The interior of the earth is hot. Geothermal Gradient: the heat within the earth steadily increases 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 elements within the magma combine and crystalize into minerals that kind an igneous rock.

You are watching: Igneous rocks that form ____ the surface are intrusive.

Magmacools either below the surface ar or at the surface ar (magma the reaches the surface ar is called lava).

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

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

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

As magma cools minerals are formed into one interlocking setup prstillproud.orgcing an igneous rock.


As magma cools that undergoes reactions that type minerals. The rate of cooling is an extremely important. If the magma cools slowly then the reactions proceed for a longer period of time and also the resulting mineral crystals (grains) end up being large. If the magma cools promptly then the reaction are much quicker and the mineral crystals (grains) are smaller.

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

Textural terms because that igneous rocks:

1. Phaneritic (coarse grained): rocks room composed of mineral grains big enough to it is in seen v the unaided eye. Indicitive of slow cooling, generally deep within the Earth.


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Aphanitic (fine grained): rocks space composed that mineral grains too small to be checked out by the unaided eye. Indicitive of quicker cooling, normally at or close to the Earth"s surface.

3. Glassy: magma cooled so easily that decision formation could not occur. Indicative of almost instant cooling in ~ the Earth"s surface.

4. Porphorytic: big mineral cyrstals surrounded by a fine grained matrix (background). This texture develops when slow-moving cooling starts to form large mineral crystals. Prior to the magma is totally cooled the magma (and its large mineral grains) move closer come the surface and also finishes cooling at a much more rapid pace.

Igneous rocks have a selection of chemical composition.

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

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

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

4. Ultramafic: greatest amount of ferromagnesium minerals and least amount of silica. Rocks are usually a dark eco-friendly to green-black.

Classification of Igneous Rocks

Igneous rocks room classified by both texture and composition. Friend will learn to identify usual igneous rocks in lab.

intrusive (phaneritic)granitedioritegabbro
extrusive (aphanitic)rhyoliteandesitebasalt