What is an example of a catastrophically formed rock layer?

Answer 1

Mt. Vesuvius is a volcano that erupted on August 24 A.D. 79 blanketing the towns and 1000s of residents of Pompeii, Stabiae, and Herculaneum. Pompeii was buried 10' deep, while Herculaneum was buried under 75' of ash. This volcanic eruption is the first to be described in detail. The letter-writing Pliny the Younger was stationed about 18 miles away in Misenum from which vantage point he could see the eruption and feel the preceding earthquakes. His uncle, the naturalist Pliny the Elder, was in charge of warships in the area, but he turned his fleet to rescuing residents and died. Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than 2 millimetres (0.079 in) in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact with water causing phreatomagmatic eruptions and ejection of entrained particles during steam eruptions causing phreatic eruptions. The violent nature of volcanic eruptions involving steam results in the magma and solid rock surrounding the vent being torn into particles of clay to sand size. Volcanic ash can lead to breathing problems, malfunctions in machinery, and from more severe eruptions, years of global cooling. Ash deposited on the ground after an eruption is known as ashfall deposit. Significant accumulations of ashfall can lead to the immediate destruction of most of the local ecosystem, as well the collapse of roofs on man-made structures. Over time, ashfall can lead to the creation of fertile soils. Ashfall can also become cemented together to form a solid rock called tuff. Over geologic time the ejection of large quantities of ash can produce an ash cone.