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Extrusive igneous rocks

Magma comes to the surface of the Earth through volcanoes where it can flow as lava and then cool and crystallises to form extrusive or volcanic rocks.

The following links show 3D examples of extrusive igneous rocks.

The models are grouped into lavas and pyroclastic rocks and then organised in rough order from the most silicon-rich rocks to the least silicon-rich rocks in each group.

Click on the models to view them in 3D and click on the numbered hotlinks in the models for more information on some models.

Lavas

∗ Rhyolite

Rhyolite is a high silicon rock that forms from low-temperature sticky lava.

Rhyolite is mainly composed of quartz and feldspar but the crystals in this sample are small and difficult to see without a hand lens or microscope. The bands/layers in the rock below are formed when the sticky lava was flowing.

This second rhyolite block is from Rotorua, New Zealand. It has clear bands due to magma flow. Zoom in to look at these bands, some of them have been bent and twisted as the lava moved.

∗ Dacite

Dacite lavas have moderate amounts of silicon and may produce lava flows or explosive eruptions. Look closely at this example of dacite lava from KareKare in New Zealand and you will see small crystals of feldspar and quartz that crystallised in the magma before it was erupted. The material between these crystals cooled rapidly after the magma was erupted and the crystals in this groundmass material are only visible with a microscope.

 

∗ Basalt

Basalt lavas have low silicon content and come from high-temperature runny magmas. They do not contain quartz or potassium feldspar and are mainly composed of plagioclase feldspar and iron and magnesium bearing minerals such as pyroxene and olivine.

This sample from western Victoria contains lots of holes called vesicles which were filled with volcanic gas when the lava solidified. The magma that formed this rock was completely liquid and did not contain any crystals. The rock cooled very rapidly and crystals are only visible with a microscope.

The green blob in this rock sample is a piece of the Earth’s mantle that was brought to the surface in a volcano. Look closely at this material and you can see the large crystals of the green mineral olivine.

 

 

... more basalt only in columns

When lava cools fractures called joints often form at right angles to the top and bottom of the flow. These joints break the lava into columns. Columns are most common in low silicon volcanic rocks but can also occur in some shallow intrusive rocks as well.

This example comes from Fingal Head in northern NSW and shows the tops of columns in a 15m thick layer of basalt.

Look down on this model and count the number of faces on a few of  these columns. What is the average number of faces?
Why do they form in this way?

... more basalt only in pillows

Pillow structures in basalt occur when the hot, runny, low silicon magma erupts underwater. The outer surface of each pillow cools extremely rapidly to produce a glassy rim. In between the pillows there may be fine grained sediment deposited from the water column.

This example shows a cross section through pillow basalt exposed on the foreshore at Sulphur Creek, northern Tasmania.

 

Pyroclastic rocks

∗ Pumice

Pumice is a volcanic rock formed during explosive eruption of high silicon content magmas that have a high gas content.

The gas expands but cannot escape before the magma solidifies because the rock cools extremely rapidly. Pumice has a large proportion of vesicles (holes) and the rock material between the holes is glassy. Crystals are hard to identify even with a microscope.

Pumice has a low density and can float on water.

∗ Tephra

Tephra is a rock made up of fragments and particles ejected from a volcano.

This example from Rotorua in New Zealand shows a series of layers composed mainly of pumice fragments with some small dark pieces of lava. Each layer represents a  volcanic event.

The thin, fine-grained layers  were likely formed by gentle settling of volcanic ash that was blown high into the atmosphere while the more coarse grained layers probably represent deposits formed by major explosive eruptions.

 

 

Volcanic Breccia

Volcanic breccias can form close to the vent of the volcano and contain large angular particles which are poorly sorted. They may form from accumulation of particles blown out of the volcano by explosive eruptions.

In this example the material that contains the large fragments is breccia made up of large basalt pieces in a much finer groundmass. The rock that the compass is sitting on is a basalt lava flow.

 

 

Now explore intrusive igneous rocks!

 

Minerals Rock Cycle Igneous Rocks Sedimentary Rocks Metamorphic Rocks More ...

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