools crystals form

• The appearance of the rock depends on the types of minerals present in the molten rock (shown are fluorite, misolite, bismuth and quartz) and the size of the crystals

• Larger crystals form when the rock cools slowly, small crystals when it cools more quickly.

​Crystals

• lava erupts at 1000 °C or more.

• Since it is erupting into air (or the sea for underwater volcanoes) it cools down relatively quickly - hours to days

• Therefore the minerals which crystalise as the rock cools are relatively small since they don't have time to grow bigger

• For example, basalt is an extrusive rock with tiny crystals, generally only visible with a loupe (magnifying glass).

• Basalt formed from underwater eruptions will generally cool quicker and have smaller crystals than basalt that cools on the ground

​

​Extrusive rocks

​Frothy rocks

​Pumice

​Scoria

• violent eruptions can occur which shoot out lava filled with gases.

• The lava cools quickly, while it is still in the air, and traps the gases inside, producing rocks full of holes.

• Two examples of this type of rock are pumice - a pale-coloured rock which can float on water; and scoria - a darker heavier rock, which contains iron and is generally found close to the volcano crater.

​

• Obsidian is a smooth, black glassy rock which forms when lava cools almost instantly.

• It cools so quickly no crystals form

• Very tiny air bubbles trapped in the rock can give it a coloured sheen.

​Obsidian

• Magma that doesn't make it to the surface cools very slowly underground, forming intrusive rocks, sometimes called plutonic rock (from Pluto - Roman god of the underworld)

• These can be exposed when the ground surface is eroded away

• Large bodies of intrusive rock are called batholiths.

• Intrusive rocks can have large crystals, which are visible by eye

​Intrusive rock

• granite is a common intrusive rock made up of white, pink, grey, black and clear crystals

• The 3 minerals it contains are quartz (clear to white or grey), feldspar (white or pink) and mica (black)

​

​Granite

Multiple Choice

Formed by melting rock:

• magma

• sediments

• sedimentary rock

• intrusive igneous

Multiple Choice

Formed by cooling & solidification of magma

• Sedimentary

• Metamorphic

• Extrusive Igneous

• Intrusive Igneous

Multiple Choice

Formed by cooling & solidification of lava

• Sedimentary

• Metamorphic

• Extrusive Igneous

• Intrusive Igneous

Multiple Choice

Has large crystals because of slow cooling

• sedimentary rock

• metamorphic rock

• intrusive igneous rock

• extrusive igneous rock

• Are rocks formed from sediments

• Sediments form when weathered rock is eroded by the wind, running water, the sea or glaciers and deposited in a new location.

• Sediments buried under further layers of deposition are tightly packed by compaction

• Chemicals dissolved in water can seep into the sediments, crystallising and gluing them together in a process called cementation.

• Many sedimentary rocks form in this way.

​

​Sedimentary rock

1. Deposition

2. Compaction

3. & 4. Cementation

​

​Sedimentary rock formation

​Types of sedimentary rock

​conglomerate

​sandstone

​siltstone and shale

• Clasts move downhill due to erosion under the influence of gravity, getting smaller & smoother.

• As running water slows down, larger pebbles and gravel fall out first, then sand and finally fine mud particles.

• Conglomerate contains larger pebbles and cemented together with smaller grains.

• Sandstone is formed from grains of sand.

• Siltstone and shale from fine grains deposited from slow moving water.

​

• Limestone is a sedimentary rock that is formed from deposits of the remains of sea organisms such as shellfish and corals.

• Coal forms from the remains of dead plants that are covered by water before they can decay and then buried by other sediments.

• The weight of the sediments above compacts the plant material

• Over millions of years compacting increases the temperature of the sediment and squeezes out the water, forming coal.

​Organic sediments

• Plant and animals remains can also be deposited in sediment layers

• When the organic materials are replaced by minerals, or decay leaving a void, this is called fossilisation.

• Trace fossils occur when tracks and burrows are preserved

​

​Fossils

​Canowindra fish fossils

​Star-shaped holes from a soft-bodied sea creature

​Dinosaur trackways Winton QLD (credit: Chris Whitelaw)

Multiple Choice

Formed by weathering & erosion

• magma

• sediments

• sedimentary rock

• intrusive igneous

Multiple Choice

Formed by compacting & cementing of sediments

• Sedimentary

• Metamorphic

• Extrusive Igneous

• Intrusive Igneous

• Igneous and sedimentary rocks which are buried deep below the Earth’s surface experience huge pressures from the weight of rock and sediments above them.

• Temperature increases by ~25°C every kilometre below the surface, so these rocks can also experience high temperatures.

• This produces changes in composition and appearance of the minerals, in a process called metamorphism and the rocks are called metamorphic rocks.

​Metamorphic rock

• Marble forms from limestone under heat and pressure.

• Metamorphic rocks that are mainly the result of great pressure on the parent rock are characterised by parallel bands of different coloured minerals or flat, leaf-like layers. For example,

• Slate forms from shale; because it is heavy and naturally forms a flat, smooth surface it is traditionally used in pool tables

• Gneiss is formed from granite

​

Types of metamorphic rock

Multiple Choice

Scientists divide rocks into groups based on how they form. The major groups of rocks are:

• sedimentary and igneous

• sedimentary, igneous and metamorphic

• metamorphic and sedimentary

Multiple Choice

Formed by heat and pressure

• Sedimentary

• Metamorphic

• Extrusive Igneous

• Intrusive Igneous

Multiple Choice

Has wavy bands.

• sedimentary rock

• metamorphic rock

• extrusive igneous rock

• intrusive igneous rock

Multiple Choice

A fossil can be formed when

• organic materials are replaced by minerals

• bones are preserved in sedimentary layers

• organic remains are metamorphosed by heat and pressure

• lava flows over plants or animals encasing them in rock.

Multiple Choice

Has small or no crystals because of rapid cooling

• sedimentary

• metamorphic

• extrusive igneous

• intrusive igneous

Multiple Choice

If heat and pressure inside the Earth cause a rock to melt, the material that formed would be

• Metamorphic

• Magma

• Sedimentary

• Igneous

Multiple Choice

You find a rock in which the grains are arranged in parallel bands of white and black crystals.  The rock is probably a(n)

• Igneous rock

• Sedimentary rock

• Metamorphic rock

• Extrusive rock

Dropdown

Fossils can only form in​ ​

Select answer

Labelling

The processes which form sedimentary rock are:

Drag labels to their correct position on the image

Deposition

Cementation

Compaction