板塊結構與海床擴展
板塊結構理論描述岩石圈的運動。岩石圈是相對堅硬的地球外層，包括全部地殼和部分地幔。
岩石圈被劃分為幾十個大小不同形狀各異的板塊，一般而言這些板塊都處於相對運動之中。一道中海脊是板塊之間的邊界，在那裡新的岩石圈的物質從下部注入。
當板塊從中海脊脫離時，它們滑向在岩石圈基部較易變形的地層上。因為地球的大小本質上是不
變的，只有同等數量的岩石圈物質在其它地方被吞沒，新的岩石圈才能生成。
銷毀舊岩石
圈的地方形成另外一種板塊邊界：一塊潛沒的區域。在這裡，一塊板塊潛沒到另一板塊的邊緣之下並結合入地幔之中。
兩種板塊邊界均與地層系統、地震以及火山活動有關，但在兩種邊界處觀察到的諸般地質活動卻迥然不同。海床擴展說實際上早於板塊結構理論。在
20世紀60年代它的理論雛形中，描述了海底的生成和毀滅，但沒有詳細介紹堅硬的岩石圈板塊。這個假定不久之後為發現所證實。
該發現表明地球磁場周期性的逆轉被記錄在海洋地殼中。當岩漿從中海脊下湧起的時候，岩漿中的磁鐵礦物質按地磁場的方向被磁化。岩
漿冷卻並凝固下來後，地磁場的方向和磁極被保留在磁化了的火山岩中。
磁場的逆轉形成一系列與斷層軸線平行的條形磁區。
這樣海洋殼就扮演了磁帶的角色，記錄下可以鑑定時間的地磁場的歷史。條形磁區的寬度表明了海底擴展的速度。
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(n.[地]岩石圈)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 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 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.