How Rocks Form
- Rocks form in three main ways:
- igneous - by crystallization of minerals from molten material or magma as it cools
- sedimentary - by accumulation of materials which have been eroded and weathered from pre-existing rocks at the earth's surface
- metamorphic - by modification of pre-existing rocks by heat, pressure or the action of chemical solutions
Igneous rocks form as molten material, called magma, cools and solidifies. As the hot magma rises to the surface and cools, the chemical elements within it combine and minerals begin to crystallize. As cooling continues, crystals get larger and more numerous and begin to interfere with the neighbouring crystals which are also growing; the result is a mass of partially formed and interlocking crystals. If cooling is slow, minerals with sharp edges and perfect faces are rare; normally so many form at once that they don't have room to grow in this way.
Magma when it cools deep within the earth cools very slowly, allowing the crystals to grow to be quite large, 3 to 10 millimetres or more. This results in a medium or coarse-grained igneous rock - termed intrusive or plutonic. Examples of this type of rock are granite or gabbro. Intrusive rocks are completely crystalline and generally unlayered. Differing proportions of light and dark minerals result in a colour range from light to dark. These colour variations, and the presence or absence of quartz, are used to distinguish the different kinds of inrusive rocks. Most igneous rocks are of intermediate composition and colour. These intermediate-coloured intrusive rocks form a series which grades between granite on the light-coloured side, through granodiorite, quartz diorite and diorite to gabbro on the dark side.
If on the other hand the magma cools close to or at the earth's surface, as for example in the Hawaiian volcanoes, cooling takes place rapidly allowing little time for crystal growth. This type of igneous rock - volcanic or extrusive - will be very fine grained. An example is basalt. Although the grains may not be clearly visible, generally the rock will still have a granular texture. In some cases the magma cools so quickly that no crystals grow and volcanic glass results. Another common scenario is where cooling of the magma begins deep inside the earth where a number of quite large crystals grow, then is moved nearer to the surface where it is cooled rapidly. The resulting rock has some large, well-formed crystals sitting within a fine-grained matrix; it is called a porphyry. The large crystals are called phenocrysts. Different types of volcanic eruptions give rise to different types of volcanic rocks. If magma pours out onto the surface, a lava with flow structures, phenocrysts, and abundant vesicles (or gas bubble cavities) may result. If the eruption is forceful, like Mauna Loa in Hawaii, or explosive, like Mount St. Helens, the rock formed may consist of a variety of fragments including glass shards, volcanic bombs, ash and pumice. These deposits are commonly layered and are termed agglomerates, volcanic breccias or tuffs.
Sedimentary rocks form at the earth's surface by the action of weathering and erosion of pre-existing rocks. Rocks are gradually broken down by physical and chemical means when they are exposed to the atmosphere. Wind-blown particles abrade; rain pounds and the rainwater dissolves some parts; alternate freezing and thawing of water in microcracks break down the surface layer; gravity and weather shift unstable particles down slope and they abrade or break other rocks that they hit. Catastrophic events also occur; earthquakes trigger massive rockslides; floods move huge amounts of materials and undercut their banks; volcanos blast cubic kilometres of rock into rubble.
Broken rocks, grains and dissolved materials wash into rivers and are carried or rolled downstream. Particles are abraded and gradually rounded off as they grind together like pebbles in a lapidary tumbler. When the rivers enter lakes or the ocean, particles are dropped and dissolved materials tend to deposit as fine precipitates of mud and ooze. Particles vary in size from boulders through gravel and sand to silt and mud. Sediment may also be transported by glacial ice or the wind.
Slowly , the layers or beds of deposited sediment are buried by more sediment and become compacted. Mineral matter precipitates from trapped water and cements the grains together forming sedimentary rocks. Apart from the layered beds, sedimentary rocks may have various other structures apparent within them, such as ripples, animal burrows or plant roots. Often plants and animal remains drop into and become buried in the accumulating sediment, becoming fossils.
Metamorphic rocks form when pre-existing rocks are subjected either to elevated temperatures or pressures, or to the actions of chemically active solutions, or some combination of these. One or more of the following changes take place:
- the rock recrystallizes to form a coarser grained rock
- new minerals form overgrowths on the original minerals
- a foliation or strong banding develops, along which the rock splits into sheets, bands or flakes. Foliation is caused by the orientaion of elongate or flattish minerals, like mica, on a planar surface.