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Types of Rock: Cycles and Classifications

Kenneth is a computer whiz who also enjoys nature and traveling. He likes researching the natural world and sharing his findings.

Learn everything you need to know about rocks. This article teaches you about petrologists, rock classifications, the rock cycle, and more!

Learn everything you need to know about rocks. This article teaches you about petrologists, rock classifications, the rock cycle, and more!

What Is a Rock?

A rock is a solid cohesive aggregate of one or more minerals. Some rocks consist of one mineral species ("monomineralic") and others consist of two or more minerals ("polymineralic").

Petrology is the study of rock. Petrologists have developed hundreds of observations from thousands of rock samples found in all parts of the world. They've discovered how rocks are formed and have also determined the variation in composition and structure of rocks.

Petrologists find difficulties in describing and classifying the rocks. They tried to find out the likeness and differences with the given separate samples of rocks. After numerous studies, petrologists group the rocks into broad categories.

Rocks are classified in several ways:

  • Origin
  • Minerals composition
  • Texture
  • Color
Types of Rocks

Types of Rocks

Rock Age Determination

Age determination of rocks is done through the radioactivity of certain chemical elements that are present within them. Geologists have reconstructed the geologic history of the earth from the time of formation up to the present. Also, rocks from the moon contributed a lot to the knowledge of the history of the earth. They can be used to study rocks from other celestial bodies.

Rock Origins

It is said that the composition of rocks reveals much of their origin. Some rocks are organic in origin. Also, colors, textures, and other properties of the minerals cause a great variety of appearances. These features of rocks give clues about the formation of respective rocks. Thus, they help petrologists indicate whether the rocks are igneous, sedimentary or metamorphic. However, sometimes the origin of the metamorphic rocks cannot be determined.

Note: There’s an exception to the requirement that rock consists of minerals. This exception is known as obsidian. It is a volcanic rock made up of glass and coal, which is a mixture of organic compounds.

Rock Cycle

When magma from the erupting volcano reaches and is exposed at the earth’s surface, it turns into igneous rock through the process of cooling and solidification. When the igneous rock is transported, deposited, consolidated and undergoes the process of lithification (cementation and compaction), it becomes sedimentary rock.

Then, through heat and pressure, it is converted into metamorphic rock—a process known as metamorphism. When metamorphic rock is deeply buried due to heat and pressure, it may be remitted to new igneous material through the process of melting.

Genetic Classification of Rock

The following categories have been developed by petrologists to help classify and order types of rock.

Igneous Rocks

It is a rock formed or crystallized from magma.; that is, they have undergone solidification from magma. Before magma reaches the earth’s surface, some of it is crystallized into layers of lava (extrusive flow rocks) and other portions are pyroclastic debris in the explosive eruption. Ingenous rock's chief elements are oxygen, silicon, sodium, aluminum, iron magnesium, calcium and potassium.

Samples of intrusive rocks that have been cooled quickly that float in water:

  • Igneous intrusions – this is a manner wherein magma cools affect the texture of the rock.
  • Batholiths – it is a large mass of intrusive igneous rock whose lower limit is indefinite.
  • Laccoliths – it is an intruding magma in which it has a definite flair.
  • Sills – it is a sheet-like mass of fluid magma that sometimes flows between the layers of existing rock.
  • Dikes – it is an intrusion of magma into vertical or nearly tissues in the bedrock.
Types of Igneous Rocks

Types of Igneous Rocks

Examples of Igneous Rock

1. Basalt (volcanic or plutonic)

  • Fine-grained
  • Fairly heavy
  • Black rock
  • Dense

2. Gabbro (plutonic)

  • Coarse-grained
  • Dark-green to black color
  • Contains pyroxene, feldspar, amphibole, hematite or magnetite

3. Granite (plutonic)

  • Coarse-grained
  • Acidic, light-colored (gray to pink)

4. Obsidian (volcanic)

  • Translucent
  • Black glass
  • Has a conchoidal fracture
  • It has a hardness of about 7.

5. Pumice (volcanic)

  • Fine-grained
  • Light-colors (almost white)
  • Frothy glass
  • Light in weight

Sedimentary Rock

A rock formed from sediment at low temperature. Groups of sedimentary rocks include:

  1. Clastic sedimentary rocks – this consists of mechanically transported particles. It has fine-grain sizes, sorting form and arrangement.
  2. Chemical sedimentary rocks – these are rocks that are precipitated from the ocean.
  3. Organic sedimentary rocks – it is silicon’s types, calcareous types and carbonaceous types.

Examples of Sedimentary Rock

1. Chert

  • Fine-grained
  • Gray, tan, or brown
  • Has a hardness of 7
  • It is a water-worn pebbles and cobbles

2. Conglomerate

  • Rounded rock
  • In size from pebbles to boulders cemented

3. Limestone

  • Fine-grained
  • Light gray to tan or black
  • Composed of calcite
  • Contains fossils
  • It has a hardness of 3
  • It effervesce from contact with diluted HCL

4. Dolomite

  • Similar to the characteristics of limestone but does not react when diluted to HCL unless powdered.

5. Sandstone

  • Coarse sand grains
  • Contain fossils

6. Shale

  • Fine particle (clay)
  • Gray or black in color
  • Contain fossils
  • Soft
  • Easily to scratched
Metamorphic Rocks

Metamorphic Rocks

Uses of Rock

  1. Used by prehistoric men as tools and weapons in hunting animals.
  2. Used as jewelry and for decorations.
  3. Used as specimens in the laboratory.
  4. Serve as territorial boundaries.
  5. Used to fertilize the land and soil.
  6. Used as food like salt.
  7. Used as a source of income.
  8. Used for building bridges, infrastructure and the like.

Metamorphic Rock

It is a rock formed by the effect of heat, pressure, and chemical action on other rocks. These rocks are a combination of igneous and sedimentary rocks.

Examples of Metamorphic Rock

1. Gneiss

  • Banded or streaked in appearance
  • Light and dark in color
  • Medium or coarse-grained

2. Marble

  • White and coarse crystalline
  • Its hardness is about 3
  • Recrystallized milestone

3. Quartzite

  • White, gray, tan in color
  • Recrystallized quarts sandstones
  • Has a hardness of about 7

4. Schist

  • Medium-grained rock with platy minerals

5. Slate

  • Extremely fine-grained rock
  • Black, blue-gray and green in color
  • It is characterized by slaty cleavage

Sources and Further Reading

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


Kenneth C Agudo (author) from Tiwi, Philippines on June 08, 2014:

Hehe thank you for loving my hubs :) Your answers were all correct, amazing! I love reading comments. I find time reading and visiting your home too sheila :)

sheilamyers on June 05, 2014:

Great hub! I learned this in college and forgot most of it because I don't use it enough. Thanks for the refresher course.