Sobai
Nov 16, 2014 12:56:19 GMT
Post by sobai2004 on Nov 16, 2014 12:56:19 GMT
Pillow lava are often considered important when trying to decipher old rock sequences because they indicate the presence of water. 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. Many of the features that supposedly can be used to tell the difference between the two don't always work. The only absolute way to know that you are looking at true pillow lavas is to find water-lain sediments between the individual pillows. 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. Pillow lavas are essentially the underwater equivalent of pahoehoe. They form from low effusion-rate eruptions of fluid basalt lava. 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). 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). Nevertheless, it is often difficult to tell the two types of lava apart in exposures. 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.
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 pillow is about 3 feet (1 m) in diameter and has a glassy rim. Figure 21.11 from Porter, 1987.
When lava cools it also forms a myriad of different shapes and types of lava. There are two main types of lava pahoehoe (pa-hoy-hoy) and a'a (ah ah). Pahoehoe lava comes out smooth and dense and can form large areas that resemble flat parking lots or smooth bumps. A'a, on the other hand, forms individual rocks on the surface anywhere from a few inches to many feet in size. The rocks are porous and very jagged. Below the surface a'a is extremely dense. In general, pahoehoe is very easy to walk on and a'a is very difficult, if not nearly impossible, to walk on (at least without getting hurt).Doc1.docx (98.72 KB)
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 pillow is about 3 feet (1 m) in diameter and has a glassy rim. Figure 21.11 from Porter, 1987.
When lava cools it also forms a myriad of different shapes and types of lava. There are two main types of lava pahoehoe (pa-hoy-hoy) and a'a (ah ah). Pahoehoe lava comes out smooth and dense and can form large areas that resemble flat parking lots or smooth bumps. A'a, on the other hand, forms individual rocks on the surface anywhere from a few inches to many feet in size. The rocks are porous and very jagged. Below the surface a'a is extremely dense. In general, pahoehoe is very easy to walk on and a'a is very difficult, if not nearly impossible, to walk on (at least without getting hurt).Doc1.docx (98.72 KB)