Volcanic Rocks and Features
Contraction during cooling of thick, extrusive basaltic lava flows causes cracks perpendicular to the cooling surface, which form the polygonal columns of the Devil's Postpile in California. The columns average about 46 cm across. In thic,k slowly cooled flows, the joints normal to the flow surface tend to develop in three directions at roughly 60 degrees to each other and form multi-sided blocks that are cut off by the joints parallel to the flow surface. Jointing of this sort may attain a high degree of regularity. The horizontal joints divide the columns into a series of segments. Such columnar jointing is found in all types of lava flows, including ash flows. It is also conspicuous in dikes and other intrusions.
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.
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Last Modified: 2015-10-14
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