Geology Lesson - Igneous Rocks

Igneous focks are formed by the solidification of molten rock material.

Intrusive igneous rocks - When molten materials from Earth's interior cool without breaking through the Earth's crust, they are surrounded by insulating rocks and they cool very slowly. Slow cooling allows the crystallizing minerals to form in chemical and crystalline purity. The slow accretion of chemicals to the crystal lattice allows large crystals to grow.

Extrusive igneous rocks - When magma is forced upwards and leaks or is blasted out onto the surface, although chemically the same as the intrusive type above, these volcanic rocks are markedly different in appearance. The molten mass is quickly chilled by the cold rock surface over which it flows. The minerals freeze before they have an opportunity to grow into large crystals. The resulting rock is dense or even glassy, with few or no visible crystals.

Fortunately, there are certain similar characeristics between the intrusive and extrusive igneous rocks. Since they share the same fundamental chemisty, their classification systems can be shared. The rocks are named on the basis of their chemistry. Some have an abundance of light-weight silica. This creates a low density rock with only a few dark iron- or magnesium-bearing minerals. Other, silic-poor, aare dense and dark. They are rich in dark ferro-magnewium minerals. The names reflect this simple difference in chemical components. But all is not black and white in the real world of rocks. There are many shades of gray, and we have names forthe gray rocks as well.

So, here comes the good stuff.

The so-called crust of the Earth is about 20 mi. thick under the continents but averages only some 4 mi. beneath the oceans. It is formed mainly of rocks of relatively low density. Beneath the crust there is a layer of denser rocks called the mantle which extends down to a depth of nearly 2000 mi. Much of the molten rocks material which goes to make up the igneous rocks is generated within the upper parts of the mantle. This material, which is called magma, migrates upward into the Earth's crust and forms rock masses which are known as igneous intrusions. If magma reaches the Earth's surface and flows out over it, it is called lava.

Within some lavas, fragment of dense, green-colored rocks are sometimes found which consist principally of olivine and pyroxene. These fragments (xenoliths) are thought to represent pieces of the mantle, carried upward by the migrating magma.

The great majority of lavas consist of the black, rather dense rock called basalt, and most petrologist consider that the primary molten rock material which comes from the mantle has a composition which is near to that of basalt. Although basalt is the most abundant of the lavas, granite is by far the commonest of the intrusive igneous rocks. Granite is mineralogically and chemically different from basalt and for many years geologist have wrestled with the problem of how the two rock types are related. If basalt is assumed to derive from the mantle, is it likely that granite, whcih is of a quite diifferent compositon, could also come from the mantle?