Extrusive rocks do cool quickly because either the crystals in the rock are very small or there are no crystals at all.
Large Crystals = Intrusive Small Crystals = Extrusive The name relates to where the minerals were cooled (at at what rate). In the case of intrusive igneous, the rocks were formed above Earth's surface and were thus cooled quickly and the minerals had little time to become defined. Extrusive rocks, therefore, were formed within the Earth's mantle and had a much longer time before being gathered to cool (as they slowly rose to the top).
Sedimentary rocks are not classified by the terms intrusive or extrusive. These are terms used specifically for igneous rock classification, referring to rock that has formed from melt either below or on the surface.
Igneous rocks are formed from the cooling and solidifying of magma. The resulting rock can be instrusive (magma cooling within the crust) and extrusive (lava cooling on the surface). The most common kind of rocks are Granite (intrusive) and Basalt (extrusive).
Some igneous extrusive rocks include obsidian, basalt, andesite, rhyolite, scoria, pumice, basaltic glass. If you want more, go to page 6 on the Earth Science Reference Tables at the related link.
Volcanic rocks are extrusive igneous rocks.
because the crystal in the rock is very small
Extrusive igneous rocks, like basalt, pumice, scoria, obsidian, and rhyolite.
They cool to quickly for crystals to form.
Extrusive rocks which cool more rapidly than intrusive rocks generally have smaller crystals.
igneous Extrusive is cool on the outside of the crust and cools quickly before crystal forms igneous Intrusive is cool on the inside of the crust and cools slowly and allowing crystal to form
Extrusive igenous rocks. The main types are: Komatite/picrite (ultramafic) Basalt (mafic) Andesite (intermediate) Rhyolite (felsic/silicic)
Lava.
Extrusive igneous rocks are igneous rocks that form above earth's surface. They are usually fine-grained due to the rate in which they cool.
Fast and slow are relative terms. If a heated igneous rock takes 3 hours to cool, is that fast or slow? During their formation, however, magma can solidify very slowly or very quickly. Rocks that have cooled quickly from magma are referred to as extrusive igneous rocks, such as obsidian and scoria. Rocks that have formed from slow cooling magma are called intrusive igneous rocks, such as granite and gabbro. The difference between the two types is in their grain size. Slow cooling magma produces large grained rock, and fast cooling magma produces fine-grained rock.
You can describe the rock by the process that allowed it to cool quickly: "Extrusive" or you can describe it by its texture: "aphanitic". An extrusive rock tends to be aphanitic = Rocks formed from lava flows tend to have small crystals.
Extrusive igneous rocks are igneous rocks that form above earth's surface. They are usually fine-grained due to the rate in which they cool.
Intrusive or extrusive are terms given to igneous rocks which describe where the rock cooled and formed, since this plays an important role in the cooling time of the rock and therefore grain size of the rock. Intrusive rocks are those that cool from magma before it reaches the surface of the earth (they form inside the earth, hence the term intrusive). Extrusive rocks form by erupting or "extruding" at the surface and cooling. Intrusive rocks take a long time to cool because the surrounding rocks act as a kind of blanket and keep it warm longer, so crystals are generally large, where extrusive rocks cool quickly being exposed to air, wind, and rain, so crystals are generally small.