The theory of plate tectonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere is divided into a few dozen plates of various sizes and shapes, in general the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.
Since the size of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a seduction zone. There one plate dives under the edge of another and is reincorporated into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcanism, but the kinds of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its original version, in the early 1960s, it described the creation and destruction of the ocean floor, but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon afterward by the discovery that periodic reversals of the Earths magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic minerals in the magma become magnetized in the direction of the magma become magnetized in the direction of the geomagnetic field. When the magma cools and solidifies, the direction and the polarity of the field are preserved in the magnetized volcanic rock. Reversals of the field give rise to a series of magnetic stripes running parallel to the axis of the rift. The oceanic crust thus serves as a magnetic tape recording of the history of the geomagnetic field that can be dated independently; the width of the stripes indicates the rate of the sea-floor spreading.
心理学家表示 别强迫自己或者别人保持乐观
《美女与野兽》的原型故事,原来很暗黑……
2017届高考英语一轮全册复习课时作业:选修6.4《Global warming》(新人教版含解析)
2017届高考英语一轮模块复习学案:模块5.3《science and nature》(解析版)
2017届高考英语一轮模块复习学案:模块8.3《the world of colours and light 》(解析版)
2017届高考英语一轮模块复习单元训练:模块1.1《school life》(原卷版)
2017届高考英语一轮全册复习课时作业:必修1.1《Friendship》(新人教版含解析)
2017届高考英语一轮模块复习学案:模块5.1《getting along with others 》(原卷版)
Unusual trip to the suburbs augurs well
你在散播负能量的最大迹象,以及如何改变这一现状
2017届高考英语一轮模块复习单元训练:模块3.3《tomorrow’s world》(解析版)
天坛公园推行“刷脸出厕纸”
2017届高考英语一轮模块复习单元训练:模块3.3《tomorrow’s world》(原卷版)
2017届高考英语一轮模块复习学案:模块7.4《public transport》(解析版)
2017届高考英语一轮模块复习单元训练:模块4.2《sports events》(原卷版)
2017届高考英语一轮模块复习单元训练:模块4.2《sports events》(解析版)
滴滴宣布专车快车动态调价双重封顶
2017届高考英语一轮全册复习课时作业:选修7.1《Living well》(新人教版含解析)
2017届高考英语一轮模块复习学案:模块6.2《what is happiness to you》(解析版)
2017届高考英语一轮模块复习单元训练:模块5.1《getting along with others 》(原卷版)
2017届高考英语一轮模块复习学案:模块5.3《science and nature》(原卷版)
英语真的应该取消吗?英语应必修!
2017届高考英语一轮模块复习学案:模块6.2《what is happiness to you》(原卷版)
2017届高考英语一轮模块复习学案:模块7.2《fit for life》(原卷版)
白色圣诞节 White Christmas
2017届高考英语一轮全册复习课时作业:选修6.5《The power of nature》(新人教版含解析)
2017届高考英语一轮模块复习单元训练:模块4.3《tomorrow’s world》(原卷版)
2017届高考英语一轮模块复习学案:模块8.1《the written word》(解析版)
特朗普前妻出书分享育儿经 谈如何培养优秀儿女
2017届高考英语一轮模块复习学案:模块8.2《the universal language》(原卷版)