Both intrusive and extrusive rocks are igneous rocks that form underground by crystallizing from molten rock. One may better understand this molten rock as magma when it sits beneath the surface in its magma chamber or lava, when it has erupted and has been exposed to the surface
Intrusive Igneous Rock: rocks that form
in Magma Chambers that is deep underground.
Extrusive Igneous Rock: otherwise known as volcanic, form when Magma makes its way to the surface. The molten rock erupts or flows above the surface as lava, and then cools forming rock.
Magma Chamber: otherwise known as a magma reservoir, is a large, underground pool of molten rock lying under the surface of the Earth’s crust.
Magma: molten rock below the surface.
Lava: magma that has been exposed to the surface of the earth.
More specifically, however, intrusive igneous rocks (plutonic rocks) form deep underground. The magma cools very slowly. This cooling can take from thousands to millions of years. It is because of this intrusive igneo
us rocks have many different minerals combined. You generally do not need a magnifying glass to see them, although there could be more microscopic minerals hidden amongst the large masses. Granite, with its small, although still visible minerals formed together, is an ex
cellent example of an intrusive igneous rock. As a matter of fact, granite is the most common type of igneous rock you will find. Upon observing granitic rocks, you will see that different granites are made of different minerals. Typically, however, granites have mica minerals, which can add a sparkle or shimmer to the granite. This is because light can easily reflect off of their flat, smooth, surfaces. They also have silicate minerals. Two excellent examples of silicate minerals are quartz and feldspar.
Mica Minerals: often found in igneous rocks such as granite, mica minerals tend to have flat surfaces that allow light to reflect off of them. This can add sparkle or shimmer to the rocks that
contain it.
Silicate Minerals: make up the largest and most important class of rock-forming minerals. They are classified based on the structure of their silicate group. Silicate minerals all contain silicon and
oxygen.
Quartz: is undoubtedly the single most common mineral in the Earth's crust, ranging from perhaps 12% of continental crust to as much as 50% of oceanic crust, as indicated by the composition of spreading-ridge volcanic lavas. Some estimates place quartz at 21% of the Earth's total lithosphere.
Feldspar: (a group of related minerals) comprises the bulk of the Earth's crust, approximately 60% of the continental crust or 49% of the lithosphere. Perhaps 75% of this is the plagioclase feldspars (mostly albite, oligoclase and labradorite) with the remainder as potassium feldspars (mostly microcline and orthoclase). Basalt is the most common extrusive igneous rock. Basalt forms most of the Earth’s surface as well as most of the ocean floors. Basalt is composed of pyroxene and olivine minerals and doesn’t have nearly as much feldspar or quartz. As a matter of fact Basalt contains less than 52% of quartz or feldspar.
Pyroxene: The pyroxenes are an important group among the single chained inosilicates. They are common rock forming minerals and are represented in most igneous and many metamorphic rocks. Their presence in a rock indicate a high temperature of crystallization with a lack of water.
Olivine: the most common mineral in the Earth, nearly 80% by volume, and that is a lot of peridot. It is the dense interior rock that the crust floats upon. Olivine is a major component of hot-spot volcanic lavas. About 15% of the crust is composed of olivines or their weathering/decomposition products.
Although intrusive and extrusive rocks are both composed of molten rock, intrusive rock is composed of magma while extrusive rock is composed of lava. This is the core difference between the two. Magma is molten rock that is underground; thus giving us the framework for our definition of intrusive rock. Intrusive rocks, while they may take thousands to millions of years from underground. Their texture, which is still small, but large enough to be seen by the naked eye, will also give us a clue. On the other than, extrusive rocks only take a few days to solidify as they form from the solidification of lava; hence the framework for our definition. Lava is exposed magma that solidifies in a matter of days. This is why extrusive rocks have a fine grained texture and it minerals usually cannot be seen by the naked eye. Basalt is the most common type of extrusive rock and granite is the most common type of intrusive rock.
http://mineral.galleries.com/minerals/by-name.htm
