Plate Tectonics and Sea-floor Spreading
[size=3][color=#29384e][font=Times New Roman]The theory of plate tectonics describes the motions of the lithosphere[/font][font=宋体]，[/font][font=Times New Roman] the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere[/font][font=宋体]（[/font][font=Times New Roman]n.[[/font][font=宋体]地[/font][font=Times New Roman]][/font][font=宋体]岩石圈）[/font][font=Times New Roman]is divided into a few dozen plates of various sizes and shapes[/font][font=宋体]，[/font][font=Times New Roman] 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.[/font][/color][/size][size=3][color=#29384e][font=Times New Roman] [/font][/color][/size]
[size=3][color=#29384e][font=宋体] [/font][font=Times New Roman]Since the size of the Earth is essentially constant[/font][font=宋体]，[/font][font=Times New Roman] 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[/font][font=宋体]：[/font][font=Times New Roman] 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[/font][font=宋体]，[/font][font=Times New Roman] earthquakes and volcanism[/font][font=宋体]，[/font][font=Times New Roman] but the kinds of geologic activity observed at the two boundaries are quite different.[/font][/color][/size]
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[color=#29384e][font=宋体][size=10.5pt] [/size][/font][size=10.5pt]The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its original version[/size][font=宋体][size=10.5pt]，[/size][/font][size=10.5pt] in the early 1960[/size][font=宋体][size=10.5pt]‘[/size][/font][size=10.5pt]s[/size][font=宋体][size=10.5pt]，[/size][/font][size=10.5pt] it described the creation and destruction of the ocean floor[/size][font=宋体][size=10.5pt]，[/size][/font][size=10.5pt] but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon afterward by the discovery that periodic reversals of the Earth[/size][font=宋体][size=10.5pt]’[/size][/font][size=10.5pt]s magnetic field are recorded in the oceanic crust. As magma rises under the mid-ocean ridge[/size][font=宋体][size=10.5pt]，[/size][/font][size=10.5pt] 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[/size][font=宋体][size=10.5pt]，[/size][/font][size=10.5pt] 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[/size][font=宋体][size=10.5pt]；[/size][/font][size=10.5pt] the width of the stripes indicates the rate of the sea-floor spreading[/size][/color]
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