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Types of Rocks: It’s Cycles and Classifications

A rock is a solid cohesive aggregate of one or more minerals. Some rocks consist of one mineral species (mono mineralic) and others consist of two or more minerals poly mineralic).

Petrology- is the study of rock. Petrologists developed hundred of observations from thousands of rock samples found in all parts of the world. By with that, they find out how rocks were formed, and also they determine the variation in composition and structure of rocks.

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

Rocks are classified into several ways:

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

Types of Rocks

Rock Age Determination

Age determination of rocks is done through radioactivity of certain chemical elements that present within them. Geologists have been reconstructed 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 and can be used to study rocks from other celestial bodies.

Rock Origins

It is said that the composition of rocks reveals much their origin. Some rocks are organic in origin. Also colors, textures, and other properties of the minerals cause great variety of appearance. Yet those features of rocks give clues under which rock masses were formed. Thus, this will indicate where the rocks are igneous, sedimentary or metamorphic. But sometimes the origin of the metamorphic rocks cannot be determined.

Note: There’s an exception to the requirements that rock consists of minerals and that is 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 reached and exposed at the earth’s surface, it turn to igneous rock through the process of cooling and solidification. And when the igneous rock transported, deposited and consolidated and undergone the process of lithification (cementation and compaction) it become sedimentary rocks. Which then through heat and pressure it is converted into metamorphic rock through the process of metamorphism. Sedimentary and metamorphic rocks may be weathered, transported and deposited it transform into younger sediments. When metamorphic rock deeply buried due to heat and pressure it may be remitted to new igneous material through the process of melting.

Genetic Classification of Rocks

Igneous rocks

It is a rock formed or crystallized from magma. These are rocks that have undergone solidification from magma. Before magma rich the earth’s surface, it has been crystallized into some as layers of lava (extrusive flow rocks) and others are as pyroclastic debris ion explosive eruption. It has chief elements which 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
  • Its hardness is of about 7.

5. Pumice(volcanic)

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

Types of Rocks (Rock-Star)

Sedimentary Rocks

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

  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 Rocks

1. Chert

  • Fine-grained
  • Gray, tan, or brown
  • Has ahardness 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 not react when diluted to HCL unless powdered.

5. Sandstone

  • Coarse sand grains
  • Contain fossils

6. Shale

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

Metamorphic Rocks

Uses of Rocks

  1. Used by prehistoric men as tools and weapon 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 Rocks

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

Examples of metamorphic rocks

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 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


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.

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