Classification of Rocks by: Audrey brumlik

Sedimentary Rocks

Rocks are classified by their origin.

The three rock groups are as followed:

  1. Sedimentary
  2. Igneous
  3. Metamorphic

Igneous and metamorphic are non-sedimentary.

A sedimentary rock can form in 3 ways.


The rock above is shale and its composed of clay and its grain size is less than 0.0004 cm.

Clastic Sedimentary Rocks that are inorganic are land derived sediments classified according to grain size. These are formed by the compaction and cementation of the fragments deposited usually by running water enter calm water. The fragments are deposited in horizontal layers called beds or strata.

Weathering is the process of wearing down the earth's surface by water, wind, glaciers and gravity. Weathering breaks down existing rocks and forms sediments. These sediments are compacted amd cemented together to form a clastic sedimentary.


Precipitation (settling out) of the minerals from a solution. They are referred to evaporites and precipitates. These are formed when the minerals in the water settle and then the water evaporates, leaving behind the rock. Sedimentary rocks formed by this process are usually called monominerallic. Monominerallic means composed of one mineral.

The image above is an photo of halite better known as rock salt. It is a type of precipitate/ evaporite. Other precipitates and evaporates are rock gypsum, dolostone, and chemical limestone.


Organic sedimentary rocks are formed from the remains of once living things. They are referred to as bioclastic. Two organic sedimentary rocks are organic limestone and coal.

The image above is coal. It is an organic sedimentary rock.

Sedimentary rocks form from deposition and burial that leads to compaction and cementation. The word clastic means fragments. A cementing agent holds sediments together in a clastic sedimentary rock. Fossils are found in sedimentary rocks because the processes that make other types of rocks would destroy the fossils.

Igneous Rocks

The two most abundant elements in minerals are silicon and oxygen. A silicate tetrahedron is a silicon surrounded by four oxygens.


All rocks contain minerals. Rocks are classified into 3 groups according to their origin. The groups are igneous, sedimentary, and metamorphic.


Igneous means " fire-formed". Rocks that form from molten rocks that solidifies (hardens) are called igneous.

There are two types of molten rock:

  1. Lava- molten rock at, or above the earth's surface
  2. Magma- molten rock below the earth's surface

Magma cools intrusively (deep in the ground). The crystals that form from magma would be large because the magma cools slowly allowing large crystals to form.

The image above is a picture of magma.

Lava cools extrusively (or at the earth's surface. The crystals are fine because the lava cools so quickly there is only time for small crystals to form.

Metamorphic Rocks

The word metamorphic means "to change."

Metamorphic Rocks

Metamorphic rocks form from already existing rocks. They can form from igneous, sedimentary, and existing metamorphic rocks. They are formed from extreme heat and/ or pressure. The rocks don't melt, they crystallize. There are two ways metamorphic rocks are formed:

Regional metamorphism happens over a large area deep inside the earth. It causes:

  1. Layers of rock to become distorted (layers become wavy)
  2. Minerals to align in bands (minerals separate out according to the density, creating bands)
The image above is a photo of gneiss, a regional metamorphic rock.

Metamorphic rocks that are distorted and/or have bands are foliated. The rocks have crystallized to form bands of minerals. The minerals were rearranged without melting.

Contact metamorphism occurs when rock is altered by coming into contact with molten rock. Rocks formed in this way are considered non-foliated metamorphic rocks. These rocks are composed of crystals.

The changes that occur in metamorphic rock are:

This an example of recrystallization.
  1. Minerals increase in size (recrystallization)
  2. Distortion
  3. Become more dense and decrease pore space
  4. Mineral banding


Created with images by Mickyboyc - "High Force" • James St. John - "Black shale" • paleo_bear - "Halite" • benscherjon - "coal cabbage burned" • skeeze - "molten volcano lava" • subarcticmike - "Cheshire Cat" • James St. John - "Acapulcoite (NWA 2989 Meteorite) 1"

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