What are the distinguishing characteristics between the two catagories of igneous rocks? I know two are texture and color. But are there any more?
Based upon what I found here:
http://geology.about.com/cs/basics_roxmin/a/aa011804a.htm ,
I would say that the other main distinctions between basaltic and granitic igneous rocks are are density and composition.
Yes, apart from texture and color, there are typically three additional characteristics that can be used to distinguish between the two categories of igneous rocks. These characteristics include mineral composition, crystal size, and the presence of vesicles.
1. Mineral Composition: Igneous rocks are commonly classified based on the type of minerals present in them. The two main categories of igneous rocks are intrusive (plutonic) and extrusive (volcanic) rocks. Intrusive rocks form from magma that cools slowly beneath the Earth's surface, allowing for large mineral crystals to form. These rocks, such as granite, are typically composed of minerals like quartz, feldspar, and amphibole. On the other hand, extrusive rocks are formed when magma erupts onto the Earth's surface and quickly cools, resulting in smaller mineral crystals. Examples of extrusive rocks include basalt and rhyolite.
2. Crystal Size: The rate at which igneous rocks cool affects the size of the mineral crystals that form within them. Intrusive rocks cool slowly, allowing for the growth of large mineral crystals visible to the naked eye. These rocks are said to have a coarse-grained texture. In contrast, extrusive rocks cool rapidly, resulting in the formation of small mineral crystals that are often too tiny to be seen without a microscope. These rocks have a fine-grained or even glassy texture.
3. Presence of Vesicles: Vesicles are small cavities or gas bubbles that form within volcanic rocks as a result of trapped gases trying to escape during rapid cooling and solidification. These bubbles can be preserved as voids in the solidified rock. Vesicular texture is commonly observed in some types of extrusive igneous rocks, particularly those rich in gas-forming minerals like pumice.
So, in summary, the distinguishing characteristics between the two categories of igneous rocks include texture (coarse-grained vs. fine-grained or glassy), color, mineral composition (types of minerals present), crystal size, and the presence of vesicles. By examining these characteristics, geologists can differentiate between intrusive and extrusive igneous rocks.