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Kids Rock Type Facts and Information
Rock Type Facts

Rocks come in many different types and look very different.  

Some rocks are very hard, easy to scratch, are crumbly, hold water and some let water run through them. 

There are different types of rock and all are used for different things.  

Some are man-made and others are natural.

Rocks get weathered by chemical and physical weathering. Below are different types of rocks and how the rocks are formed.


rocks photo


Rock Classification

Rocks can be divided into three groups:

1.  Igneous:  Rocks with crystals that have fomed by moten magma cooling above or below the earth's surface.

2.  Sedimentry:  Broken fragments of older rocks that have been cemented together.

3.  Metamorphic:  Rocks that hae been altered by the effects of heat and pressure.

1.  Igneous rocks : Igneous means made by fire. These rocks are found deep underground near to molten rock from inside the earth.

Igneous rock is formed through the cooling and solidification of magma or lava.  Molten rock is called magma.  When it is forced to the Earth's surface it is also called lava.  Lava cools quickly on the surface to form igneous rocks with small crystals.

Some magma gets trapped and cools underground to form igneous rocks which have large crystals. Some igneous rocks are changed into metamorphic rocks.  Granite and basalt are very hard, expensive, impermeable and have muticolored, small particles.

Granite & Basalt:

Granite - is a common, coarse-grained, light-colored, hard igneous rock consisting chiefly of quartz, orthoclase or microcline, and mica.  It is used in monuments and for building.

Basalt - is a common extrusive volcanic rock.  It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of the planet. Unweathered basalt is black or grey.

           Basalt  (Igneous)                                   Granite (Igneous)

2.  Sedimentary rocks : is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. These rocks are made from sediment on the sea bed.

Particles that form sedimentary rock by accumulating are called sediment. These rocks are soft and crumbly, often have layers and fossils can be found in them. Sediment is formed by weathering and erosion in a source area and then transported to the place of deposition by water, wind, mass movement or glaciers which are called agents of denudation.

The layers of sediment gradually get squashed and eventurally form sedimentarty rock.   They are permeable.  Some sedimentary rocks get heated and squashed under the surface of the Earth.

This changes the rock into metamorphic rock.

Chalk, Conglomerate, Limestone, Mudstone & Sandstone:

Chalk - is a soft, white, gray, or yellow limestone consisting mainly of calcium carbonate and formed primarily from the accumulation of fossil microorganisms such as foraminifera and calcareous algae.

Conglomerate - rock has pieces of other rocks glued together to form one larger rock.  These rocks are found along rivers, beaches and glaciers where water or ice drops them.  This rock is made up of pebbles and other small gravel and sand glued together with iron oxice, calcium carbonate or silica.  

Limestone - is a sedimentary rock consisting mostly of calcium carbonate. It is used as a building stone and in the manufacture of lime, carbon dioxide, and cement.

Mudstone - is a fine grained sedimentary rock whose original constituents were clays or mud. Individual grains are too small to be seen without a microscope - grain size is up to 0.0625 mm (0.0025 in).

Sandstone - this rock can be scratched easily.  It is formed by the consolidation and compaction of sand and held together by a natural cement, such as silica.

                     Limestone (Sedimentary)                       Chalk (Sedimentary)

                         Conglomerate                                               Sandstone
                            (Sedimentary)                                          (Sedimentary)

                                                        Mudstone (Sedimentary)

3.  Metamorphic rocks – these rocks characteristics have been altered or changed (Metamorphism) over time by great pressure from layers of rock above them and heat below them or chemical changes affecting pre-existing rocks. Metamorphism is the process by which an existing rock is changed through the action of heat and pressure.   These rocks have been changed in the earth by heat and pressure. They are impermeable. These rocks were once igneous or sedimentary rocks.  They are hard and commonly used in building materials.

Gneiss, Marble, Quartzite & Slate :

Gneiss - ["nice"] is a rock of great variety with large mineral grains arranged in wide bands. It means a type of rock texture, not a composition.  It is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.

Marble - A metamorphic rock formed by alteration of limestone or dolomite, often irregularly colored by impurities, and used widely in architecture and sculpture.

Quartzite - is a hard metamorphic rock which was originally sandstone.  Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide. Other colors, such as yellow and orange, are due to other mineral impurities.

When sandstone is metamorphosed to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals.

Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism that occured. Small amounts of former cementing materials, iron oxide, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite.

Slate - is grey and quite smooth fine-grained metamorphic rock..  Easily splits into thin and smooth-surfaced layers. 

                       Marble (Metamorphic)                      Gneiss (Metamorphic)

                            Quartzite (view 1)                              Quartzite (view 2)
                               (Metamorphic)                                 (Metamorphic)

                                                Slate (Metamorphic) 

Permeable Rocks - allow water to pass through them.

Impermeable Rocks - are rocks that are waterproof.

Words associated with rocks:

Acid, Cliff, Cracks, Crumble, Crystals, Deposited, Earth
Erosion, Fossil, Frost, Geologist, Geology, Glacier, Grains,
Granite, Hard, Humus, Igneous, Limestone, Loam
Marble, Metamorphic, Mineral, Molten, Rock, Rough,
Sample, Sedimentary Sediment, Smooth, Soft,
Soil, Stalactite, Temperature Transport, Weathering

Coal - is a rock which can be burned as fuel.  Coal is called 'fossil fuel' because it has formed from remains of trees and plants from millions of years ago.

A fossil  - is the remains of an animal or plant that has been preserved in rocks. The shapes and details of fossils show us what the creature or plant looked like when it was alive.

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