Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock (called country rock); the magma cools slowly and, as a result, these rocks are coarse grained. The mineral grains in such rocks can generally be identified with the naked eye.
https://en.wikipedia.org/wiki/Igneous_rock
Some extrusive igneous rocks cool so quickly that they have a glassy texture. Common extrusive igneous rocks are: andesite, basalt, dacite, pumice, rhyolite, and obsidian. The crystals are so fine in obsidian, that paleo-people and modern surgeons alike use/used the rock to manufacture razor-sharp tools.
http://www2.pvc.maricopa.edu/~douglass/v_trips/wxing/introduction_files/rocktypes.html
Similarly, liquid magma also turns into a solid &#8212 a rock &#8212 when it is cooled. Any rock that forms from the cooling of magma is an igneous rock. Magma that cools quickly forms one kind of igneous rock, and magma that cools slowly forms another kind. When magma rises from deep within the earth and explodes out of a volcano, it is called lava, and it cools quickly on the surface. Rock formed in this way is called extrusive igneous rock.
http://www.learner.org/interactives/rockcycle/change2.html
The same thing happens to a rock when it is heated enough. Of course, it takes a lot of heat to melt a rock. The high temperatures required are generally found only deep within the earth. The rock is pulled down by movements in the earth's crust and gets hotter and hotter as it goes deeper.
http://www.learner.org/interactives/rockcycle/change2.html
Extrusive or volcanic rocks crystallize from lava at the earth's surface. The texture of an igneous rock (fine-grained vs coarse-grained) is dependent on the rate of cooling of the melt: slow cooling allows large crystals to form, fast cooling yields small crystals.
http://www.columbia.edu/%7Evjd1/igneous.htm
Sedimentary and metamorphic rocks, which we will study later, may be thought of as derivative rocks. Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into two categories. Intrusive or plutonic rocks crystallize from magma beneath the earth's surface.
http://www.columbia.edu/%7Evjd1/igneous.htm
In this Module, you will encounter a LOT of sandstone. Sandstone is a sedimentary rock that is highly resistant to weathering. Other sedimentary rocks you will encounter are mudstone and siltstone. These are soft and easily weathered (lots in the Painted Desert).
http://www2.pvc.maricopa.edu/~douglass/v_trips/wxing/introduction_files/rocktypes.html
In addition to texture, igneous rocks may are classified according to their chemical composition. The most general classification is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic (magnesium and ferrum or iron) minerals.
http://www.columbia.edu/%7Evjd1/igneous.htm
Extrusive igneous rocks, also known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. They are formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate.
https://en.wikipedia.org/wiki/Igneous_rock
Bowen's Reaction Series can be thought of as an idealized order of crystallization of a cooling magma. However, not all of these minerals will be crystallized together in the same rock. A mafic magma will begin crystallizing olivine and continue with pyroxenes and calcium rich plagioclase feldspar.
http://www.columbia.edu/%7Evjd1/igneous.htm
which rock would cool quicker
Extrusive Igneous rocks would cool quicker. Extrusive igneous rocks would cool quicker.