Types of rocks

Collecting and classifying rocks can be a rewarding pastime. Both rocks and minerals play an important part of our lives through the many products made from them. Rocks provide most of our material needs, much of our power, and most of our national and state wealth. The study of rocks is called petrology and forms part of the science of geology.

Rocks are classified according to their origin.

The main classes of rocks are:

  • Igneous rocks which crystallise or solidify from a molten state.
  • Sedimentary rocks made up or fragments of pre-existing rock, biogenic material or chemical precipitates.
  • Metamorphic rocks formed from the alteration of pre-existing rocks due to heat and pressure in the earth.

Igneous Rocks

Igneous rocks all form by the cooling of molten material known as magma. This can occur at or near the surface, at shallow depths in the earth’s crust, or deep in the crust.

Igneous rocks which form at or very close to the earth’s surface are called volcanic rocks. They generally have small sized or indiscernible crystals or glassy textures. Their classification depends on the chemistry of the rock, importantly the amount of silicon dioxide in the rock. Examples of volcanic rocks are basalt, rhyolite and obsidian.

Igneous rocks which form at shallow depths in the earth are known as hypabyssal rocks. They generally have smaller crystals. Examples of hyabyssal rocks include dolerite, microgranite and microdiorite.

Igneous rocks which cool and solidify deep in the earths crust are called plutonic rocks. Examples of plutonic rocks are granite, gabbro, and granodiorite.

In general igneous rocks are named depending on their minerals and hence chemistry. Those with a high proportion of silicon dioxide are known as felsic, those with an intermediate amount of silica are called intermediate and those with lower amounts of silica are called mafic. A fourth group very low in silica is called ultramafic. A formal classification scheme for igneous rocks is defined by the International Union of Geological Sciences.

There are a group of rocks which fall halfway between volcanic rocks and sedimentary rocks. These rocks are the products of volcanic explosive activity and are called pyroclastic and volcaniclastic rocks. These include tuff, ignimbrites, and volcanic ash.

Sedimentary Rocks

Sedimentary rocks are rocks formed from the accumulation of sediment on the earth’s surface or the chemical precipitation of minerals on or near the surface.

Sedimentary rocks are divided into:

  • Clastic rocks made up of the fragments of weathered and eroded pieces of pre-existing rocks. Examples of this are sandstone, claystone, and conglomerate
  • Biogenic rocks formed from the accumulation of animal or plant remains. Examples are coal and limestone.
  • Chemical precipitates which form by the deposition of chemicals near or on the earth’s surface. Examples are bauxite, silcrete and ironstone.


Metamorphic rocks

Metamorphic rocks form by the alteration of pre-existing rocks by pressure and or heat in the crust. The rock is changed in solid state, with the heat and pressure changing the minerals in the rock. The rocks reflect the amounts of heat and pressure and the original material present. Metamorphic rocks are classified according to their fabric (rock texture) and their mineralogy. Examples of metamorphic rocks are marble, schist, phyllite, quartzite, slate, amphibolite and gneiss.

By examining the mineral species present in a metamorphic rock, geologists can tell the pressures and temperatures at which it formed. Thus geologists can determine at what depth the rock has been buried prior to uplift.

If you would like a rock identified, please contact us.

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