Acid Rain
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AA-LAVA Robert Buchwaldt Massachusetts Institute of Technology, Cambridge, MA, USA
Synonyms aʻa; ʻaʻa; a-aa Definition Aa (derived from the Hawaiian word a’a’ for rough lava) lavas is the term for cooling textures of a highly viscous lava flow and was introduced as a technical term by Clarance Dutton in the year 1883. Aa is characterized by a rough rubbly surface composed of broken lava blocks called clinkers. When the molten rock cools, the lava flow increases its viscosity due to degassing and crystallization of minerals. At some point, the lava flows so slowly that it allows a thick crust to form. As flow continues to move, the cooler and brittle skin surface breaks into rough jagged blocks, or clinkers. The clinkers are carried along at the surface. At the leading edge of an aa flow, however, these cooled fragments often tumble down the steep front and are subsequently buried by the advancing flow. This process produces a layer of lava fragments both at the bottom and top of an aa flow. Accretionary lava balls as large as 3 m are common on aa flows (Figure 1). Aa flows are emitted from vents at high rates ranging up to 50 km/h, often with much lava fountaining. They are characteristic of viscous magmas. Aa flows are animated
AA-LAVA, Figure 1 An aa flow front on the coastal plain of Isabella, one of the islands of the Galapagos archipelago, Ecuador (Robert Buchwaldt).
with sporadic bursts of energy. Such flows may overturn houses, walls, and forests.
Bibliography Dutton, C. E., 1883. 4th Annual Report U.S. Geological Survey, 95.
Cross-references Eruption Types (Volcanic) Lahar Shield Volcano Stratovolcano Volcanoes and Volcanic Eruptions
P.T. Bobrowsky (ed.), Encyclopedia of Natural Hazards, DOI 10.1007/978-1-4020-4399-4, # Springer Science+Business Media Dordrecht 2013
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ACCELEROMETER
ACCELEROMETER
ACID RAIN
Zhengwen Zeng1, Lin Fa1,2 1 University of North Dakota, Grand Forks, ND, USA 2 Xi’an Institute of Post and Telecommunications, Xi’an, Shaanxi, China
Mary J. Thornbush University of Birmingham, Edgbaston, Birmingham, UK
Synonyms Geophone; Gravimeter Definition An accelerometer is defined as a transducer whose output is proportional to acceleration. An accelerometer measures the proper acceleration it experiences relative to freefall. This is equivalent to inertial acceleration minus the local gravitational acceleration, where inertial acceleration is understood in the Newtonian sense of acceleration with respect to a fixed reference frame, which the Earth is often considered to approximate. An accelerometer can be used for detecting magnitude and direction of the acceleration as a vector quantity. An accelerometer consists of a proof mass, a reference frame, a sensor (an induction coil or a potentiometer and so on), a spring, a damper, and a casing. Because of the constraint of the reference frame, the proof mass can only move along an axis-line. According to Newton’s law, when the casing experiences an acceleration in the axis, the proof mass with a certain inertia resists a change in its sta
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