Earth has three main layers. They are the crust, the mantle, and the core. The crust is Earth's hard surface where all humans live. It is the outermost and the thinnest of Earth's layers. Underneath the crust is the mantle, Earth's thickest layer. The rocky material in the upper mantle is rigid. However, beneath this rigid upper mantle lies a very hot and soft
rock zone. Below the mantle is the core. The core has two parts: a liquid outer core and a solid inner core. The crust and the upper mantle form a rigid layer of rock called the lithosphere. Below the lithosphere is the asthenosphere. It is very hot and soft and can flow like a heated liquid. Thus, the rocky material in the mantle is constantly in motion. It rises and pushes against the bottom of the crust. This movement causes the thin crust to break into pieces called plates. In fact, Earth's crust is composed of many of these plates. The plates vary in size and shape. The major plates are named after the continents or oceans that they support. Some
of them are the Eurasian Plate, the North American Plate, the Pacific Plate, and the Caribbean Plate. These plates are constantly in motion. Remember that Earth's lithosphere sits on top of the mantle. Because the mantle flows, it makes the plates in the lithosphere move. As a result, Earth's crust is constantly moving. The theory that Earth's crust is divided into plates that are constantly moving is called plate tectonics. However, these plates do not move very quickly. They may only move a few centimeters a year. But, over many years, these movements can cause major changes in the plates. When plates collide, they may form mountain chains and ocean trenches. The movement of the plates is also what causes earthquakes and volcanoes.
地球內部有三層構造，分別是地殼、地函和地核。地殼是人類所生活的堅硬地表，是地球最外面、最薄的一層。地殼底下是地函，是地球最厚的一層，上地函是堅硬的岩石，在堅硬的上地函底下卻是熾熱軟質的岩層。地函下是地核，包括兩部分：液態的外地核和固態的內地核。 地殼和上地函構成堅硬的岩石層，稱為岩石圈，底下是軟流圈。軟流圈非常高溫，且質地柔軟，可呈熔融狀態流動，故地函的岩石物質時時刻刻在移動。它會升高並推擠地殼底部，這種運動使薄地殼分裂成數塊，稱為板塊。 事實上，地球的地殼由許多板塊組成，這些板塊的大小、形狀不同。主要板塊皆由它們所支撐的大陸或海洋來命名，包含歐亞板塊、北美板塊、太平洋板塊和加勒比板塊。這些板塊時時在漂移。前面提到地球的岩石圈位於地函的頂層，地函的流動帶動岩石圈的板塊，地殼也因此隨時都在移動。地殼分裂成板塊並遷移的理論稱為「板塊構造學說」。 儘管板塊的移動速度非常緩慢，一年可能只移動幾公分，但經年累月下來，卻能造成重大的板塊改變。板塊碰撞可能形成山脈和海溝。板塊移動也是地震和火山的成因。 | 
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