Volcano exhibiting possible pre-eruption activity.
To understand why diverse rocks occur near erupted volcanoes, you need to understand something of various geological processes, the basics of classifying rocks generally known as volcanic rock and the fact that volcanic eruptions involve interactions between the surrounding landscape including existing rock with both lava and ash. While certain types of rocks most commonly occur near erupted volcanoes, because the lava can carry existing rock with it you can find almost any type of rock near an erupted volcano.
While people tend to think of volcanic eruptions as dramatically explosive events, scientists recognize two types of volcanic eruptions–explosive and effusive. Hawaiian active volcanoes have nonexplosive eruptions involving lava fountains, spatter cones, and gentle lava flows. Other volcanoes have more explosive interruptions. In both types, the lava flow itself will eventually cool into volcanic rock, and the lava flow and ash will interact with existing rock on the Earth’s surface in the affected area.
Igneous rock forms when superheated volcanic liquid, also called magma, cools and thus hardens through a process of crystallization in one of two manners: by cooling and crystallization on the Earth’s surface such as after a volcanic eruption; or by cooling more gradually through a process of rising as it cools through the magma while still below the Earth’s surface. Scientists further classify such rocks based on their mineral characteristics: pumice has a light weight and a light color and is made up mostly of holes known as vesicles left behind by the past presence of gases; vesicular rock also has many holes previously occupied by gas and has a Swiss cheese appearance; and obsidian is a darker glassy-looking volcanic rock.
Igneous Rock Subcategories
Another way of classifying igneous rock based on composition involves four categories of composition: silica content; iron and magnesium content; color; and density, or viscosity. Scientists classify such volcanic rocks as: basalt, which contains low silica but high iron and magnesium content, has a black color and low viscosity; andesite, which has a moderate silica content as well as a moderate iron and magnesium content along with a color ranging from gray to green and a moderate viscosity; dacite, which incorporates a high silica but low iron and magnesium content also with color ranging from gray to green as with andesite but accompanied by a high viscosity; and rhyolite, which has a very high silica but low iron and magnesium content accompanied by a light color and high viscosity.
A metamorphic rock forms through geological forces applied to the rock that have caused it to turn from one type to another, leaving evidence behind of the process. While most commonly this involves conversion through pressure from a sedimentary to a metamorphic rock, this can also involve a process whereby the flow of superheated lava over the area surrounding an erupted volcano changes the form of some of the existing rock from both sedimentary and igneous rocks into a metamorphic rock such as is the case with some forms of granite.