36 Plate Tectonics and Sea-floor Spreading
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 belows. 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 subduction 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 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.
【复习方略】2017版高考英语一轮外研版课时提升作业:选修6 Module 2《Fantasy Literature》(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修3 Module 5《Great People and Great Inventions of Ancient China》(含解析)
2017届广东省惠阳县高考英语语法填空一轮选练:2(含答案)
2017年高考英语一轮复习讲练测:必修4 语法专题复习 动词时态语态复习(2)(讲)(原卷版)
2017年高考英语一轮复习讲练测:必修4 语法专题复习 情态动词与虚拟语气(1)(练)(解析版)
2017届广东省惠阳县高考英语语法填空一轮选练:3(含答案)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修3 Module 2《Developing and Developed Countries》(含解析)
2017年高考英语一轮复习讲练测:必修4 语法专题复习 动词时态语态复习(2)(练)(解析版)
2017届广东省惠阳县高考英语语法填空一轮选练:7(含答案)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修7 Modules 5、6(含解析)
2017年高考英语一轮复习讲练测:必修1 名词性从句复习(2)(讲)(原卷版)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修1 Module 4《A Social Survey—My Neighbourhood》(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修6 Module 1《Small Talk》(含解析)
2017年高考英语一轮复习讲练测:必修4 语法专题复习 情态动词与虚拟语气(1)(讲)(解析版)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修7 Modules 1、2(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修8 Modules 5、6(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修6 Module 5《Cloning》(含解析)
消费者投诉新渠道:全国12315互联网平台正式上线
2017届广东省惠阳县高考英语语法填空一轮选练:4(含答案)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修2 Module 6《Films and TV Programmes》(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修2 Module 3《Music》(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修3 Module 6《Old and New》(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修7 Modules 3、4(含解析)
【复习方略】2017版高考英语一轮外研版课时提升作业:选修6 Module 3《Friendship》(含解析)
2017届广东省惠阳县高考英语语法填空一轮选练:1(含答案)
2017届广东省惠阳县高考英语语法填空一轮选练:6(含答案)
2017届广东省惠阳县高考英语语法填空一轮选练:5(含答案)
2017年高考英语一轮复习讲练测:必修1 名词性从句复习(2)(测)(原卷版)
2017年高考英语一轮复习讲练测:必修4 语法专题复习 动词时态语态复习(2)(练)(原卷版)
【复习方略】2017版高考英语一轮外研版课时提升作业:必修1 Module 1《My First Day at Senior High》(含解析)
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |