Category The Earth, Earth Science, Planet Earth

The rock cycle is the process through which all the Earth’s rock is continually changing.

The rock cycle is a process in which rocks are continuously transformed between the three rock types igneous, sedimentary and metamorphic. Rocks of any type can be converted into any other type, or into another rock of the same type, as this diagram illustrates:

Conversion to metamorphic rocks requires conditions of increased temperature and/or increased pressure, conversion to sedimentary rocks occurs via the intermediate stage of sediments, and conversion to igneous rocks occurs via the intermediate stage of magma:

Increased temperature and pressure occurs in subduction zones and in areas where two plates of continental lithosphere collide to produce a mountain range, while increased pressure without increased temperature is produced when sedimentary rocks are deeply buried under more sediments. Sediments are produced when rocks are uplifted, weathered and eroded, and the resulting detrital material deposited in marine or terrestrial basins. If the sediments are buried under further layers of sediment, they can become lithified to produce a sedimentary rock. Magma is produced when rocks are melted. This melting can occur when a lithospheric plate descends into the Earth’s crust at a subduction zone, or when a mid-ocean ridge opens up and produces decompression melting in the athenosphere under the ridge. When the magma solidifies, it becomes an igneous rock.

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The earth’s rocks are divided into three main types. Igneous rock is the original material that makes up the Earth, formed when magma rises to the surface and cools. The planet’s oldest rocks are all of the igneous type. Sedimentary rock is made up of particles of other rock that has been affected by contact with the atmosphere. Erosion caused by water, wind and ice breaks the rock down into tiny particle that are carried away and settle in rivers, lakes and other areas. Over time, the particles compress to form sedimentary rock. Metamorphic rock is formed by the natural effects of heat and pressure changing igneous and sedimentary rock.

The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed.

Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils. Examples of this rock type include conglomerate and limestone.

Metamorphic
Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface. Examples of this rock type include gneiss and marble.

Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian.

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All rocks are made of various natural substances called minerals. Each mineral has its own chemical make-up, and the different minerals combine together in various ways. Most rocks contain around six different minerals that grow together in a crystal structure.

A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition and the way in which it is formed. Rocks are usually grouped into three main groups: igneous rocks, metamorphic rocks and sedimentary rocks. Rocks form the Earth’s outer solid layer, the crust.

Igneous rocks are formed when magma cools in the Earth’s crust, or lava cools on the ground surface or the seabed. The metamorphic rocks are formed when existing rocks are subjected to such large pressures and temperatures that they are transformed—something that occurs, for example, when continental plates collide. The sedimentary rocks are formed by diagenesis or lithification of sediments, which in turn are formed by the weathering, transport, and deposition of existing rocks.

The scientific study of rocks is called petrology, which is an essential component of geology. Rocks are composed of grains of minerals, which are homogeneous solids formed from a chemical compound arranged in an orderly manner. The aggregate minerals forming the rock are held together by chemical bonds. The types and abundance of minerals in a rock are determined by the manner in which it was formed.

Many rocks contain silica; a compound of silicon and oxygen that forms 74.3% of the Earth’s crust. This material forms crystals with other compounds in the rock. The proportion of silica in rocks and minerals is a major factor in determining their names and properties.

Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. These physical properties are the result of the processes that formed the rocks. Over the course of time, rocks can transform from one type into another, as described by a geological model called the rock cycle. This transformation produces three general classes of rock: igneous, sedimentary and metamorphic.

Those three classes are subdivided into many groups. There are, however, no hard-and-fast boundaries between allied rocks. By increase or decrease in the proportions of their minerals, they pass through gradations from one to the other; the distinctive structures of one kind of rock may thus be traced gradually merging into those of another. Hence the definitions adopted in rock names simply correspond to selected points in a continuously graduated series.

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Lava differs between volcanoes according to the type of rock it is made from, the gases it contains, and where it erupts. Pahoehoe lava moves quickly and looks rather like coils of rope when it cools. The thicker, lumpier as lava cools into chunky rocks.

Lavas, particularly basaltic ones, come in two primary types: pahoehoe (pronounced ‘paw-hoey-hoey”) and aa (pronounced “ah-ah”). Both names, like a number of volcanological terms, are of Hawaiian origin. A third type, pillow lava, forms during submarine eruptions. The adjacent picture of a dark pahoehoe flow on a lighter brown aa flow illustrates the difference between the two (photo from Galapagos, Islands Lost in Time by T. De Roy Moore, Viking Press, 1980). The difference in color is in this case is a reflection of age. The older aa in the photo has weathered and the iron in it has oxided somewhat, giving it a reddish appearance (even young aa flows are occasionally slightly brown or reddish, due to the oxidation that occurs during flow). The pahoehoe flow has a comparatively smooth or “ropy” surface. The surface of the aa flow consists of free chunks of very angular pieces of lava. This difference in form reflects flow dynamics.

A forms when lava flows rapidly. Under these circumstances, there is rapid heat loss and a resulting increase in viscosity. When the solid surface crust is torn by differential flow, the underlying lava is unable to move sufficiently rapidly to heal the tear. Bits of the crust are then tumbled in and coated by still liquid lava, forming the chunks. Sometimes the crust breaks in large plates, forming a platy aa. Pahoehoe forms when lava flows more slowly. Under these circumstances, a well-developed skin can form which inhibits heat loss. When a tear in the skin does form, it is readily healed. Both magma discharge rate and the steepness of the slope over which the lava flows affect the flow rate. Thus aa lavas are associated with high discharge rates and steep slopes while pahoehoe flows are associated with lower discharge rates and gentle slopes. The steep slopes of the large western Galapagos volcanos thus generally consist of aa, making ascent very difficult (and occasionally painful!). The less common pahoehoe flows on these volcanos are erupted from vents on the gently sloping apron or the caldera floor. Flows which begin as pahoehoe can convert to aa when a steep slope is encountered.

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When water is heated by volcanic activity, strange and spectacular landscapes are created. Known as hydrothermal areas, they can feature_ steaming hot springs, gurgling pools of mud and jets of water spouting hundreds of feet into the air.

Volcanoes mark vents where molten rock achieves the Earth’s surface — often in violent fashion. From subtle fissures to skyscraping peaks, these landforms are both destructive and constructive: They can smother terrain and ecosystems with lava, mudflows and ash, but also nourish biological communities with fertile soil and — significantly — create new topographic features.

Volcanoes, of course, are themselves landforms: sometimes subtle, sometimes unmistakable and dramatic. The steeply conical silhouette of a composite or stratovolcano — the classic image of a volcano in most minds — derives from intermixed layers of viscous lava, ash and other “pyroclastic” materials accumulated over many eruptions and emissions. In sharp contrast, a shield volcano — such as enormous Mauna Loa and Mauna Kea in Hawaii — assumes a much gentler slope from easily flowing basaltic lava. Volcanoes may also assume the shape of cinder cones and lava domes. Where weathering and erosion have stripped outer layers from extinct volcanoes, all that may be left on the landscape are resistant remnants of their “throats” and conduits in the form of volcanic necks (or plugs) and dikes. A world-famous example of the former is Shiprock in New Mexico. In the oceans, volcanic seamounts and island arcs are major features marking volatile tectonic margins.

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