Geology And Geophysics

Chemistry of Volcanic Eruptions



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Contradictory to common beliefs, a volcanic eruption does not just produce lava. Actually, in a volcanic eruption, the volcano spews out different gases, lava, and ash. The ash consists of volcanic rocks such as pulverized rock and pumice, known as projectile materials. Before lava erupts out of the volcano, it is considered to be magma. Once the lava spews out, it quickly hardens to form igneous rock.

Magma is made up of the mantle of the Earth, water, high pressure, and high temperature. The mantle is the innermost layer of the earth which is molten and is at a very high temperature. Of course, there are many factors to the genetic makeup of magma and other volcanic materials. For example, in underwater volcanoes, the depth and distance from a trench, the thickness of the crust, and composition of oceanic sediments all have an impact of the genetic makeup of lava and igneous rock. 

Another element produced in a volcanic eruption is igneous rock. This forms from hardened lava, which in turn came from magma. Here is an image of the cycle of magma. As you can see, the magma (in this state actually lava) quickly hardens to rock because of the cooling of the earth. This “hardening” of lava is the effect of minerals in the magma crystallizing into igneous rock at different temperatures. The minerals in the magma which harden are usually made up of silicon, oxygen, aluminum, sodium, potassium, calcium, iron, and magnesium. So in actuality, very few elements actually make up this hardening mineral. The elements combine to form silicate minerals. Silicate minerals account for ninety percent of all igneous rocks. The part of the magma that does not harden is referred to as the residual melt. The genetic makeup of silicate minerals is as follows.

When a volcano erupts, seldom are smoke and fire emitted despite how it appears to human’s eyes. It is possible, however, for combinations of hydrogen, carbon, oxygen, and sulfur to ignite and flare, but it is uncommon. What accounts for the so called “fire” is actually a large amount of very fine dust mixed with steam. The erupting materials produce a glare off of the dust and steam mixture. Because of their high temperatures, the erupting materials produce a very convincing glare, giving the materials an appearance of being on fire.

Many gases are also produced in a volcanic eruption. The most common volcanic gases are water vapor, carbon dioxide, sulfur dioxide. Other kinds of gases that are usually emitted are hydrogen sulfide, hydrogen chloride, hydrogen fluoride. Rarer gases involved are hydrogen, carbon monoxide, and volatile metal chlorides. As you can see, many of these are greenhouse gases. Sulfur dioxide emissions can be measured to predict eruptions and to predict the depravity of eruptions. Also, the sulfur dioxide emissions can be present after eruptions to be measured to know when it is safe to return to an area. During an eruption, these gases, along with ash can be ejected into the air as high as 15-20 miles above the volcano’s surface. After this, the sulfur dioxide and hydrogen sulfide combine to form sulfate aerosols. The sun’s radiation is then reflected back into space which cools the earth below. Some eruptions can cause a decline in temperature of up to half a degree on the Fahrenheit scale for periods of one to three years. The aerosols also undergo complex chemical reactions with chlorine and nitrogen, producing chlorine monoxide. The chlorine monoxide works to destroy ozone. The gases produced can help to keep our Earth cool and clean.

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