The coarse graining of intrusive igneous rocks is usually due to the presence of pyroxenes and plagioclase. Pyroxenes are small, unidirectional, and orange-green in color, while plagioclase is a blue-gray mineral that is often found in association with other minerals.
With the slower cooling, the minerals that are within intrusive igneous rock will have more time to get larger in size. Minerals are generally formed by crystallization through heat and precipitation which can lead to some very large crystals. The common characteristic of an intrusive igneous rock is that they are a coarser grained rock.
Due to this characteristic, some fine grained extrusive igneous rocks have a very glossy like texture and can be visually identified by a smooth and sometimes glassy surface. Obsidian is an extrusive igneous rock, not an intrusive igneous rock. Obsidian is a great example of a fine grained extrusive igneous rock.
Why are intrusive rocks coarse grained? Not all of them are coarse grained, however, in general when magma is intruded into other rocks rather then being extruded onto the earth's surface and cooling in air or water, it cools down more slowly. This slow cooling allows time for crystals to grow and this makes the rocks coarse grained.
The rapid cooling of the extrusive igneous rock happens on the surface or very near the surface of the earth. The lava that erupts from the volcano will cool and solidify quickly in the contrasting temperature at the surface of our planet. Due to this quick cooling, the minerals in the rock do not have much time to form very large crystals.
Extrusive Igneous Rocks: Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.
Intrusive igneous rocks form when magma cools and solidifies within Earth. Extrusive igneous rocks form when lava cools and hardens at the surface. How to coarse-grained igneous rocks form? Coarse-grained igneous rocks form when magma cools slowly within Earth.
If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving the intrusive igneous rock a coarse-grained or phaneritic texture.
Extrusive igneous rocks are typically finer grained than intrusive igneous rocks. Why? The extrusive magma cools quickly so the mineral grains do not have time to grow.
Igneous rocks may be simply classified according to their chemical/mineral composition as felsic, intermediate, mafic, and ultramafic, and by texture or grain size: intrusive rocks are course grained (all crystals are visible to the naked eye) while extrusive rocks may be fine-grained (microscopic crystals) or glass ( ...
The two main categories of igneous rocks are extrusive and intrusive. Extrusive rocks are formed on the surface of the Earth from lava, which is magma that has emerged from underground. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.
Coarse grain varieties (with mineral grains large enough to see without a magnifying glass) are called phaneritic. Granite and gabbro are examples of phaneritic igneous rocks. Fine grained rocks, where the individual grains are too small to see, are called aphanitic.
Because extrusive rocks cool quickly, they only have time to form very small crystals such as basalt or none at all. On the other hand, intrusive rocks grow larger crystals because they take longer to cool. Extrusive rocks are usually fine-grained or glassy while intrusive rocks are coarse-grained.
Intrusive igneous rocks cool from magma slowly because they are buried beneath the surface, so they have large crystals. Extrusive igneous rocks cool from lava rapidly because they form at the surface, so they have small crystals. Texture reflects how an igneous rock formed.
(a) Said of a crystalline rock, and of its texture, in which the individual minerals are relatively large; specif. said of an igneous rock whose particles have an average diameter greater than 5 mm (0.2 in.).
Intrusive igneous rock is formed when magma cools and solidifies within small pockets contained within the planet's crust. As this rock is surrounded by pre-existing rock, the magma cools slowly, which results in it being coarse grained – i.e. mineral grains are big enough to be identifiable with the naked eye.
The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture.