Types of volcanic rock | Sciencelearn Hub Types of volcanic rock Rocks are not all the same. Some are heavy, some are light. Others are dark, while some can be almost pure white. Even igneous rocks that are all formed from magma in the Earth’s mantle can look very different. Igneous rocks Rocks are broadly classified into three groups – igneous, sedimentary and metamorphic . Igneous rocks are formed from magma in the Earth’s mantle. They generally don’t contain fossils, don’t react with acids, don’t usually contain obvious layers, can be made of different minerals, sometimes have holes or bubbles and may be glassy in appearance. Volcanologists look for these igneous rocks so that they can learn more about where these rocks have come from and whether they were formed during a volcanic eruption. Geologists use the visual appearance of the rock as an initial clue to its composition but will then verify their ideas using specialised techniques. For example, scientists at The University of Auckland use an electron microprobe to measure the exact quantities of silica , iron , magnesium and many other chemicals that are in rock samples they collect. This information helps them to classify the rock and may give them direct clues about the volcano and the eruption that formed the rock.

Lava solidifies to rock New Zealand has 3 main types of volcanoes, and each has been formed from a different type of magma. Once the lava has erupted, it cools and solidifies into rock: Basalt magma often forms shield volcanoes. Andesite magma often forms cone volcanoes. Rhyolite magma often forms calderas . Depending on how much gas the magma contains, it can also form cone volcanoes. Basalt Image: Columnar basalt The Earth’s crust is mainly basalt rock. It is a heavy, dark, grainy rock. Basalt is associated with great rock columns that are found in many places around the Earth, for example, the Organ Pipes in Dunedin or the Giant’s Causeway in Ireland. Basalt magma is formed at high temperatures (around 1,200ºC). When it comes out of the volcano, it is hot and liquid. It contains very little silica (less than 50%) and a lot of magnesium and iron, which makes the rock look dark.

The Auckland volcanic field has erupted this type of hot, runny iron-rich lava, and the landscape is dotted with mountains made from basalt and scoria (a red-coloured rock that contains large amounts of iron-rich minerals). Both rock types are excavated for building materials and landscaping. Andesite Image: Andesite Andesites are lighter coloured than basalt because they contain less iron and more silica (50–60%). Some scoria rocks fall within the andesite classification because of their chemical composition. Magma that contains andesite is generally around 800–1,000ºC and forms steep-sided cone volcanoes (stratovolcanoes). Mount Ngāuruhoe is an example of an andesite volcano. Rhyolite Image: Pumice Rhyolite is light-coloured or white – this is a clue that the rock contains a lot of silica (more than 70%) and not much iron or magnesium. Rhyolitic magmas are associated with low temperatures (750–850ºC) and are often thick, which means gases can’t escape.

Some rhyolitic rocks are quite light, for example, pumice , which may still have evidence of the bubbles of gas trapped as the rock solidified. Nature of Science Classification helps scientists organise things into groups. In rock classification, such grouping can help geologists see patterns and perhaps explain the reasons for rocks looking similar. Metadata Volcanic Minerals Volcanic Minerals Can your knowledge of volcanic rocks make you rich? Yes!! That is, if you combine it with knowledge of mineral deposits and global economic factors. Volcanoes directly or indirectly produce or host deposits of aluminum, diamonds, gold, nickel, lead, zinc, and copper. We use most of these materials everyday and, over the course of a lifetime, consume some of them (via the products we buy and use) in great amounts. This page provides a overview of the mineral deposits hosted in volcanic rocks. . Photo courtesy of Grant Boxer. Bauxite Aluminum ore, called bauxite, is most commonly formed in deeply weathered rocks. In some locations, deeply weather volcanic rocks, usually , form bauxite deposits. This sample of bauxite ore is from Western Australia. Uses for Aluminum Aluminum has a wide range of common uses.

It is lightweight, strong (especially with alloys), and conducts heat well. Many kitchen items (pots and pans, foil, dishes) are made of aluminum. Most materials used for transportation use large amounts of aluminum: cars, trucks, boats, aircraft, and aircraft engines. Road signs and high-voltage power lines are also made mostly of aluminum. World Supply of Aluminum About 110 million tonnes of aluminum was produced in 1994. Australia produces most of the world's aluminum. Diagram from ITAM Bauxite by the Minerals Council of Australia. Nickel Deposits of nickel sulfides are mined from greenstone belts in many ancient volcanic terranes. The ore is associated with . Komatiites have a unique texture called spinifex. The texture is an intergrowth of long, skeletal crystals of olivine and/or pyroxene. The texture is named for its resemblance to spinifex grass in Australia. Photo courtesy of Rob Hill, CSIRO. Simplified from Hill and others, 1989. Cross-section of igneous lithologies and associated massive nickel sulfides in a komatiite lava flow. Simplified from Hill and others, 1989.

Core in komatiite lava with adcumulate texture. Photo courtesy of Rob Hill, CSIRO. Uses for Nickel: Nickel is commonly alloyed with other metals to provide resistance to corrosion and heat, and to add strength and hardness . These alloys are used for industrial and consumer goods. Most of the nickel produced is used to make stainless steel. Stainless steel contains about 8% nickel. Monel metal, a highly corrosion-resistant alloy, contains 67% nickel and is usedin ship building, food-processing equipment and in hospitals. Nickel is used by the military for armor plate. Nickel is used in alkaline batteries, dyes, insecticides and as a catalyst. Nickel is commonly used in coins. World Supply of Nickel The Sudbury District in Canada is the world's largest producer of nickel. Another major nickel producer is in Western Australia and includes the Kambalda and MurrinMurrin deposits. MurrinMurrin consists of five deposits with a total of 11.9 million tonnes at 1.01% Ni and 0.07% Co.

In 1993, a large deposit of nickel-, copper-, and cobalt-sulfides (~150 million tons) was discovered at Voisey Bay on the northeast coast of Labrador, Canada. This deposit formed in magma chambers that fed large basaltic eruptions. In 1994, about 875,000 tonnes of nickel was produced worldwide. Major producers are the Commonwealth of Independent States (former Soviet Union), Canada, New Caledonia, Indonesia, and Australia. Diagram from ITAM Nickel by the Minerals Council of Australia. Gold of Vesuvius. Gold forms in close association with volcanoes or is hosted in volcanic rocks. Three environments/styles are most common: gold in greenstone belts, gold in porphyry deposits, and gold in epithermal deposits. Gold in Greenstone Belts Gold is found in Archean (rocks older than 2.5 billion years) greenstone belts in Australia, southern Africa, and Canada. Greenstone belts are volcanic-sedimentary sequences, which include shearing . Gold is most commonly found along the edges of greenstone belts and associated with structural features. Intensely altered and fractured basalt is a common host rock.

The gold is though to be mobilized by hydrothermal solutions during regional metamorphism. The solutions probably contain only a few parts-per billion gold but great volumes of solution can precipitate their gold in a small zone with favorable chemical conditions. The deposit itself is usually a quartz vein that carries the gold or adjacent altered rock. A classic example of gold hosted in greenstone is the Golden Mile in Kalgoorlie, Western Australia. The top diagram is a simple map that shows folds, faults, rock type, and mineralization. The bottom diagram is a cross-section of the deposit. Dolerite refers to rocks with plagioclase crystals in pyroxene crystals. By 1993, 40 million ounces of gold has been mined from the Golden Mile. Diagram from ITAM Gold by the Minerals Council of Australia. Another example of gold hosted in greenstone is the St. Ives deposit near Kambalda, Western Australia. Three mines have produced more than 2.1 million ounces of gold from 1980-1993. Another 5.4 million ounces of gold remains in the deposits. The gold is found in altered rocks in all parts of the sequence.

Diagram from ITAM Gold by the Minerals Council of Australia. Gold and copper are found in ore bodies associated with porphry. Porphry is a general term applied to igneous rocks of any composition that contain conspicuous phenocrysts (crystals) in a fine-grained groundmass. The term is from a Greek word for purple dye and was first applied to a purple-red rock with phenocrysts of alkali feldspar that was quarried in Egypt. Diagram from ITAM Copper by the Minerals Council of Australia. This type of deposit forms beneath stratovolcanoes and is associated with subduction zones . Erosion strips off overlying rocks to expose the mineralization. Gold and copper are found in sulfide minerals disseminated throughout the large volumes of intrusive rock (strictly speaking, this ore is associated with volcanic systems, usually not the volcanoes themselves). This requires large amounts of rock to be mined, often in open pits. The deposits are commonly 3-8 km across and copper may be less than 1% of the rock. Porphyry deposits are zoned in alteration (potassic ® sericitic ® argillic ® propylitic) and mineralization.

Epithermal refers to mineral deposits that form in association with hot waters. The deposits form within 1 km of the surface and water temperatures are about 50-200 degrees C. Shallow bodies of magma supply heat. The rising hot water carries dissolved gold and other elements. The water boils about 300 m below the surface and hydrogen sulfide gas escapes. This causes the gold to precipitate. The boiling zone is the target for mineral exploration. Veins commonly host the economic minerals. Diagram from ITAM Gold by the Minerals Council of Australia. Lihir Island in Papua New Guinea Lihir Island is an epithermal deposit discovered in 1982. The island is made of three volcanoes including Luise trachybasalt lava flows, breccia, and tuffs.The mineralized rocks are highly altered. Most of the ore is in breccia thought to have been a boiling zone for rising fluids. The deposit formed between 350,000 and 100,000 years ago. It is estimated that the deposit contains 21.3 million ounces proven and probably another 42 million ounces as a geological resource.

Most of the gold is fine particles in pyrite (FeS) grains. Hot springs and fumaroles are still active on the caldera floor. Uses for Gold Gold is rare, durable, chemically inert, and beautiful. These qualities make it useful for monetary exchange and investment, jewelry, and art.Gold’s high electrical conductivity, malleability, and ductility make it useful as an industrial metal. Gold is alloyed with silver, copper, nickel, palladium, zinc, and titanium to increase its tensile strength. Note: for gold, 1 carat = 1/24 part. Pure gold is 24 carat. World Supply of Gold Below is a pie diagram of major gold producers. In 1994, 2,296 tonnes of gold was produced. South Africa is the largest producer, followed by the USA, Australia, and the CIS (former Soviet Union). Diagram from ITAM Gold by the Minerals Council of Australia. Diamonds are crystalline carbon and the hardest known substance. This photo shows diamonds from the Argyle Mine in northern Western Australia. xenoliths are found in some kimberlite and provide clues to the magma's origin.

The source depth for kimberlite magmas is estimated at 200 km, more than twice as deep as the source region for most magmas. At a depth of 200 km the pressure is 60,000 times greater than the surface and the temperature is about 1500 C. Kimberlite magmas are rich in carbon dioxide and water which brings the magma quickly and violently to the surface. Most kimberlites occur as multiple intrusive events. Kimberlite was named for the rock associated with diamonds in Kimberley, South Africa. Right: Simplified from Hawthorne (1975). Kimberlite magmas form "pipes" as they erupt. A deposits. In the subsurface, a funnel-shaped body narrows to a depth of hundreds of meters. The pipe (also called a diatreme) is filled with kimberlite, with or without diamonds (only 1 in 5 of the pipes at Kimberley contain diamonds). Just how many diamonds are needed to make aa pipe economical? Some South African mines operate at 25 carats of diamond per 100 cubic meters of rock or about 2 grams of diamonds per 100 tons of rock.

Because diamond has a specific gravity of 3.5 grams per cubic centimeter, 1 cubic centimeter of diamond weighs 16 carats. Picture a giant 100-ton ore truck full of kimberlite - that truck contains only half of a cubic centimeter of diamonds! Only about 35% of those diamonds are gem quality. Macles diamonds from the Argyle pipe, Western Australia. Photo courtesy Grand Boxer. Uses for Diamonds Most diamonds are used in drill bits and diamond tools. A small number are used for glass cutters and surgical instruments. Only the finest are used as gems. World Supply of Diamonds Australia is currently the world's largest producer of diamonds. Most of these diamonds are low quality and used for industrial purposes. Most of the diamonds are from the Argyle diamond pipe in northern Western Australia. The pipe at Argyle is made of lamproite, not kimberlite. The mine produced 27.8 million carats (1 ct = 200 mg; 5 carats = 1 gram) of low grade diamonds in 1993-1994. Because of the high rate of production at Argyle, mining operations will end within the next few years.

The mines at Kimberley, South Africa have produced a total of more than 200 million carats since the 1870s. About half of South Africa’s diamonds are gem quality. Base Metals Zinc-Copper volcanogenic massive sulfide (VMS) deposits have been observed as they form at mid-ocean ridges. Chimneys formed at the ridges have as much as 29 weight % zinc and 6 weight % copper in sulfide minerals (pyrrhotite, pyrite, sphalerite, and chalcopyrite). The minerals are dissolved in fluids at temperatures as high as 380 C flowing at 1-5 m/sec. The minerals precipitate as the hot solution comes in contact with cold sea water. Photography courtesy of Woods Hole Oceanographic Institution and members of the Adventure dive (Principle Investigators: D. Fornari, R. Haymon, K. Von Damm, M. Perfit, M. Lilley, and R. Lutz). Lead, zinc, and cooper are found in VMS deposits. The deposits form in deep ocean water by the precipitation of sulfide minerals released by submarine volcanoes.

Volcanic fluids and hot seawater move through the volcanic rocks and leach metals. The deposits are associated with lava flows, breccia, water-deposited tuffs, cherts, sulfates, and limestones. VMS deposits are usually associated with quartz, anhydrite, gypsum, and barite. This photo shows pyrite in silicified tuff from a VMS deposit in eastern Java, Indonesia. The sample contains 0.55 ppm gold. There are three types of VMS deposits: ga, sp, py, some ch ch, py volcanic arc ) basins. Kuroko deposits are zoned from copper-rich near the center, to zinc-rich, to lead-rich at the outer edges of the deposit. The fluids that form Kuroko deposits have twice the salinity of average ocean water and temperatures of 250-300 C. This cross-section of a typical Kuroko deposit is from Sato (1974) and Franklin and others (1981).

Modern-day VMS deposits of the Kuroko-style have been observed as they form in the back-arc basin of the Okinawa Trough (Halbach and others, 1989). The deposit at Bathurst in Canada is an example of lead-zinc-copper VMS. Hydrothermal solutions associated with Keweenawan flood basalts have produced copper deposits in the Lake Superior area. Between 1845 and 1968, over 13 billion pounds of copper and 16 million ounces of silver were produced from the Keweenawan district. The porphry (not VMS) mine at El Abra in Chile will produce 500 million pounds of copper annually. Uses for Copper, Lead, and Zinc Copper was used as long ago as 8000 BC for tools, weapons, and ornaments. The discovery of bronze (copper and tin alloy) about 3500 BC marked the onset of the Bronze Age. Romans used lead for the plumbing and sewage systems. In Modern times, the electrical conductivity, ductility, and resistance to corrosion of copper, lead, and zinc, make them very useful in alloys.

Copper is used for electrical applications (50%), general and industrial engineering applications (20%), building and construction (15%), transportation (11%) and other applications. Lead is used in storage batteries, paints, dyes, explosives, insecticides, and rubber products. Zinc is used in galvanized steel, protective coatings for steel, and die casting. Zinc compounds are used for luminous dials, cosmetics, plastics, rubber products, soaps, and inks. World Supply of Copper In 1991, 9,167,000 tons of copper was produced. Chile (19.8%), USA (17.8 %), and the CIS (9.8%) are the main producers. USA, Japan, and the CIS are the main consumers. Obsidian By Jim Miller, B.Sc., M.Sc. Geology* Obsidian...even the name is exotic. Ever since I had my first rock collection as a child, I've loved obsidian. Sharp and shiny, obsidian is so different from other rocks. But until a few years ago when I made my first obsidian collecting trip to Glass Buttes, Oregon, I thought obsidian was pretty much just black glass.

That amazing trip really opened my eyes. The ancient volcanic hills called Glass Buttes hold a dazzling variety of gem-quality obsidian, including: mahogany, red, flame, midnight lace, jet black, pumpkin, brown, rainbow, gold sheen, silver sheen, green, lizard skin, snowflake and more. My goal in this article is to increase your awareness of some of the more fascinating aspects of this incredible stone. matrix . When I say "crystals," don't visualize those beautiful pointed prisms of quartz found in geodes. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms that comprise a mineral become arranged in regular, geometric patterns that are unique to the specific mineral. Crystal faces form only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free faces. Granite is composed entirely of intergrown crystals of quartz, feldspar, mica and other minerals. These relatively large mineral crystals (easily visible to the naked eye) give granite a rough surface. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic "conchoidal" fracture.

This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor. This had obvious advantages for our Stone Age ancestors, who used obsidian extensively for tool making. rhyolite , which also were originally molten. Because obsidian is not comprised of mineral crystals, technically obsidian is not a true "rock." It is really a congealed liquid with minor amounts of microscopic mineral crystals and rock impurities. Obsidian is relatively soft with a typical of 5 to 5.5 on the mineral hardness scale. In comparison, quartz (crystallized silicon dioxide) has a hardness of 7.0. Obsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the of obsidian.

Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon. If obsidian is similar in composition to granite and rhyolite, both of which were originally molten, then why is obsidian glassy? The answer relates to the original cooling rate and water content of the magma. Granite cools very slowly miles below the surface of the earth; this slow cooling over millions of years allows for the formation of sizable mineral crystals within the slowly cooling mass of molten rock. Rhyolite typically cools more rapidly near the earth's surface and contains smaller mineral crystals than granite. When rhyolite magma approaches the earth's surface and the pressure of burial decreases, most of the water in the magma is lost as steam. The resulting silica-rich magma with little remaining water becomes very viscous (thick and pasty) obsidian magma. This magma is so viscous that sizable mineral crystals cannot grow before chilling of the magma "freezes" crystal development. Some obsidian is erupted as lava flows at the ground surface. These surface flows are so viscous that they flow very slowly.

One article I read indicated that "an ant could probably outrun an obsidian , a glassy, bubble-rich, lightweight rock that develops when water vapor (steam) escapes rapidly from the molten glass at or near the ground surface. Sometimes obsidian of excellent quality develops as surface lava flows. However, the best quality obsidian often forms below the ground surface around volcanic vents. Silica-rich magma squeezes into rock fractures to form layers and lenses of obsidian that are relatively free of dirt, viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colors. Each of these streaks or lines may represent a distinct pulse of an obsidian eruption. You can visualize the process that results in streaked obsidian if you consider two blobs of green and red taffy (a viscous candy) that are mixed together. Distinct streaks of red and green taffy result as the blobs are mixed. In the case of obsidian, the slow flow of stiff, viscous magma away from the source vent provides the mixing needed to create the layered or streaked varieties of obsidian.

The "midnight lace" variety of obsidian often has incredibly contorted streaking, apparently formed as the obsidian layers were stretched and rolled with slow movement of the magma. The various colors of obsidian are a result of several factors. Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals. Red or brown obsidian generally results from tiny crystals or inclusions of hematite or limonite (iron oxide). Abundant, microscopic crystals of minerals like magnetite, hornblende, pyroxene, plagioclase and biotite, combined with tiny fragments of rock, likely produce the jet-black varieties of obsidian. Microscopic crystals of various types of feldspars may yield the unique blue, green, purple or bronze colors associated with rainbow obsidian. The reflectance of rainbow obsidian is likely attributed to a preferred orientation of microscopic crystals of feldspar or mica oriented along flow layers. A certain amount of water always is present in obsidian. Very small inclusions of water vapor in the form of bubbles often are trapped in the glass. Tiny gas bubbles that have been stretched nearly flat along the flow layers in obsidian generally cause the reflectance of gold sheen and silver sheen obsidian.

Some of these bubbles are visible to the naked eye. The bubbles can be seen readily with a strong magnifying glass or a microscope. Obsidian is relatively unstable from a geologic perspective. It is rare to find obsidian older than about 20 million years, which is very youthful in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock in a process known as "devitrification." In this process, the silica molecules within the glass slowly rearrange into organized crystal patterns. The "snowflakes" in snowflake obsidian are quartz crystals that have formed through devitrification of the original obsidian. The crystals that develop through devitrification cause obsidian to lose its conchoidal fracture and glassy texture. Native Americans discovered almost all of the obsidian locations in North America. Each obsidian source area has a unique assemblage of trace elements, allowing identification of the original source locality for the obsidian used in prehistoric artifacts. Trace element analyses have shown that native peoples traded this valuable commodity many hundreds of miles from the volcanic source areas along numerous trade routes. The fact that obsidian was transported great distances attests to the mystique of this unique material.

My favorite place to collect obsidian is Glass Buttes in central Oregon. A trip to Glass Buttes is well worth the effort for the natural scenery, abundant and beautiful gem-quality obsidian, and exploration of a fascinating geological area. These sagebrush- and juniper-covered hills are remote but accessible by automobile. Located in central Oregon south of Highway 20 and about 80 miles east of Bend, a visitor will be rewarded with many different varieties of obsidian through exploration of the area. Fist-sized pieces are abundant and can be collected with no digging. For large pieces of high quality obsidian (ranging up to several hundred pounds), considerable effort with a shovel and pry bar are necessary. Further information on Glass Buttes can be found in several rockhound guidebooks that include the central Oregon area. * Jim Miller is a practicing geologist and an avid flint knapper residing in Bothell, Washington. Examples of obsidian from Glass Buttes and stone tools made from those materials by the author are included with this article. Several varieties of obsidian from Glass Buttes, Oregon and tools made by the author from those materials.

From left to right across the top, the obsidian varieties are silver sheen, midnight lace, pumpkin and red mahogany. Source material and stone tools of red mahogany obsidian. Source material and stone tools of red mahogany obsidian. Pencil gives scale. Source material and stone tool of silver sheen obsidian. Source material and stone tools of pumpkin obsidian. Pencil gives scale. Multi-colored streaks oriented along flow lines in midnight lace obsidian. Contorted streaks within midnight lace obsidian. viscosity that has the greatest control on the ordered crystalline pattern, the measure of viscosity is dependent on the temperature, crystal content and chemical composition . Viscosity is a measure on the ability of substance to flow, high viscosity means poor ability to flow and low viscosity means good ability to flow, an example of rhyolite . For obsidian to form, magma is trapped below the eutectic, point of crystallization, by loss of heat. Therefore leaving a magma that is unable to crystallize will form (glass) obsidian. For this process to occur during a lava flow the lava is caught just below crystallization temperature, thus forming a glass due to the inability to form a crystalline solid.

The formation of obsidian could also be the melt, liquid remaining from a magma after crystallization, of a rhyolite magma that has been erupted before any crystals can form as stated earlier. The gas content of obsidian is very low so for this to occur the gas has to be released in some way before the dacite . This difference is due to the difference in composition, specifically volatile, gas, content. Volatiles within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily so as they rupture, burst, they release an enormous amount of pressure producing an eruption such as the Mount. St. Helens eruption on May 18th 1980. For these explosive types of eruptions not occur in the emplacement of obsidian the volatile content for the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977).

The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the may property controling the viscosity over any of the other properties. The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian.

As volatile as pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C is than released as the obsidian at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma. This information comes to the conclusion that the flow of obsidian is dependent on the time for the obsidian magma melt to cool, and that amount of time for cooling is very short because if allowed to cool to a temperature lower than initial temp it will not flow. Bibliography: Bakken, B., (1977), Obsidian and Its Formation. North West Geology.

6-2, (88-92) Obsidian and its Groovy Flow Emplacement A model for rhyolite composition approaches the surface volatiles exsolve, separate, from the magma and rise to the plumes top. The decrease in the confining pressure leads to frothing of the volatile rich top section of the plume as ascension of the stratified mass occurs. This process of vesiculation leads to some number of explosive eruptions depositing . The transition from the explosive eruption to the pumiceous lava occurs once gas pressure in vesicles can no longer exceed strength of magma. Later in the eruption following the pumiceous lava flow the event of the emplacement of obsidian occurs, this process happens with the bubble free obsidian flowing out over the earlier emplaced coarsely vesicular pumice (highly inflated pumiceous lava). The Obsidian is slightly less viscous but more dense than the coarse pumice. There remains a gradual transition from the coarse pumice to obisidian due to volatile decrease with the progression from pumice phase to obsidian phase. Essentially while obsidian is erupted vesicular material continues to be extruded.

As the obsidian rides over the coarse pumice, volatiles continue to evolve from the cooling upper flow surface of the obsidian flow. This process of the volatiles exsolving from the obsidian flow forms a finely vesicular layer that keeps the flow interior of the obsidian insulated and warm; this contact is also gradational, just as the coarse pumice to the obsidian was. Within the volcanic vent that the obsidian flowed from the remaining magma from within cools and crystallizes and then erupts as a crystal rich rhyolite flow capping the vent mouth as a rhyolite pyroclastic flow occurred essentially taking the place of the pumiceous lava flow, however there remain deposits of course pumice within the region of Big Obsidian Flow which many indicate a short event of pumiceous flow. The final stage was the emplacement of Big Obsidian Flow that moved very slowly and according to Sherrod et al (1997) this lava flow probably moved a few meters to tens of meters a day. From observations in the field of the Big Obsidian Flow the presence of a thin visculated layer coats some of the surface of the obsidian flow.

The final stage of the emplacement of a rhyolite dome is not present the vent is actually plugged by the obsidian flow indicating that the rhyolite stage was not present in the eruption or possibly the rhyolite magma was already to cool and harden to come to the surface of the flow. This is an image of the Big Obsidian Flow of Newberry Volcano,a USGS Photograph taken in October 1987 by Willie Scott. The structure of the deposit indicates that the obsidian flowed over being erupted explosively. Fink (1983) provided most of the information regarding the flow emplacement model of obsidian as well as the figures displayed. Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite . This difference is due to the difference in composition, specifically gas content. Gases within highly viscous magmas can produce eruptive events due to the inability for the volatiles to escape easily from the magma so as they rupture they release an enormous amount of pressure producing an eruption such as the Mount. St. Helens eruption on May 18th 1980.

For these explosive types of eruptions not occur in the emplacement of obsidian the gas content of the obsidian must be low. The average water content of obsidian is (0.3 wt %) where as crystalline rhyolite is <2.0 wt % water (Bakken Barbara., 1977). The low amount of water in comparison with rhyolite pumice indicates that the flow of obsidian must take place at the end stage of the explosive eruption phase of rhyolite magma (Bakken Barbara., 1977) end stage indicating after the vent of the volcano has released a large amount of gas through the explosive eruption stage. When it comes down to it the temperature ot the obsidian magma is the property controling the viscosity over any of the other properties, higher the temperature the lower the viscosity. The viscosity of obsidian must be lower than rhyolite so it can flow; difference in eruption temperature is the greatest control over this difference. Initially the magma erupts at a temperature around 900 degrees C however this first eruption is rich in volatiles producing pumice, so this stage still has a greater viscosity than obsidian.

As volatile rich pumice is released and the obsidian melt from the rhyolite is built up at a temperature around 900-700 degrees C the obsidian melt can be erupted at a low viscosity of a magnitude of 10^8 Pa s, which is an order of 3 magnitudes less than that of the original rhyolite pumice eruption, this therefore indicates that the eruption of the obsidian has a similar temperature of eruption as the initial pumice only difference is in the magma's gas content. After obsidian has been erupted what to follow is typically a rhyolite dome rich in crystals due to slow cooling of the magma; this event makes it clear that the reason for lack of crystals in the obsidian are due to being erupted at a temperature greater than the final rhyolite magma. This information comes to the conclusion that the flow of obsidian is dependent on the time the obsidian is erupted during the eruption, the temperature of the obsidian and the gas content. Bibliography: Fink, J.H., (1983), Structure and emplacement of a Rhyolite Obsidian Flow: Little Glass Mountain, Medicine Lake Highland, Northern California.

Geological Society of America Bulletin. 94 (362-380) Sherrod., Mastin., Scott., Schilling. (1997). Newberry , Oregon Big Obsidian Flow - Latest Eruptive Event Excerpt from: Sherrod, Mastin, Scott, and Schilling, 1997, Volcano Hazards at Newberry Volcano, Oregon. U.S. Geological Survey Open-File Report, 97(513) The Fly Style of Obsidian Flow Textures and Features Obsidian flows usually consist of fold surfaces, explosion craters, flow banding, and cavities underling the craters. The explosion craters are commonly located on the surface of the flow and can have diameters of 10-25m (36-91ft), at 5-15m (15-55ft) deep; the accompanying cavities can be 5-15m in length. The cavities form near the surface by a buckling mechanism.

Buckling occurs by contrast of stiff upper layer relative to the near liquid flow interior, which forces the upper layer to make an almost triangular shape at the surface, separating it from the lower layer. These cavities can collect /meteoric water*, and as vapor pressure builds up and exceeds the strength of the surface crust of cavity an explosion results producing the explosion caters. *Meteoric water: rain water/ground water, not from the magma . Image depics the event of cavities forming by buckling and exploding due to gas pressure exceeding the strength of the rock. This illustration comes from Castro's Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano . Flow banding in obsidian is a distinctive characteristic and the abundance of the flow banding. Even with the abundance of these flow bands within obsidian little is known about the origins of these features, but what is known is that these freatures derive from both crystallization and deformation processes. According to a paper by J.M. Castro the flow bands develope from the deformation of microlites (very small ) by shear strains during the flow of the obsidian. The picture below shows the flow banding as pointed out by the arrow.

Bibliography Castro, J., Cashman, K., Jaslin, N., Olmsted, B., (2002) Structural origin of large gas cavities in the Big Obsidian Flow, Newberry Volcano. Journal of Volcanology and Geothermal Research. 114 (3-4), (313-330) Castro, J. (2005). New insignts on the origin of flow bands in obsidian. Special Paper - Geological Society of America, 39655-65. doi:10.1130/2005.2396(05). Water Plus Obsidian Obsidian After an obsidian flow has been emplaced it is subject to the atmosphere that causes weathering to the obsidian. One specific type of weathering done on to obsidian is called hydration, which occurs by the water within the atmosphere being absorbed by the obsidian thus increasing the water content within the rock. The product of a hydrated obsidian is called devitrified obsidian, (de- to remove) (vitrify- glassy), so essentially the removal of the glassy property of the obsidian.

The products of devitrified obsidian can produce secondary fiberous mineral crystals that can form ball like shapes called spherulites that are embedded within the obsidian, these secondary minerlas can develope into what is called snowflake obsidian as well, which makes the obsidian look as if it is decorated with snowflackes due to the snowflake shapes the secondary minerals produce. The snowflake obsidian can be used as jewlery and many rockhounds search for it, Yellowstone Manicure and Pedicure Products | Cosmetics Info Manicure and Pedicure Products Nail Products Nail Products are products that are used to color the nails, to protect them against damage, to soften and condition cuticles, and to supplement the nails. They include nail polish and enamels and nail polish and enamel removers. Manicure and Pedicure Products are designed to cleanse, exfoliate and moisturize hands and feet. They may contain a large variety of ingredients. Many products have multiple functions and may fall into more than one category. The major categories are as follows: Soaks are designed to cleanse and soften skin nails in preparation for the rest of the manicure or pedicure service.

Soaks may be sold as liquid concentrates or as crystals, both of which are added to water to make the soak solution. In either case, some sort of surfactant (= soap) is usually present to clean the skin. Some products also contain minerals, honey, and a variety of herbal extracts and oils to condition the skin. As with any product skin is exposed to, if you experience any redness or other skin irritation, rinse the affected area thoroughly with warm running water and discontinue any future use of the product that caused the irritation. Exfoliating products are designed to remove dead skin cells for smoother feeling skin and are typically water based emulsions, with alpha-hydroxy acids (AHAs) and/or beta-hydroxy acids, and a low pH to break down the dead skin cells and soften calluses. Pedicure exfoliants tend to be a bit stronger as feet tend to have thicker, tougher calluses. Some pedicure exfoliants use a high pH (alkaline) rather than low pH (acidic) to achieve exfoliation; either will work to soften calluses. Sunscreen should always be used on areas of the body that are exposed to sun within a week of an exfoliation service.

Scrubs achieve the same exfoliation goal with mechanical abrasives such as sugar crystals, salt, pumice, polymer beads, or finely ground nutshells. Sugar and salt crystals are broken down by water and some alcohols, so these products are often oil-based, with added surfactants to enable the oil to wash off on contact with water. Due to the tougher skin on the feet, pedicure products are often formulated with coarser grit than comparable manicure products. As with exfoliating products, sunscreen should always be used on areas of the body that are exposed to sun within a week of an exfoliation service. Masks are designed to moisturize and soothe skin. They are typically composed of clays or other minerals (“mud”), infused with moisturizing ingredients, essential oils, menthol, and/or herbal extracts. Self heating masks are water-free with butylene glycol or other ingredients that cause warming on application. Moisturizing lotions and creams are designed to moisturize, condition and address a variety of cosmetic conditions of the skin.

They are commonly water-based emulsions whose major ingredients, besides water, are typically oils (vegetable or mineral), waxy alcohols (cetyl alcohol , stearyl alcohol, etc.), and esters (isopropyl palmitate, glyceryl stearate, etc.). Humectants such as propylene glycol or glycerin are often included, along with antioxidant vitamins or other product enhancing ingredients. Massage lotions are similar, but have enhanced “slip” properties due to a careful choice of silicones, oils, or esters to increase lubrication. Some specialized manicure lotions contain skin lightening or sun protection (SPF) ingredients. Common Ingredients: Pedicure Pictures: Safety, Toenail Polish Colors, Calluses, Cuticles, and More 2 / 15 Do Soften Calluses Carefully Calluses and corns are hardened skin that can crack and hurt if they get too thick. During a pedicure, don't let the salon technician use a razor on your feet. Razors raise the risk of infection. At home, soak your feet in warm water for at least five minutes.

Then use a foot file, pumice stone, or exfoliating scrub to remove calluses. Don't go barefoot, either -- it can cause calluses. Swipe to advance 3 / 15 Do Cultivate Beautiful Cuticles Cuticles keep out germs, so treat them well! Tell the nail technician not to push back or cut your cuticles during a pedicure. This lowers your risk for infection. In between visits, never tear your cuticles. You can soften ragged cuticles by rubbing in moisturizer. If you notice that your cuticles are red or irritated, see your doctor. Swipe to advance 4 / 15 Do Erase Memories of Pedicures Past Dark, dramatic shades such as red and purple can stain nails yellow if you leave them on for more than two weeks. Use an acetone nail polish remover (or a nonacetone remover followed by lemon juice) to remove stains. When you polish, use a clear base coat first. But if your discolored nails are also thick, brittle, or curled, you may have a fungal infection. Skip the polish and head over to your doctor's office instead.

Swipe to advance 5 / 15 Don't Shave Before a Pedicure No matter how embarrassed you are, don't shave your legs before a pedicure. Bacteria are more likely to get into your body through tiny nicks or cuts on freshly shaved legs. Certain kinds of bacteria live in tap water and can thrive in a dirty footbath. They can cause boils or other hard-to-treat infections, so wait until after your pedi to shave. Swipe to advance 6 / 15 Do Bring Your Own Tools You can take your own tools to a nail salon, even if you feel self-conscious. Germs can linger on salon tools that aren't cleaned, like emery boards. If you choose to use the salon's tools, they should be heat sterilized, soaked in clear antibacterial solution, or come prepackaged. Ask for a new tool if one falls on the floor during your appointment. You don't want to leave the salon with a skin or nail infection. Swipe to advance 7 / 15 Do Trim Toenails Right In between pedicures, cut your toenails straight across to trim them. This helps prevent ingrown toenails and keeps them strong.

Filing your nails slightly so they're rounded in the center is fine. Before you polish your nails, gently clean underneath them with a manicure stick. If you have diabetes, your health care provider will do regular foot exams and teach you how to care for your feet. Swipe to advance 8 / 15 Don't Get Hooked on Fish Pedicures Some salons let a certain kind of fish nibble the calluses off your feet rather than scrubbing them off. Some states have banned these "fish pedicures." Salon workers can't clean the tubs well in between customers while the fish are in them. And it's not possible to clean the fish, so they may spread infections from other customers. Swipe to advance 9 / 15 Do Protect Your Pedi in Winter In cold weather, bring plastic wrap to your pedicure to help avoid smudges. Before you leave the salon, ask the technician to put a layer of cuticle oil on your newly polished toes. Then wind the plastic wrap around them before you put tights, socks, or shoes back on. Pedicure socks have openings for toes and can be worn with flip-flops.

Or slip your feet into special pedicure boots that have a front flap that leaves your toes exposed. Swipe to advance 10 / 15 Do Go Natural Once in a While You might think bare toenails are ugly, but nails need to breathe and have moisture, and polish acts as a barrier. So skip the nail polish for a week or longer each month and enjoy the low-maintenance look. It's a good idea to remove your nail polish if you can't fix it as soon as it chips. Your toenails are vulnerable to damage in the spots where polish is missing. If by some miracle your polish lasts two weeks, go ahead and change it anyway. Swipe to advance 11 / 15 Consider a Pedi During Pregnancy When you're pregnant and feel bloated, and even your ankles are swollen, a pedicure may sound heavenly. Good news! It's OK to pamper your feet. Pedicures are safe during pregnancy. Your nails don't absorb the chemicals in nail polish. If the odors from nail products make you feel ill, try sitting near a window or an open door.

Swipe to advance 12 / 15 Do Keep Your Tools Squeaky Clean Whether you bring your own nail tools to the salon or use them at home, keep them clean. After each pedicure, give the tools a good scrub with soap and water. Then soak them in a disinfectant solution like rubbing alcohol for at least 10 minutes. Swipe to advance 13 / 15 Don't Knock the 'Man' Pedicure There's no reason a guy can't have neat toenails. Some salons and barbershops offer special guy-friendly pedicures, with or without the nail polish. But everyday salons commonly cater to both sexes. Swipe to advance 14 / 15 Do Pick the Right Polish Remover Nail polish removers with acetone work well if your polish is very dark. They're also a good choice if you have a gel or shellac finish, which is hard to remove. Acetone can be drying and harsh, so if your nails or skin are sensitive, go for a nonacetone remover. It's gentler, but it'll take more work to remove the polish.

Swipe to advance 15 / 15 Don't Use a Sketchy Nail Salon Always check out a new nail salon beforehand, even if you're desperate for a pedicure. Be sure employees wash their hands and clean their equipment between clients. Most salons have an odor from nail products, but it shouldn't be overpowering. A very strong smell may mean that the ventilation system doesn't work. And look for the nail technician's state cosmetology license. It should be current and on display.

Swipe to advance 15) Shelby Jean Gates/Workbook Stock SOURCES: ABC News: 'Mansome': It's Now a Man’s World Among Women's Products American Academy of Dermatology: "Below-the-belt dermatological conditions," "Nails." American Academy of Family Physicians: "Nail Fungal Infections." American Podiatric Medical Association: "Dos and Don'ts for a Fabulous Pedicure." American Society for Dermatologic Surgery: "How to Avoid a Nail Infection." CBSNews.com: "Getting A Safe Manicure." CDC: "Fish Pedicures and Fish Spas." Cleveland Clinic: "Calluses." Medline Plus: "Diabetes Foot Care." Nails Magazine: "17 Ways to Knock the Socks Off Your Winter Pedicure Clients [Literally]," "How to Choose a Quality Nail Salon," "Foot Huggies Pedicure Socks." The New York Times: "Beauty Spots." Parents.com: "Pregnancy Pampering: What's Safe, What's Not." U.S. Department of Health and Human Services, Office on Women's Health: "Body changes and discomforts." Vugia DJ, et al.

Emerging Infectious Diseases [serial on the Internet]. April 2005. WomansDay.com: "9 Things Your Nail Technician Wants to Tell You." How Is Pumice Formed? | Sciencing How Is Pumice Formed? Photo by MPH. Pumice, Volcanoes and Density Pumice is a unique rock, noted for its light weight and low density (dry pumice can float in water). It is commonly used in cement, concrete and breeze blocks and as an abrasive in polishes, pencil erasers, exfoliants and to produce stone-washed jeans. Pumice is also used to remove dry skin from the bottom of the foot during the pedicure process at some beauty salons. Pumice is a type of igneous rock full of bubble holes and can be formed when lava comes into contact with water, cooling rapidly. Formation Pumice is formed by lava coming into contact with water. This occurs most often with volcanoes near or under water. When the hot magma comes into contact with the water, rapid cooling and rapid de-pressurization creates bubbles by lowering the boiling point of lava.

The cooling of the rock below the rock's melting point means that when the rock changes almost immediately to a solid after coming into contact with the water, the bubbles are trapped inside. Because pumice is igneous, it is at times glass-like, and the bubbles are trapped between the thin translucent bubble walls of rock. Volcanic Gases and Density Depending on the amount of volcanic gases coming from the lava before it cools rapidly, either pumice or scoria can be created. Pumice is a lighter color, has a porosity near 90 percent and is less dense; scoria is denser with larger bubbles and thicker bubble walls and sinks rapidly unlike pumice, which initially floats. If there are large amounts of gas present, pumice is created; when there is less gas, associated with less viscous magma, scoria is formed. Pumice can be formed rapidly and, in the past, large pumice rafts have been created from underwater volcanic eruptions like during the volcanic activity near Tonga in 2006.

References NASA Earth Observatory: New Island and Pumice Raft in the Tongas About the Author Collin Fitzsimmons has been writing professionally since 2007, specializing in finance and the stock market. He serves as a financial analyst at AMF Bowling Centers, Inc. Fitzsimmons earned a bachelor's degree in economics from the University of Virginia. What Are the Different Types of Pedicures? (with pictures) What Are the Different Types of Pedicures? Last Modified Date: 25 December 2016 Copyright Protected: Top 10 amazing movie makeup transformations Besides a regular pedicure, there are several methods for grooming and pampering feet. Most of these can be done as a professional pedicure or a home pedicure , with the exception of some of the more unusual ones. Some treatments involve covering the feet with melted wax, while others include a massage with oils and hot stones. Individuals can also opt for a miniature pedicure, French pedicure , or margarita pedicure. Some pedicures are slightly unusual and involve soaking the feet in things like fish tanks, chocolate , or wine. A regular pedicure is one of the most common types.

This usually involves soaking the feet in warm, scented water for a period of time. Rough calluses are then sloughed away with a pumice stone, and the toenails are trimmed and filed. Moisturizer is applied, usually during a foot massage, and polish is then applied to the toenails. A miniature pedicure, or mini pedi, is basically the same as a regular pedicure, but it takes less time. The feet are soaked for a shorter period of time, for instance, and the calluses are not sloughed away. Usually, the nails are just trimmed, filed, and polished. These types of pedicures are good for people who are short on time. Ad Paraffin treatments are other popular types of pedicures and manicures. During a paraffin pedicure, the feet are covered with several layers of melted paraffin wax after a thick moisturizer is applied. The feet can either be dipped into the wax, or the wax can be brushed onto the feet with a brush. After the wax dries, it is peeled away from the feet.

A paraffin pedicure is known for being very moisturizing, and people with very dry feet will often benefit from them. Stone pedicures are usually more of a massage. This type of pedicure involves oil being massaged into the feet and lower legs with hot stones. The toenails may also be trimmed, filed, and polished as well. French pedicures are also very popular. These are usually very similar to regular pedicures, except for the colors of the toenail polishes. Typically, the toenails will either be left natural or covered with a light pink polish. White polish is then applied just to the tips of the toenails, and the nails are then covered with clear polish. More usual foot treatments also exist, including fish pedicures, which involve soaking the feet in a tank full of fish. The traditional type of fish used for these uses are garra rufa, which are native to the rivers of parts of the Middle East. These fish are also sometimes referred to as nibbler fish or doctor fish. Some spas have hundreds or thousands of these fish in tanks, and when people soak their feet in the tanks, the fish eat the dead dry skin .

Many experts claim that these fish will do wonders for people suffering from psoriasis and eczema. Margarita pedicures are also becoming increasingly popular. They usually begin with the feet being scrubbed with a coarse salt scrub . Limes are then added to the water that the feet are soaked in, and lime moisturizers are also used. Chocolate and wine pedicures are also considered somewhat usual. These involve soaking the feet in chocolate or wine, respectively. These, however, are usually only offered in high-end salons and spas. Ad Icelandic volcanic rock - Spa Brands Spa Products Icelandic volcanic rock Heritage Icelandic volcanic rock These sand particles can actually float, there are a great number of small gas bubbles from chunks of cooled lava can float. Pumice is the name of the most common volcanic rock that floats. We use various % grains for various procedures. Particularly the naturally occurring species Zeolite, heulandite, clinoptilolite and chabazite. These applications include de toxicants and de contaminants, they have been shown to remove heavy metals, toxins and other compounds from the body, balance the body’s pH levels; and support a healthy immune system.

Pumice results from volcanic activity and is used in Natural Skin Care Products as an exfoliant. Pumice powder is obtained by finely milling Pumice stone to produce a mildly abrasive powder. Pumice stone results from volcanic activity which forms an aerated, almost frothy substance which ‘floats’ on certain types of lava. “Pumice stones” are often used in beauty salons during the pedicure process to remove dry and excess skin from the bottom of the foot Finely ground pumice powder is added to some beauty products, and are ground very fine that it does not harm the skin, it actually helps and assists the skin. Pumice has many cosmetic applications and is probably best known in its rock form where it is used to smooth rough skin on the feet, knees and elbows. The volcanic sand and pumice powder used in Purete’s body polish and body scrub is very fine and will not injure the skin. volcanoes A Volcano is an opening in the earth surface through which lava, hot gases, and rock fragments erupt burst forth. The name comes from Vulcan, Roman god of the forge. Such an opening forms when melted rock from deep within the earth blast through the surface.

Most volcanoes are mountains, particularly cone shaped, which are built around the opening by lava and other materials thrown out during eruptions. The beginning of a volcano. A volcano begins as magma, melted rock inside the earth, Magma results from the extreme heat of the earth's interior. The eruption of a volcano. The gas filled magma in the reservoir is under great pressure from the weight of the solid rock around it. This pressure causes the magma to blast or melt a conduit (channel) in a fracture, or weakened part of the rock. The magma moves up through the conduit to the surface. When the magma nears the surface, the gas in the magma is released. The gas and magma blast out an opening called the central vent Most magma and other volcanic materials then erupt through this vent The materials gradually pile up around the vent, forming a volcanic mountain, or volcano. After the eruption stops, a bowl like crater generally forms at the top of the volcano. The vent lies at the bottom of the crater. Types of volcanic materials Three basic kinds of materials may erupt from a volcano: 1) lava, (2) rock fragments, and (3) gas. The material that erupts depends chiefly on how sticky or fluid a volcano's magma is.

Lava is the name for magma that has reached the earth's surface. When lava comes to the surface, it is red hot and may have a temperature of more than 2012 F. (1100 C). Highly fluid lava flows rapidly down a volcano's slopes. Sticky lava flows more slowly. As the lava cools, it hardens into many different formations. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehoe (pronounced (pa-hoy-hoy). Stickier lava cools into rough, jagged sheets of rock called aa (AH ah). Pahoehoe and aa cover large areas of Hawaii, where the terms originated. The stickiest lava forms flows of boulders and rubble called block flows. It may also form mounds of lava called domes. Rock fragments, generally called tephra (TEHF ruh), are formed from sticky magma. Such magma is so sticky that its gas cannot easily escape when the magma approaches the surface or central vent. Finally, the trapped gas builds up so much pressure that it blasts the magma into fragments. Tephra includes, from smallest to largest, volcanic dust, volcanic ash and volcanic bombs.

Volcanic dust consists of particles less than 0.25 millimeter in diameter. Volcanic dust can be carried great distances. In 1883, the eruption of Krakatau in Indonesia shot dust 27 kilometers into the air. The dust was carried around the earth several times and produced brilliant red sunsets in many parts of the world. It is likely that large quantities of volcanic dust can affect the climate by reducing the amount of sunlight that reaches the earth. Volcanic ash is made up of fragments less than 0.5 centimeter in diameter. Most volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may reach speeds of 97 kilometers per hour and can be highly destructive. Volcanic bombs are large fragments. Most of them range from the size of a baseball to that of a basketball. The largest bombs may measure more than 1.2 meters across and weigh up to 91 metric tons. Small volcanic bombs are generally called cinders. Gas discharges out of volcanoes in large quantities during most eruptions. The gas is mainly made up steam.

But it also includes carbon dioxide, nitrogen, sulfur dioxide, and other gases. Most of the steam comes from a volcano's magma. Types of volcanoes Volcanoes are divided into three main groups based on the shape of the volcano and the type of material they are made of: 1) shield volcanoes, 2) cinder cones, and 3) composite volcanoes. Shield volcanoes are formed when a large amount of free-flowing lava spills from a vent and spreads widely. The lava gradually builds up a low, broad, dome-shaped mountain. The famous Mauna Loa in Hawaii is a shield volcano. Thousands of separate, overlapping lava flows, each less than 115 meters thick, formed Mauna Loa. Cinder cones build up when mostly tephra erupts from a vent and falls back to earth around the vent. The accumulated tephra, which is generally cinders, forms a cone-shaped mountain. Paricutin in western Mexico is a well-known cinder cone. It began in 1943, when a crack opened in the ground of a cornfield. When the eruptions ended in 1952, the top of the cone was 410 meters above its base.

Composite volcanoes are formed when both lava and tephra erupt from a central vent. The materials pile up in alternate layers around the vent and form a towering, cone-shaped mountain. Composite volcanoes include Japan's Mount Fuji; Mayon Volcano in the Philippines; and Italy's Vesuvius. Mount St. Helens, which has erupted several times since 1980, is one of the most active composite volcanoes in the United States. Plate tectonic setting of volcanism: Why volcanoes occur in restricted areas Most volcanoes are found along a belt, called the Ring of Fire that encircles the Pacific Ocean. Volcanic activity also occurs in such places as Hawaii, Iceland, and southern Europe and at the bottom of the sea. The theory of plate tectonics explains why most volcanoes as well as most earthquakes and mountains occur only in certain places. According to this theory, the earth's outer shell is divided into a number of rigid sections of rock, called plates. The plates slide or drift about continuously over a layer of partly melted rock. Relative movement at the boundary between plates is generally about l to 10 centimeters a year.

As the plates move, their boundaries collide, spread apart or slide past one another. Most volcanoes occur at the plate boundaries. Most volcanoes are formed at convergent plate boundaries where two plates collide. One of the plates is then forced under the other. As the plate sinks, friction and the earth's heat cause part of it to melt. This melted part then rises as magma. When it reaches the surface, it produces a volcano. Volcanic activity also occurs at divergent plate boundaries when two plates spread apart. Most such movement takes place on the ocean floor. As the plates move apart, magma below the crust moves up between the plates. Large amounts of lava pour onto the surface and build up the ocean floor. Magma sometimes creates an underwater mountain range, such as the huge Mid-Atlantic Ridge that runs down the length of the Atlantic Ocean. Iceland and the volcanic islands nearby are exposed parts of this ridge. A number of volcanoes, for example, those in Hawaii lie far from plate boundaries. Such volcanoes develop at hot spots, when a huge column of magma rises from inside the earth toward the surface.

This column, called a mantle plume, may measure up to 160 kilometers in diameter and rise 13 to 25 centimeters yearly. In some cases, the plume comes close enough to the surface so that part of the magma breaks through and forms a volcano. Classification of prediction of volcanic activity. This classification is based on the activity of a volcano according to how often it erupts: (1) active, (2) intermittent, (3) dormant, or (4) extinct Active volcanoes erupt constantly. The eruption is generally quiet but occasionally becomes violent. A well-known active volcano is Stromboli, which lies on an island off the coast of Italy. Intermittent volcanoes erupt at fairly regular periods. Such volcanoes include Mount Asama in Japan, Mount Etna in Sicily, and Hawaii's Hualalal Dormant volcanoes have become inactive, but not long enough to know whether they will erupt again: Such "sleeping" volcanoes include Lassen Peak in California and Parcutin in Mexico. Extinct volcanoes have been inactive since the beginning of recorded history Aconcagua in Argentina and Mount Kenya in Kenya are extinct volcanoes. They probably will not erupt again.

Classification of volcanic eruptions. Volcanic eruptions can be divided into four basic groups based on the violence of the eruption and the type of material that erupts: (1) Hawaiian, (2) Strombolian, (3) Vulcanian/Peleean, and (4) Plinian. Other variations include steam-blast (phreatic euruptions). Hawaiian eruptions are named after the volcanoes in Hawaii and are the least violent type. In such eruptions highly fluid lava flows quietly from several vents and gradually builds up a shield volcano. Strombolian eruptions are named after Stromboli. Such eruptions result from the continuous escape of gas from the magma. As the gas escapes, it produces tephra that piles up into a cinder cone. Vulcanian eruptions get their name from Vulcano, a volcanic island off the Italian coast. These eruptions occur when sticky magma plugs up the central vent. The magmatic gas gradually builds up pressure until it blasts the magma into volcanic dust and bombs. Vulcanian eruptions are some time referred to as Pelean eruptions are the most violent.

Their name comes from the eruption in 1902 of Mont Pelee on Martinique, an island in the West Indies. The eruption killed about 38,000 people. A Peleean eruption occurs when the gas in highly sticky magma builds up tremendous pressure. This pressure causes violent explosions that produce glowing clouds of hot ash and dust. In Peleean eruptions, much of the volcano blows apart. Peleean eruptions are violent; their name comes from the eruption in 1902 of Mont Pelee on Martinique, an island in the West Indies. The eruption killed about 38,000 people. A Peleean eruption occurs when the gas in highly sticky magma builds up tremendous pressure. This pressure causes violent explosions that produce glowing clouds of hot ash and dust In Peleean eruption, much of the volcano blows apart. Steam-blast (Phreatic) eruptions are violent, explosive, glowing avalanches; include powerful blasts of rock, magma, and steam, rising magma come into contact with seawater or groundwater; usually steep-sided domes or collapse calderas. They often erupt viscous, silica-rich magma.

Examples include: Surtsey (Iceland); and Krakatau (Indonesia) Plinian eruptions are exceptionally powerful blasts from the eruptive column; includes viscous, gas-rich magma and voluminous pyroclastics. Usually stratovolcanoes. Examples include: Mount Pinatubo (Philippines); Mount St. Helens (Washington State, USA). Predicting volcanic eruptions is one of the chief concerns of volcanology. When a volcano erupts, little can be done to prevent property damage in the surrounding area. But many lives can be saved if people in the area are evacuated before the eruption begins. Most volcanic eruptions cannot be predicted. However, some volcanoes, such as those in Hawaii, have a built-in warning system. Before such a volcano erupts, it expands slightly as magma collects in the magma chamber. Then as the magma rises, many small earthquakes occur. The temperature in the surrounding area also begins to increase, and clouds of gas start to pour from the vent Several devices are used to predict when such a volcano will erupt. An instrument called a tiltmeter to measure the expansion of a volcano is often used. A seismograph helps detect earthquakes.

Thermometers check temperature increases in the area, and gas detectors measure the amount of gas. Benefits of volcanoes Volcanoes are among the most destructive natural forces on the earth; however, they can also produce benefits, for example, many volcanic materials have important industrial and chemical uses. Rock formed from lava is commonly used in building roads. Pumice, a natural glass that comes from lava, is widely used for grinding and polishing stones, metals, and other materials. Sulfur deposits from volcanoes are used in making chemicals. Weathered volcanic ash greatly improves soil fertility. In many volcanic regions, people use underground steam as a source of energy. This geothermal energy is used to produce electricity in such countries as Italy, Mexico, New Zealand, and the United States. In Reykjavik, Iceland, most people heat their homes with water piped from volcanic hot springs. Volcanoes serve as 'windows" to the earth's interior. The materials they erupt help scientists learn about conditions within the earth. Also, they help to release excessive pressure from the earthï¿½s interior.

Question: What type of volcanic stone is commonly used during pedicures?
Answer: Pumice