關于托福聽力課程的自然科學類，常常會出現(xiàn)一個名詞：Plate Tectonics板塊運動/地殼構造論?？赡茉S多考生都會對這個學說感到熟悉又陌生，接下來給大家聊聊我們腳下的土地到底是怎么運動的。

　　Plate tectonics is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large of tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift. Earth's lithosphere is broken into seven or eight major plates and many minor plates. Earthquakes,and oceanic trench formation occur along these plate boundaries. Tectonic plates are composed of the continental plate and the denser oceanic plate. If two plates collide, the denser one will subduct beneath the less dense one. The three different types of plate boundaries are divergent boundaries, convergent boundaries, and transform boundaries.

　　板塊構造學是一個已經(jīng)被普遍接受的科學理論，它認為地球的巖石圈lithosphere是由許多自大約 34 億年前以來一直在緩慢移動的大型構造板塊組成。這個模型主要是建立在大陸漂移continental drift的概念基礎之上。地球的巖石圈共被分成7至8個主要板塊和許多小板塊，在這些板塊相遇的地方，我們經(jīng)常能夠見到地震、火山、造山和海溝形成等活動。構造板塊是通過大陸板塊和密度更大的海洋板塊一起組成。當兩個板塊碰撞時，密度較大的板塊將俯沖到密度較小的板塊下面。地球上一共存在三種主要的板塊邊界類型，分別為擴張邊界、聚合邊界和轉(zhuǎn)換邊界。

　　分離型邊界 Divergent Boundary

　　Divergent boundaries occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, the ocean basin expands. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin.

　　The most significant example of divergent boundaries is the East African Rift.

　　擴張邊界出現(xiàn)在兩個板塊往相反方向移動的地方。在海洋間的裂谷帶，海底擴張形成了不同的邊界。隨著海洋板塊的分裂，一道洋脊在擴張中心形成，海洋盆地也逐漸擴大。在大陸間的裂谷帶，隨著大陸分裂、擴張，中央裂谷漸漸塌陷，逐漸的形成了新的海洋。擴張邊界最顯著的例子便是東非大裂谷。