Over the last several million years the Hawaiian Islands have been built of successive lava flows. They are the most recent additions in a long line of volcanoes that extends up the intersection ofthe Aleutian Island chain with the Kamchatka peninsula. This set includes very colorful imagesof lava fountains, lakes, cascades, flows, spatter and lava entry to the sea from eruptionsoccurring over the last 30 years. Most of these volcanoes are no longer visible above the sea surface. These islands and sea mounts formed as the Pacific plate moved over a hot spot in Earth's mantle. The amount of lava that has erupted here is difficult to comprehend. Mauna Loa, on the Island of Hawaii (Big Island) is the largest volcanic structure in the world with a volume estimated at 42,000 km3. It rises from the ocean floor, 5,000 m below sea level, to a height above sea level of 4,172 meters. In addition to eruptions at the summit, Hawaiian volcanoes have flank eruptions with lavaflowing several kilometers from the vent. The height of such volcanic structures (known as shield volcanoes) increases only slightly while they continually grow in width. Hawaii's usually non-explosive eruptions are characterized by the relatively quiet outpouring of lava known as effusive eruptions. High temperature, a low gas content, and exceptionally fluid lava are typical of these eruptions. The high fluidity of Hawaiian lava comes from its basaltic composition. They are contrasted to the more viscous dacite erupted explosively at Mount Saint Helens in 1980. Hawaiian eruptions usually start with lava issuing vertically from a central vent or a fissure in a rhythmic jet-like eruption, called a lava fountain. The lava fountains vary widely in form, size and duration depending on the shape of the vent, volume of lava, and other conditions. Fountains spouting from a series of nearly continuous fissures are called curtains of fire. As the eruption proceeds the lava fountain activity is confined to a single vent or opening. The lava may form lava lakes of fluid rock in summit craters or in pit craters on the flanks of the volcanoes. If the lava lake forms around an active vent, the crust breaks up in response to circulation and sloshing of the molten lavabeneath. Lava falling from fountains and flowing from vents often forms glowing lava streams or lava flows. During some Mauna Loa eruptions flows rushed down the steep slopes at 58 km per hour. As the eruption continues, the lava solidifies along the edges of the flow building levees or ramparts that allow the level of the lava to be raised. If the roof of the channel hardens and forms a solid crust the molten lava may continue to flow within what has become a lava tube. Lava tubes generally have arched roofs but their floors may be flat, formed by the surface of the last liquid lava to move through them. The walls of such tubes become thermal insulators allowing the lava to flow greater distances from the vent. Lava streams that plunge over cliffs or the steep walls of craters form lava cascades or lava falls. There are two main types of lava flows: pahoehoe, and aa. The Hawaiian names refer to the surface character of the lava. Many flows consist ofpahoehoe upstream and change to aa downstream. However, aa flows do not change into pahoehoe. The type of lava is determined by the initial gascontent of the lava, the changes in lava viscosity and the rate of deformation (shear strain of the lava during flow and cooling). Pahoehoe has a smooth surface. In some areas it is wrinkled and twisted resembling folds in heavy cloth. This appearance results from the dragging and twisting of the thin, hot, still-plastic crust of the flow by movement of the liquid lava underneath. The surfaces of most pahoehoeflows are rolling or undulating. One can walk across a moving flow, and although the crust may bend, it does not break. The crust of a lava flow is a poor conductor of heat so the part of the flow beneath the crust may remain hot

The word volcano is used to refer to the opening from which molten rock and gas issue from Earth's interior onto the surface, and also to the cone, hill, or mountain built up around the opening by the eruptive products. This slide set depicts ash clouds, fire fountains, lava flows, spatter cones, glowing avalanches, and steam eruptions from 18 volcanoes in 13 countries. Volcano types include strato, cinder cone, basaltic shield, complex, and island-forming. Perhaps no force of nature arouses more awe and wonder than that of a volcanic eruption. Volcanoes can be ruthless destroyers. Primitive people offered sacrifices to stem the tide of such eruptions and many of their legends were centered around volcanic activity. Volcanoes are also benefactors. Volcanic processes have liberated gases of the atmosphere and water in our lakes and oceans from the rocks deep beneath Earth's surface. The fertility of the soil is greatly enhanced by volcanic eruptive products. Land masses such as islands and large sections of continents may owe their existence entirely to volcanic activity. The "volcano" is used to refer to the opening from which molten rock and gas issue from Earth's interior onto the surface, and also to the cone, hill, or mountain built up around the opening by the eruptive products. The molten rock material generated within Earth that feeds volcanoes is called magma and the storage reservoir near the surface is called the magmachamber. Eruptive products include lava (fluid rock material) and pyroclastics or tephra (fragmentary solid or liquid rock material). Tephra includes volcanic ash, lapilli (fragments between 2 and 64 mm), blocks, and bombs. Low viscosity lava can spread great distances from the vent. Higher viscosity produces thicker lava flows that cover less area. Lava may formlava lakes of fluid rock in summit craters or in pit craters on the flanks of shield volcanoes. When the lava issues vertically from a central vent or a fissure in a rhythmic, jet-like eruption, it produces a lava fountain. Pyroclastic (fire-broken) rocks and rock fragments are products of explosive eruptions. These may be ejected more or less vertically, thenfall back to Earth in the form of ash fall deposits. Pyroclastic flows result when the eruptive fragments follow the contours of the volcano and surrounding terrain. They are of three main types: glowing ash clouds, ash flows, and mudflows. A glowing ash cloud (nuee ardente) consists of an avalanche of incandescent volcanic fragments suspended on a cushion of air or expanding volcanic gas. This cloud forms from the collapse of a vertical ash eruption, from a directed blast, or is the result of the disintegration of a lava dome. Temperatures in the glowing cloud can reach 1,000 deg C and velocities of 150 km per hour. Ash flows resemble glowing ash clouds; however, their temperatures are much lower. Mudflows (lahars) consist of solid volcanic rock fragments held in water suspension. Some may be hot, but most occur as cold flows. They may reach speeds of 92 km per hour and extend to distances of several tens of kilometers. Large snow-covered volcanoes that erupt explosively are the principal sources of mud flows. Explosions can give rise to air shock waves and base surges. Air shock waves are generated as a result of the explosive introduction of volcanic ejecta into the atmosphere. A base surge may carry air, water, and solid debris outward from the volcano at the base of the vertical explosion column. Volcanic structures can take many forms. A few of the smaller structures built directly around vents include cinder, spatter, and lava cones. Thick lavas may pile up over their vents to form lava domes. Larger structures produced by low viscosity lava flows include lava plains and gently sloping cones known as a shield volcanoes. A stratovolcano (also known as a composite volcano) is built of successive layers of ash and lava. A volcano may consist of two or more cones side by side and is

Volcanoes have contributed significantly to the formation of the surface of our planet. Volcanism produced the crust we live on and most of the air we breathe. The remnants of an eruption reveal as much as the eruption itself, for they tell us many things about the eruption. Included here are examples of several volcanic products and other magmatic features, with descriptions of how they were formed and what they tell us about volcanism. Most volcanic rock material begins as molten rock material formed within Earth and is called magma. Eruptive products include lava (fluid rockmaterial) and pyroclastics or tephra (fragmentary solid or liquid rock material). Tephra includes volcanic ash, lapilli (fragments between 2 and 64 mm), blocks, and bombs. Perhaps the best known volcanic product is lava, the fluid rock material that flows rather quietly from volcanic vents. The external and internal structures of lava flows are the result of the physical properties of the magma from which it was derived. Of these physical properties viscosity is the most important and it is in turn affected by the temperature and chemical composition of the magma. Lavas of low viscosity can spread great distances from the vent. Greater viscosity produces thicker lava flows that generally cover less area. The rate of supply of magma relative to the velocity of the lava as it flows from the vent and the external environment through which the lava flows also affect the structure of the solidified lava. Products of explosive eruptions include pyroclastic (fire broken) rocks and rock fragments. The force that produces explosive eruptions is the release of trapped gas. Ejecta from these explosions may be derived from the magma or from rocks in the vicinity of the volcanic conduit that are blasted out in the eruption. These may be ejected more or less vertically, then fall back to earth in the form of ash fall deposits. Pyroclastic flows result when the eruptive fragments follow the contours of the volcano and surrounding terrain. They are of three main types: glowing ash clouds (nuee ardente), ash flows, and mudflows. Volcanic structures can take many forms. A few of the smaller structures built directly around vents include cinder, spatter, and lava cones. Thick lavas may pile up over their vents to form lava domes. Larger structures produced by low viscosity lava flows include lava plains. The erosion of volcanoes leaves volcanic remnants, interesting reminders of the volcano's former fury. Erosion of the layers of lava and ash that built the volcano leaves the congealed magma in the conduit. This feature, sometimes referred to as a plug or the volcanic neck or throat, is a dramatic pillar of rock rising above the surrounding plain. These plugs or necks may be composed partially of fragments of the walls of the pipe and partially of congealed magma. They may be as more than a kilometer in diameter. Magma flowing into cracks in the rocks produces dikes, sills and laccoliths. This intrusive rock is generally resistant to erosion and often remains after the surrounding rock has eroded away. These exposed intrusive rocks give us a glimpse of the complex underground network of piping in active volcanoes. These igneous features are constant reminders of the timelessness of the processes that relentlessly form, and reform, the surface of planet Earth.

Volcanic and Seismic Hazard Intensity Level in the coastal zone of Kauai, Hawaii

Volcanic and Seismic Hazard Intensity Level in the coastal zone of Lanai, Hawaii

Volcanic and Seismic Hazard Intensity Level in the coastal zone of Lanai, Hawaii

Volcanic and Seismic Hazard Intensity Level in the coastal zone of Oahu, Hawaii

Volcanic and Seismic Hazard Intensity Level in the coastal zone of Maui, Hawaii

At dawn on June 15, 1991, a cataclysmic eruption began with a tremendous explosion that destroyed ten deserted villages. This eruption deposited approximately 5 to 7 km3 of volcanic fragments in pyroclastic flows on the slopes of the volcano and over neighboring towns and agricultural areas. It is this material that continues to threaten structures and lives in the area, in the form of lahars (debris flows) during heavy rainstorms. The lahars from the Mount Pinatubo volcano have been particularly damaging to the surrounding area. This set of slides shows how the disaster that began at Pinatubo in 1991 continues to threaten the population in the area. Mount Pinatubo is located on the Island of Luzon in the Philippines, about 100 km northwest of Manila. The volcano, with K-Ar datings of approximately 1.1 million years, and with the youngest carbon-14 dating of + 400 years B.P. (before present), is the youngest volcano in the western Luzon volcanic arc. On April 2, 1991, Pinatubo, which had been rumbling for months, stirred to life. Over the next six weeks earth tremors and minor explosions occurred. These natural warnings led to the evacuation of personnel at Clark Air Base and of 55,000 people in nearby towns and villages. At dawn on June 15, 1991, a cataclysmic eruption began with a tremendous explosion that destroyed ten deserted villages. The eruption deposited approximately 5 to 7 km3 of volcanic fragments in pyroclastic flows on the slopes of the volcano and over neighboring towns and agricultural areas. It is this material that continues to threaten structures and lives in the area, inthe form of lahars (debris flows) during heavy rainstorms. The Pinatubo deposits are subdivided into two general groups based on the lithology and age of emplacement: the "ancestral" and the "modern." The ancestral Pinatubo (+/- 1 million to +/-35,000 years B.P.) is an andesite-dacite stratovolcano of mostly laval flows and breccia deposits oflaharic origin. On its slopes are numerous elongated to sub-rounded hills made up of breccia, created mostly by the ancestral lahars. The modern Pinatubo (+/- 35,000 years to present) shows signs of repeated, very explosive eruptions which have produced large volumes of pumiceouspyroclastic flows. Pyroclastic flows, also known as nuee ardentes, or glowing avalanches, are extremely hot (+/-1,000 degrees Celsius), often incandescent, highly fluid, gravity-driven density currents of gas and volcanic fragments that sweep down slope and travel at hurricane speed (+/-100 km per hour). Pyroclastic flows are generated when the density of the rising column of volcanic fragments and gas exceeds that of the surrounding atmosphere. Gravity causes a portion or all of the column to collapse and flow down the flanks of the volcano. Most of the 1991 pyroclastic flow deposits were emplaced during the June 15 eruption. The pre-eruption magma temperature of Pinatubo was about 800 degrees Celsius and the temperature of the emplaced pyroclastic flow was on the order of 600 degrees Celsius. The deposits are non-welded, dry, and very loose. The accumulated thickness of the pyroclastic flows varies, depending on the proximity to the crater and the pre-eruption morphology. It reaches more than 200 meters along deep pre-eruption valleys. The pyroclastic flow deposits of 1991 affected eight major watersheds around the slopes of the volcano and radically altered the hydrological regimes, leading to unprecedented amounts of erosion and sediment delivery in the form of destructive lahars. Lahars predominantly occur during the rainy season in the monsoon period, which lasts from June until November. Long-duration and high-intensity rainfall, associated with the occurrence of strong typhoons, are responsible for the production of large-magnitude lahars. Other factors contributing to the rapid erosion of lahars are: failure of lahar dams, secondary explosions produced by rapid vertical and lateral erosion of the pyroclastic flow

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