编辑点评: 研习GRE作文考试的优秀范文不仅可以学到有用的词汇和句子,更能从中学习逻辑的构建方式和作文框架的结构等内容,这里为大家提供GRE的范文,希望对同学们的备考有所帮助。
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 1960 s, 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 Earth s 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.
英文如何表达“拍马屁”或“巴结”
男生女生:我们可以只当朋友吗?
实用口语:你以为你是谁啊?
2011年实用口语练习:取钱那些事
趣味英语:搭讪十大妙招
实用口语情景轻松学:我想买个数码摄像机
2011年实用口语练习:Join a club 社团活动
英语口语主题:交际英语热门话题47个(11--表达歉意)
2011年实用口语练习:表达鼓励的10句英语
“潜规则”之职场八条
实用口语情景轻松学:国外理发店实用对话
2011年实用口语练习:学习疑问
2011年实用口语练习:“淘金热”
实用口语情景轻松学:您要的早餐送上来了
英语口语主题:交际英语热门话题47个(14--同事之间)
疯狂口语要素精选 6
英语口语主题:交际英语热门话题47个(25--竞选和辩论)
实用口语情景轻松学:交通高峰期影响车速
如何用英文表达“你活该”
大运会必备接待口语
实用口语:浪漫 Romance
20条地道实用英语句型(1)
如何用英语表达“你得减肥了”
实用口语情景轻松学:奶奶过生日美颠儿颠儿的
疯狂口语要素精选11
英语口语主题:交际英语热门话题47个(3--邀请)
英语口语主题:交际英语热门话题47个(6--闲聊)
实用口语:Singing With Friends
实用口语情景轻松学:Take baby steps 慢慢来
实用口语情景轻松学:有假钞的时候要送到银行去
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |