Rocks: Definition, Rock Cycle, Sedimentary, Metamorphic and Igneous Rocks

Rocks: Definition, Rock Cycle, Sedimentary, Metamorphic and Igneous Rocks


Rocks are solid aggregates or mixtures of minerals, mineraloids, or organic compounds that occur naturally. They are the fundamental constituents of the crust of the Earth and can be made up of one or several minerals. The rock cycle is a geological process that causes rocks to continuously change throughout time, moving from one type to another. Geology is the study of rocks and the processes that shape the Earth.

The Rock Cycle

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle.

The rock cycle is driven by two forces:

(1) Earth’s internal heat engine, which shifts material in the mantle and core and causes gradual but substantial changes in the crust;

(2) the hydrological cycle, which is driven by the sun and involves the movement of air, ice, and water at the surface.

Rock Types

The three primary classes, or types, of rock, are igneous, sedimentary, and metamorphic,

Igneous Rocks

They are formed when magma that is located deep within the ground cools. They frequently feature big crystals that are visible to the unaided eye. Magma or molten rock found deep within the earth, cools and solidifies to produce igneous rocks.

The core of every igneous rock is magma. Gases and other volatile substances are mixed with melted or semi-melted rock to form magma. The temperature increases as you descend below the surface; eventually, you will reach the Earth’s mantle, a massive layer of magma that envelops the planet’s core.

Magma naturally cools to become rock, but if it cools below the surface at high temperatures (which are still lower than the temperature of the magma), the cooling process will be sluggish, allowing crystals to form. Because the magma had time to cool, this is why rocks like granite have large crystals. As you can see here, the crystals are likewise distinct.

However, if the magma erupts or is cooled rapidly, you instead get a volcanic rock –– not igneous, but also originating from lava. The classical example here is basalt, which can have many small crystals or very few large ones. Volcanic rocks are also called extrusive igneous rocks, as opposed to intrusive igneous rocks. Some volcanic rocks (like obsidian) don’t have any crystals at all.

There are differences in the chemical composition, temperature, and gas content of different magmas, and these variations can result in the formation of several types of rocks. You thus receive an amazing variety. Igneous rocks, of which there are over 700 hundred varieties, are often the hardest and heaviest forms of rocks. But volcanic rocks can also be exceedingly light; pumice, for instance, is so light that ancient sailors referred to it as “the foam of the sea.” When a volcano erupts violently and leaves air pockets in the rock, pumice is produced.

Igneous rocks that are most frequently found are andesite, basalt, dacite, dolerite (also known as diabase), gabbro, diorite, peridotite, nepheline, obsidian, scoria, tuff, and volcanic bomb.

Sedimentary Rocks

Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Sediment is the collective term for all of these particles. The sediment builds up gradually in layers and eventually turns into rock over a lengthy period. Sedimentary rock typically has a fragile texture and is prone to crumbling. The only form of rock that typically contains fossils is the kind that you can see in it, such as sand, pebbles, or stones.

Examples of this rock type include conglomerate and limestone.

The most widely used categorization for sedimentary rocks is as follows, though they can be classified according to several factors, including their chemical composition, the process of sedimentation, and whether or not they include organic or inorganic material.

• Clastic sedimentary rocks — small fragments of rock, mostly silicates, carried and deposited by fluids (bed flows, water). The size and makeup of the clastic crystals that are present in the sedimentary rocks—typically quartz, feldspar, mica, and clay—further categorize these rocks. 

• Conglomerates (and breccias) — Round gravel makes up the majority of conglomerates, whereas sharper, angular gravel makes up breccias.

• Sandstones — It is a rock composed of grains of rock and minerals the size of many sands, as the name suggests. Since quartz is the most prevalent mineral in the Earth’s surface crust, it is the most prevalent mineral in sandstone.

• Mudrocks — They are formed of cemented mud rocks. They usually contain very tiny particles that are carried by turbulent flow in water or air as suspended particles and deposit after the flow settles.

• Biochemical rocks — It may surprise you to learn that the majority of the limestone on Earth originates from biological sources. In other words, the majority of limestone that is found in the world today is derived from the bones of creatures like foraminifera, corals, and molluskos. An additional biochemical rock is coal.

• Chemical rocks — These rocks, which are predominantly composed of salt (halite) and gypsum, are mostly the result of water evaporation.

Examples: Argillite, breccia, chalk, chert, claystone, coal, conglomerate, dolomite, limestone, gypsum, greywacke, mudstone, shale, siltstone, and turbidite are a few examples of common sedimentary rocks.

Metamorphic Rocks

The metamorphosis (change) that results from extreme heat and pressure (squeezing) is what forms metamorphic rocks beneath the earth’s surface. These processes produce rocks with ribbon-like layers and sometimes with shiny crystals on their surface, which are the consequence of minerals forming slowly over time.

There are two types of metamorphism (change) that can cause this:

• contact metamorphism (or thermal metamorphism) — • Contact metamorphism, also known as thermal metamorphism, occurs when rocks get so close to magma that they begin to partially melt and undergo physical changes. Re-crystallization, crystal fusion, and numerous other chemical processes are possible. Here, temperature has a major role.

• Regional metamorphism, also known as dynamic metamorphism, is a process that usually occurs when rocks are compressed extremely hard and deep underground. As a result, the original features of the rocks are often destroyed and they become elongated. Here, pressure plays a major role (sometimes in conjunction with temperature).

When rocks undergo metamorphosis, they might retain new minerals that are the product of the transformation together with crystals and minerals from the original rocks. A metamorphic process is indicated by certain minerals, though. Of them, garnet, chlorite, and kyanite are the most commonly found. Two metamorphic processes that result from changes in the chemical environment are equally important: chemical recrystallization, which occurs when changes in temperature and pressure cause some crystals to become unstable and change into other crystals, and mechanical dislocation, which physically alters the rock or certain minerals.

Although they can be categorized in a variety of ways, they are usually separated into:

• Foliated metamorphic rocks: A definite preference alignment is produced when pressure elongates or compresses the crystals.

• Metamorphic rocks that are notfoliated; there is no preferred alignment of the crystals.

Some rocks, such as limestone, are made of minerals that simply don’t elongate, no matter how much stress you apply. Metamorphic rocks can form in different conditions, in different temperatures (up to 200 °C) and pressures (up to 1500 bars). By being buried deep enough for a long enough time, a rock will become metamorphic. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion; they can also form when the rock is heated up by the intrusion of magma from the Earth’s interior.

The most common metamorphic rocks are amphibolite, gneiss, marble, eclogite, and schist (blue, green, and micaschist, among others). Slate, migmatite, phyllite, quartzite, and serpentinite

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