Temperature and gases certainly influence whether the lava becomes aa or pahoehoe. However, if it cools quickly and moves fast it can tear into clinkery pieces called a'a. If lava cools slowly and does not move too fast it forms smooth ropy lava called pahoehoe. During the early episodes of the current eruption of Kilauea volcano, aa flows up to 36 feet (11 m) thick surged through the Royal Gardens subdivision at rates as great as 108 ft/min (33 m/min). Aa lava flows tend to be relatively thick compared to pahoehoe flows. (Kilauea) A'aĪ'a is characterized by a rough, jagged, spinose, and generally clinkery surface. In map-view the flows tend to be narrow and elongate. Pahoehoe flows tend to be relatively thin, from a few inches to a few feet thick. Pahoehoe lava is characterized by a smooth, billowy, or ropy surface. Pahoehoe is the second most abundant type of lava flow. The pillow is about 3 feet (1 m) in diameter and has a glassy rim. Pillow lavas are also found near the summit of Mauna Kea These pillow lavas were produced by a subglacial eruption that occurred 10,000 years ago. ![]() The only way to absolutely know that the flows you're looking at are pillow lavas rather than pahoehoe toes, is to find submarine sediments (such as hyaloclastite debris formed from the violent reaction of lava and water) between the pillows. Nevertheless, it is often difficult to tell the two types of lava apart in exposures. Also in contrast to pahoehoe toes, pillow lavas tend to have thicker skins of glass (because they are quenched more quickly), less vesicular skins (because even shallow water pressure is able to prevent bubbles from expanding very much), and generally radial fractures (in contrast to the generally concentric flow banding seen in pahoehoe toes). They have a rounder form than pahoehoe toes, mainly because of the ability of water to help buoy them up (gravity doesn't flatten them out so much). They form from low effusion-rate eruptions of fluid basalt lava. Pillow lavas are essentially the underwater equivalent of pahoehoe. You might think "wait a minute, sediments are lain down really slowly, how are they going to get between the pillows while they're active?" Actually, when lava is flowing under water, there is a lot of sediment generated as pieces of the lava fall off during the rapid collapsing of the pillows as the quickly chill. The only absolute way to know that you are looking at true pillow lavas is to find water-lain sediments between the individual pillows. Many of the features that supposedly can be used to tell the difference between the two don't always work. However, you have to be very careful to make sure that you are not looking at regular old pahoehoe toes, which of course, indicate dry land. Pillow lavas are often considered important when trying to decipher old rock sequences because they indicate the presence of water. Pillow lavas have elongate, interconnected flow lobes that are elliptical or circular in cross-section. Image Credit: Gordon Tribble/USGSĮruptions under water or ice make pillow lava. Pillow lavas are volumetrically the most abundant type because they are erupted at mid-ocean ridges and because they make up the submarine portion of seamounts and large intraplate volcanoes, like the Hawaii-Emperor seamount chain. A mudflow containing volcanic material, called a lahar, may also form when the rock of the pyroclastic flow mixes with water to become a quickly moving slurry.There are three types of basalt lava flows: pillow, pahoehoe, and a'a. Floods may also occur when the flow of hot material melts snow and ice, swelling rivers and streams beyond their banks. Pyroclastic flows may result in flooding as streams are blocked or rerouted by the flow. ![]() Not only does it destroy living material in its path, it often leaves behind a deep layer of solidified lava and thick ash. Such a flow can transform the landscape drastically in a short period of time. ![]() Above this, a thick cloud of ash forms over the fast-moving flow. Along the ground, lava and pieces of rock flow downhill. Pyroclastic flows often occur in two parts. Pyroclastic flows can also form when a lava dome or lava flow becomes too steep and collapses. Another cause is when volcanic material expelled during an eruption immediately begins moving down the sides of the volcano. A common cause is when the column of lava, ash, and gases expelled from a volcano during an eruption loses its upward momentum and falls back to the ground. It may move at speeds as high as 200 m/s. A pyroclastic flow is extremely hot, burning anything in its path. It occurs as part of certain volcanic eruptions. ![]() A pyroclastic flow is a dense, fast-moving flow of solidified lava pieces, volcanic ash, and hot gases.
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