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Different Types of Rocks

Rocks are naturally formed, non-living materials made from collections of mineral grains bound together in a stable, firm mass. The presence of different mineral grains causes a difference in color and texture.

The mineral grains forming a rock can be tiny and microscopic or as big as a fingernail, depending on the rock type. Different rocks have different textures and a distinctive set of mineral composition. Rocks can be easily identified based on their mineral composition.

How Many Types of Rocks Are There?

Rocks are naturally occurring mineral composed constituting the basic unit of the solid earth’s composition and form. Rocks are classifiable into three main groups depending on their mode of formation. They can be studied in hard samples in outcrop, at their original location, or moved from their original location.

Rocks are commonly classified into igneous rock, sedimentary rocks, and metamorphic rocks, depending on the process that contributed to their formation. Igneous rocks are made of a solidified molten material known as magma, while sedimentary rocks consist of fragments extracted from precipitated material or pre-existing rocks.

Metamorphic rocks are derived from either sedimentary or igneous rocks under such conditions that result in changes in texture, internal structure, and mineral composition.

The three classifications of rocks are further subdivided into various groups, and types are based on chemical textural and mineralogical attributes. Rocks come in different sizes, and the earth’s crust and surface keep evolving through a process known as the rock cycle, which is responsible for the formation of most rocks.

Most rocks are formed using minerals consisting of oxygen and silicon, which are the most abundant elements found in the earth’s crust.

Rock Facts for Kids

  • The study of rocks is called geology, and there are three different types of rocks known today.
  • Rocks are made from mineral composition and have been used for millions of years for different purposes.
  • Minerals making rocks are made from a composition of elements joined together in different ways, e.g., silver.
  • Rocks from space are called meteorites, and scientists use them to study the solar system.
  • It takes millions of years for rocks to change from one form to another through the rock cycle.
  • Rocks made from minerals compose of such essential elements as silver and gold are called ores.

The Rock Cycle

It’s possible for a rock to begin as one type then change later as many times through a rock cycle process. The changes occurring during this process are slow and difficult to see, and such things as pressure and heat can cause them.

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It takes thousands of years for rocks to weather and be a move by erosion. Weathering is a process that breaks down rocks into smaller pieces as a result of rain, wind, chemicals, ice, plant roots, freezing, throwing, and running water.

The Three Types of Rocks

Igneous Rocks

Igneous Rocks

The word igneous is derived from a Latin word meaning fire. Igneous rocks are formed when magma cools and hardens. Magma refers to the molten rock deep in the earth, and it’s the heart of igneous rocks.

Magma is composed of a mixture of gases, some volatile elements, and molten or semi-molten rocks. These rocks solidify from magma, and they are formed at high temperatures when their constituent materials crystallize from molten material. Ingenious rocks originate from processes at depths of the earth, usually in the upper mantle or the mid to lower crust.

Igneous rocks are further classified into two subcategories; extrusive and intrusive igneous rocks, depending on their formation. When igneous rocks are formed inside the earth, they are known as igneous intrusive or plutonic rocks.

Some common examples of intrusive igneous rocks are diorite and granite. These rocks have large mineral grains and a very coarse texture, which is an indication that they spent millions of years inside the earth cooling down. This time course is what causes the formation and growth of large mineral crystals.

When these rocks are formed outside or on the crust of the earth, they are called volcanic or extrusive igneous rocks. Some examples of extrusive igneous rocks are pumice, a rock with a vesicular texture. Extrusive igneous rocks get their texture when the magma ejected contains gases. When this magma cools, the gas bubbles end up trapped, which is what gives these rocks a bubbly texture. 

Despite all igneous rocks forming from magma, not all magma is, made of similar composition. Different magma varies in quantities of gas, temperature, chemical compositions, and they include an incredible variety of rocks.

There are more than 700 types of igneous rocks, which are basically the heaviest and hardest of all rocks, with the exception of volcanic rocks. Other common types of igneous rocks are basalt, peridotite, andesite, tuff, gabbro, scoria, and obsidian, among others.

Metamorphic Rocks

Metamorphic Rocks

Just as the name suggests, metamorphic rocks are formed as a result of a metamorphosis, meaning that they changed from sedimentary or igneous rocks or even metamorphic. Metamorphic rocks go through a transformation by either pressure, heat, or both through such processes as mountain building.

There are two types of changes that lead to the formation of these rocks; dynamic and thermal metamorphism. Metamorphic rocks form as a result of changes in pre-existing rocks and from processes deep within the earth using recrystallization.

When contact metamorphism or thermal metamorphism occurs, the stones that are close to the magma start to melt partially, and their properties are altered. This results in chemical reactions, recrystallization, and fusing of crystals, as long as the temperature is the primary influence.

Dynamic or regional metamorphic changes; on the other hand, happens when rocks found deep underground are exposed to immense pressure. This pressure is significant enough to cause elongation of the stones, which ruins these rocks’ original and physical features.

Metamorphic rocks contain minerals and crystals derived from the initial rocks and some from the new minerals. There are some minerals such as garnet, kyanite, chlorite, which are clear indications of metamorphosis.

Specific significant changes in the chemical environment also result in metamorphic processes like chemical recrystallization and mechanical dislocation. Metamorphic rocks can be further classified into foliated and non-foliated metamorphic rocks.

Foliated metamorphic rocks have a precise preferential alignment as a result of the pressure squeezing and elongating the crystals. Non-foliated metamorphic rocks have no preferential alignment, with such stones as marble being composed of minerals that don’t elongate, regardless of their stress.

Metamorphic rocks are formed in different conditions, pressures, and temperatures. These rocks are buried deep in the earth’s surface and allow for metamorphosis to occur for a long time. These rocks form from tectonic processes like continental collisions, which result in friction, horizontal pressure, and distortion.

Metamorphic rocks also form when rocks are heated up by magma intrusion from the interior of the earth. Rocks resulting from metamorphic processes usually have ribbon-like layers, and sometimes their crystals are shiny. The most known metamorphic rocks are marble, slate, gneiss, quartzite, eclogite, phyllite, hornfels, and serpentinite. 

Sedimentary Rocks

Sedimentary rocks form by compacting and cementing sediments of sand, clay, silt, or rock-like gravel together. The rocks derive their name from their sediment nature. Sediment refers to a naturally occurring material that has been broken down by erosion and weathering and is transported naturally.

The sediments can be formed from the erosion and weathering of pre-existing rocks, and these sedimentary rocks have some chemical residues as well. Sedimentary rocks have a variety of defining qualities, but they all form through the deposition of materials on the earth’s surface and within water bodies.

The chemical makeup, inorganic/ organic material, and the sedimentation process behind the formation of sedimentary rocks are imperative in the classification of these rocks. Sedimentary rocks are formed after years and years of sediments compacting together and hardening.

Usually, small pieces of rocks and minerals sediments are carried by a river or stream and deposited to a larger water body where the pieces settle at the bottom. With time, these pieces compact and form solid rocks such as shale and Limestone. The most common classifications of sedimentary rocks are sandstones, biochemical rocks, conglomerates, classic sedimentary rocks, mudrocks, and chemical rocks.

Conglomerates are made of rounded gravel and angular gravel, while chemical rocks are formed through water evaporation. Mud rocks are made from solidified mud, and they are made of very fine particles.

These rocks are transported in suspended particles using a raging flow in air or water and deposit once the flow has settled. Clastic sedimentary rocks are small rock fragments that have been transported and deposited by water flow and bed flows through erosion and weathering.

Clastic sedimentary rocks are classified further depending on their composition and size of clastic crystals that make up these rocks, mostly clay, feldspar, quartz, and mica.

Biochemical rocks come from biological processes and sources. For instance, Limestone is a biochemical sedimentary rock formed from skeletons of organisms like foraminifera, corals, and mollusks.

There are other types of sedimentary rocks, such as those formed in hot springs. Some common examples of sedimentary rocks are coal, chalk, gypsum, claystone, siltstone, and dolomite. Most of the rocks found around are sedimentary, and they can take millions of years to form.  

Space Rocks

Meteorites

A special classification of rocks comes from space, commonly known as meteorites or space rocks. This isn’t a major classification of rocks as they are mostly composed of iron elements, and they may have different mineral makeup and elements compared to typical earth rocks.

Several processes involve information about different types of rocks, with the major ones being metamorphism, sedimentation, crystallization, and erosion. Rocks are all around us in the form of statues, tombstones, bricks, pebbles, sandpaper, and glass, among others.

Rocks have been used since the beginning of human civilization to build homes, machinery, and jewelry, among other things.