volcanic ash 3

volcanic ash 3

Volcanic ash is made up of tiny, dust-like fragments of jagged rock, minerals and volcanic glass. Ash particles are 2 millimeters (.08 inches) across or smaller. These particles are sometimes called tephra.

Coarse ash looks and feels like grains of sand, and very fine ash is powdery. Ash forms as lava is thrown into the air during an explosive volcanic eruption. Gases in the volcano's molten lava expand during the eruption and shatter the lava into the tiny ash particles.

Unlike the soft ash created by burning wood, volcanic ash is hard, abrasive, and does not dissolve in water. It can conduct electricity when it is wet. This ability allows ash to "recharge" itself as it drifts into storms.

During an eruption, the wind can carry small ash particles great distances. Ash has been found thousands of kilometers away from an eruption. The smaller the particle, the further the wind will carry it. Ash deposits tend to be thicker or deeper closer to the eruption, and thinner as distance from the volcano increases.

In addition to shooting volcanic ash into the atmosphere, an explosive eruption can create an avalanche of ash, gases and rock, called a pyroclastic flow. These incredibly fast avalanches of volcanic debris can be impossible for humans to outrun. They are capable of razing buildings and uprooting trees.

After a violent eruption, the ash in the air can be thick enough to block sunlight. If inhaled, ash can cause breathing problems or suffocation. It also can disable machinery. Severe eruptions that have sent ash into the atmosphere blocked some sunlight and lowered temperatures worldwide for years. This phenomenon is an example of global cooling.

One of the most famous explosive volcanic eruptions occurred in 79 CE, when Mount Vesuvius buried the Roman (now Italian) cities of Pompeii and Herculaneum.

Pompeii was buried under 18 meters (60 feet) of ash. The ash buried the cities so completely that it preserved entire buildings, paintings, and artifacts. It also created very detailed molds around the bodies of people who were killed.

Starting in the 18th century, archaeologists began excavating Pompeii. They discovered the hollow impressions left by bodies in the hardened ash and developed a way to inject them with plaster to create casts of the bodies. Today the excavated city and its gruesome models of dead and dying people and animals are popular tourist attractions.