2014年12月6日托福考试阅读真题回顾解析(第一版)

signalling to other individuals of the same species, such as to attract mates.

　　第二篇：板块漂移学说

　　题材划分：地质类

　　主要内容：

　　LS专家认为fossil是一样的，另一个认为tidal force让整体漂移然后分开，他的观点是错的。第三人是第一个develop drift学说的，最后一个人支持前一个人，他发现一个水生生物在两处都有。

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　　OG- Geology and Landscape

　　相关知识背景

　　Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building")]is a scientific theory that describes the large-scale motion of Earth's lithosphere. This theoretical model builds on the concept of continental drift which was developed during the first few decades of the 20th century. The geoscientific community accepted the theory after the concepts of seafloor spreading were later developed in the late 1950s and early 1960s.

　　The lithosphere, which is the rigid outermost shell of a planet (on Earth, the crust and upper mantle), is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary; convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trenchformation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.

　　Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some supporters) propose gradual shrinking (contraction) or gradual expansion of the globe.

　　Tectonic plates are able to move because the Earth's lithosphere has greater strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust, which result in differences in gravitational forces) and drag, with downward suction, at the subduction zones. Another explanation lies