The type of magma, how long it takes to cool, and the gas present during a volcanic eruption impact what an igneous rock looks like after cooling and solidifying. Solidification of molten rock is called crystallization. Igneous rocks that cool underground cool more slowly, giving mineral crystals time to grow within the rock.
In these rocks, the different minerals are large and easy to see with the naked eye. A volcanic rock made entirely of glass, called obsidian, forms from lava that cools with little to no crystals at all. If lava is flung into the air during an eruption, or if it comes in contact with water, the edges of the lava will also cool to form a glass.
When volcanoes violently erupt, many gas bubbles form in the lava, so it crystallizes into rocks filled with holes where the gas used to be.
Rocks with holes formed from gas trapped in the lava are called vesicular rocks. Gases can sometimes be trapped in the rocks, allowing volcanologists and geologists studying old vesicular rocks to determine which gases were present in ancient volcanoes [ 6 ]. Lava that contains very little gas tends to have no gas bubbles.
Volcanologists have names for all the various kinds of rocks that are formed from volcanic eruptions. These rocks are first classified into categories, including felsic, mafic, and intermediate, based on the types of minerals they contain Figure 3. Basalt is the most common type of igneous rock made from lava. Basalt is a dark-colored rock in the mafic category, with very little crystal formation. In contrast, andesite, an intermediate rock, is much lighter in color and has many crystals visible without a magnifying glass.
Granite is a type of non-volcanic igneous rock, formed from magma cooling slowly underground. Granite is often used for kitchen or bathroom countertops and can be found in a variety of colors, all sharing similar crystal size and mineral compositions. Rocks without obvious crystals also vary a lot in color and texture. Pumice is an example of a light-colored, felsic, vesicular rock that can float in water. Scoria is also vesicular but is dark, mafic, and denser, unable to float in water.
Obsidian, composed of volcanic glass, is a felsic rock that cooled with no crystals. Although obsidian appears to be dark, like basalt, it is felsic in composition.
Its dark appearance is due to impurities like iron or magnesium present within the rock. Many factors play a part in how rocks are created from volcanic eruptions, and what types and shapes of rocks volcanoes can produce. Volcanologists can look at igneous rocks that formed from a volcano to understand how it erupted, to study its geological history, and to predict future eruptions. Rock samples from multiple locations around a volcano can be dated to help construct an eruption history for the volcano.
Using satellite imaging, geologic maps can be created to show how each of the volcanic materials are distributed. Using this information, as well as many other pieces of evidence, volcanologists can create a timeline of events for a volcanic region [ 7 ]. Not every site will have only one kind of igneous rock, and some sites may have had multiple volcanic eruptions, which makes this process extremely complex.
For example, did volcanic activity cause any of the known mass extinctions? Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding unmelted rocks, and dissolved gases, but it is primarily a liquid composed principally of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Magmas also contain many other chemical elements in trace quantities.
Upon cooling, the liquid magma may precipitate crystals of various minerals until solidification is complete to form an igneous or magmatic rock. T he diagram below shows that heat concentrated in the Earth's upper mantle raises temperatures sufficiently to melt the rock locally by fusing the materials with the lowest melting temperatures, resulting in small, isolated blobs of magma. These blobs then collect, rise through conduits and fractures, and some ultimately may re-collect in larger pockets or reservoirs "holding tanks" a few miles beneath the Earth's surface.
Mounting pressure within the reservoir may drive the magma further upward through structurally weak zones to erupt as lava at the surface. In a continental environment, magmas are generated in the Earth's crust as well as at varying depths in the upper mantle. The variety of molten rocks in the crust, plus the possibility of mixing with molten materials from the underlying mantle, leads to the production of magmas with widely different chemical compositions.
An idealized diagram of a volcano in an oceanic environment left and in a continental environment right. I f magmas cool rapidly, as might be expected near or on the Earth's surface, they solidify to form igneous rocks that are finely crystalline or glassy with few crystals. Accordingly, lavas, which of course are very rapidly cooled, form volcanic rocks typically characterized by a small percentage of crystals or fragments set in a matrix of glass quenched or super-cooled magma or finer grained crystalline materials.
If magmas never breach the surface to erupt and remain deep underground, they cool much more slowly and thus allow ample time to sustain crystal precipitation and growth, resulting in the formation of coarser grained, nearly completely crystalline, igneous rocks. Subsequent to final crystallization and solidification, such rocks can be exhumed by erosion many thousands or millions of years later and be exposed as large bodies of so-called granitic rocks, as, for example, those spectacularly displayed in Yosemite National Park and other parts of the majestic Sierra Nevada mountains of California.
Deaths caused directly by lava flows are uncommon because most move slowly enough that people can move out the way easily. Death and injury can result when onlookers approach an advancing lava flowtoo closely or their retreat is cut off by other flows. Deaths attributed to lava flows are often due to related causes, such as explosions when lava interacts with water, the collapse of an active lava delta that forms where lava enters a body of water, asphyxiation due to accompanying toxic gases, pyroclastic flows from a collapsing dome, and lahars from meltwater.
Other natural phenomena such as hurricanes, tornadoes, tsunami, fires, and earthquakes often destroy buildings, agricultural crops, and homes, but the owner s can usually rebuild or repair structures and their businesses in the same location. Lava flows, however, can bury homes and agricultural land under tens of meters of hardened black rock; landmarks and property lines become obscured by a vast, new hummocky landscape.
People are rarely able to use land buried by lava flows or sell it for more than a small fraction of its previous worth. Skip to main content. Search Search. Volcano Hazards Program. Lava flows destroy everything in their path.
Lava flow ignites and burns a building near Kalapana, Hawaii.
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