IGNEOUS ROCKS

Formation and characteristics

The term igneous is from the Latin word “ignis" which means FIRE.

When most people think about igneous rocks they imagine a volcano erupting ash and lava. Igneous rocks are produced this way but most igneous rocks are produced deep underground by the cooling and hardening of magma. Magma is molten (melted) rock under the surface of the Earth. It is produced in the upper parts of the mantle or in the lowest areas of the crust usually at a depth of 50 to 200 kilometres.

IGNEOUS

This diagram shows where magma is produced at a subduction zone.

MAGMA is less dense than the surrounding rock which causes it to rise.

When magma reaches the surface it is then called LAVA and the eruptions of lava and ash produce volcanoes.

When lava reaches the surface of the Earth through volcanoes or through great fissures the rocks that are formed from the lava cooling and hardening are called EXTRUSIVE or VOLCANIC IGNEOUS ROCKS.

Some of the more common types of extrusive igneous rocks are lava rocks, cinders, pumice, obsidian, and volcanic ash and dust.

This diagram shows a large intrusive igneous body called a batholith. A batholith is the largest of the intrusive bodies. They are larger than 100 square kilometres and usually form granite cores.

• Millions and even billions of years ago molten rock was cooling and thus hardening into igneous rocks deep under the surface of the Earth.

• These rocks are now visible because mountain building has thrust them upward and erosion has removed the softer rocks exposing the much harder igneous rocks.

• These are called INTRUSIVE or PLUTONIC IGNEOUS ROCKS because the magma has intruded into pre-existing rock layers. Types of intrusive igneous rocks are granite and basalt.

The composition of igneous rocks falls into categories determined by the amount of silica (SiO2) that the rocks contain – the less silica, the darker the rock. The four main categories are acidic, intermediate, basic and ultrabasic.

Acidic rocks have a high silica content (65% or more) along with a relatively high amount of sodium and potassium.

These rocks are composed of the minerals quartz and feldspar.

RHYOLITE and GRANITE are the two most common types of acidic rock.

The composition of igneous rocks falls into categories determined by the amount of silica (SiO2) that the rocks contain. The four main categories are acidic, intermediate, basic and ultrabasic.

• Intermediate rocks contain between 53% and 65% silica.

• They also contain potassium and feldspar with a small amount of quartz.

• DIORITE and ANDESITE are the two most common types of intermediate rock.

INTERMEDIATE ROCKAndesite Diorite

The composition of igneous rocks falls into categories determined by the amount of silica (SiO2) that the rocks contain. The four main categories are acidic, intermediate, basic and ultrabasic.

• Basic rocks are composed of less than 52% silica and a large amount of feldspar and very rarely quartz.

• The two most common types of basic rocks are BASALTS and GABBROS.

The composition of igneous rocks falls into categories determined by the amount of silica (SiO2) that the rocks contain. The four main categories are acidic, intermediate, basic and ultrabasic.

• Ultrabasic rocks are composed of less than 45% silica and contain no quartz or feldspar.

• They are composed mainly of a dense iron and magnesia mineral called olivine and the mineral pyroxene.

• The most common ultrabasic rock is PERIDOTITE. It is a dark green, coarse-grained igneous rock that many scientists believe is the main rock of the mantle.

ULTRABASIC ROCKPeridotite

The most widespread igneous rock

Basalts are dark coloured, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.

This is a vertical columnar basalt formation. When basaltic lava cools it often forms hexagonal (six sided) columns. Some famous examples of columnar basalt formations are the Giant's Causeway in Northern Ireland (bottom left) and the Devil’s Postpile National Monument in California (bottom right).

Giant’s Causeway

SCORIA

Scoria is a type of basalt that's full of bubble holes.  The bubbles formed as the lava was blasted out of a volcano, and were trapped as the lava cooled and hardened.

The bubble holes are often uniform in size and shape.

Despite all the holes, scoria doesn't float in water.

Scoria can be black, dark gray, or red.

PUMICE the rock

that floats

PUMICE is a very light coloured, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes.

Pumice is so light that it actually floats on water. Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals.

Pumice is ground up and used today in soaps, abrasive cleaners, and also in polishes.

• RHYOLITE is very closely related to granite. The difference is rhyolite has much finer crystals.

• These crystals are so small that they can not be seen by the naked eye.

• Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite giving it a glassy appearance.

• The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.

RHYOLITE

GRANITE

Granite is one of the most common igneous rocks.  Many headstones are made of granite.

You can't scratch granite with a nail or knife. 

Some broken surfaces have flat surfaces that shine in sunlight.

Granite is made mostly of the minerals feldspar and quartz.  (Reddish feldspars give this granite its colour and break to form flat surfaces.  The quartz crystals may be a semi-clear greyish or purplish colour.)   

Many granites also contain small crystals of mica or darker minerals.

• GABBROS are dark-coloured, coarse-grained intrusive igneous rocks.

• They are very similar to basalts in their mineral composition.

• They are composed mostly of the mineral feldspar with smaller amounts of pyroxene and olivine.

GABBRO

• OBSIDIAN is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct smooth shell-like curved fracture. Look at the fractures in the photo above.

• Obsidian is produced when lava cools very quickly.

• When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way. It is the result of volcanic lava coming in contact with water - often the lava pours into a lake or ocean and is cooled so quickly that no crystals can form, producing a glassy texture in the resulting rock.

OBSIDIAN

• OBSIDIAN is usually black or a very dark green caused by the iron and magnesium in the rock, but it can also be found in an almost clear form and can also contain white 'snowflake' crystal patterns of the mineral Cristobalite as shown in the photo above.

• Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations.

OBSIDIAN

IGNEOUS ROCKS - review

IGNEOUS rocks are formed from the cooling and consolidation of magma

• PLUTONIC (intrusive) – cooled below the surface

• VOLCANIC (extrusive) – cooled on the surface

IGNEOUS ROCKS - review

IGNEOUS ROCK textures are formed by the rate of cooling and the chemical composition of the magma

• GLASSY – no minerals present e.g. obsidian

• CRYSTALLINE – rocks made of mineral grains e.g. granite

• VESICULAR – with bubble holes e.g. pumice, scoria

• PORPHYRITIC – mixture of coarse and fine grains e.g. basalt

Subduction-related igneous rocks

http://geollab.jmu.edu/Fichter/PlateTect/igevol.html