STRUCTURE OF THE EARTH

·         The three main layers of Earth: crust (outermost, rigid, very thin, brittle), mantle (dense, hot layer of semi-solid rock), and core (nearly twice as dense as the mantle because its composition is metallic – iron-nickel alloy, made up of liquid outer core and a solid inner core).

·         Lithosphere is the rigid outer part of the earth composed of the crust and the upper mantle.

·         Asthenosphere - lies below the lithosphere, relatively narrow, mobile zone in the mantle, composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure over geologic time. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere

·         In geologic terms, a plate is a large, rigid slab of solid rock. Plate tectonics, refers to how the Earth's surface is built of plates. The Earth’s crust is made up of 16 some are big like the North American plate and others are small like the Philippine plate.

·         The Continental Drift hypothesis proposes that the continents were once joined in a large continent, Pangaea. This supercontinent split apart and the continents drifted to their present positions. This theory is the forerunner to the theory of plate tectonics.

·         The Seafloor Spreading hypothesis proposes that the seafloor is in motion caused by the movement in the mantle beneath the seafloor.

·         The Theory of Plate Tectonics states that the Earth's outermost layer is fragmented into a dozen or more, large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.