The internal of the planet is hot. Geothermal Gradient: the warm within the earth steadily boosts with depth; theheat is from radiation decay and also remnant warm of the Earth"s formation.

Magma is molten rock material. As magma cools the facets within the magma combine and also crystalize right into minerals that type an igneous rock.

You are watching: Glassy igneous rocks form when magma cools too fast for mineral grains to grow.

Magmacools either below the surface or in ~ the surface (magma the reaches the surface is referred to as lava).

As magma cools igneous rock is formed. There space two species of igneous rock:

1) Extrusive: magma get the surface ar of the Earth prior to cooling and the lava cools rapidly.

2) Intrusive: magma cools under the Earth"s surface. The magma cools very slowly.

As magma cools minerals are created into an interlocking arrangement prjajalger2018.orgcing an igneous rock.


As magma cools the undergoes reaction that type minerals. The rate of cooling is really important. If the magma cools slowly then the reactions proceed for a longer period of time and the result mineral crystals (grains) become large. If the magma cools quickly then the reactions are lot quicker and also the mineral crystals (grains) are smaller.

Igneous rocks will have actually a distinctive appearance (this is described as texture) based on the rate of cooling (fast or slow).

Textural terms because that igneous rocks:

1. Phaneritic (coarse grained): rocks room composed the mineral grains large enough to be seen with the unaided eye. Indicitive of slow cooling, generally deep in ~ the Earth.


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Aphanitic (fine grained): rocks space composed the mineral seed too small to be seen by the unaided eye. Indicitive of quicker cooling, generally at or near the Earth"s surface.

3. Glassy: magma cooled so conveniently that decision formation could not occur. Indicative of practically instant cooling at the Earth"s surface.

4. Porphorytic: big mineral cyrstals surrounded by a good grained procession (background). This texture develops when sluggish cooling starts to kind large mineral crystals. Before the magma is totally cooled the magma (and its large mineral grains) moves closer come the surface and also finishes cooling at a much more rapid pace.

Igneous rocks have actually a variety of chemical composition.

1. Sialic (granitic) or felsic: rocks have highest silica, contain feldspar and ferromagnesium minerals. Rocks space light in color.

2. Intermediate (andesitic): rocks have actually less silica, save feldspar and also ferromagnesium minerals. Rocks are medium gray in color.

3. Mafic (basaltic): rocks have greater amounts of ferromagnesium minerals and also less silica and feldspar. Rocks space dark in color, black or dark gray.

4. Ultramafic: highest amount the ferromagnesium minerals and least lot of silica. Rocks space usually a dark eco-friendly to green-black.

Classification the Igneous Rocks

Igneous rocks space classified by both texture and also composition. You will find out to identify typical igneous rocks in lab.

intrusive (phaneritic)granitedioritegabbro
extrusive (aphanitic)rhyoliteandesitebasalt