andesite: a dark gray fine-grained volcanic rock containing about 60% silica, plus iron and magnesium. photo

archipelago:a group or chain of islands. photos

asthenosphere: the upper part of the Earth's mantle, approximately 200 km thick. The asthenosphere is hot: 1400°C: but it is not molten. The rock of the asthenosphere can be deformed by pressure, resulting in movement of the crust above it. diagram

basalt:hard black volcanic rock formed by lava that has flowed from volcanoes or cracks in the Earth's crust and hardened. photo 1, photo 2

continental drift: a theory that movement of the Earth's surface causes the continents to move slowly toward or away from each other. According to this theory, the continents were once part of a giant land mass. Over time, the land mass broke up, and the continents drifted apart to their present locations. See Tectonic Plate Theory. diagram

convection: a process by which heat energy may be circulated in gases and liquids. As the material is heated, it expands, becomes less dense, and rises. Cooler material moves in to take its place. Meanwhile, the heated material cools off, becomes less dense, and sinks. The movement of heating and cooling material sets up a circular convection current. diagram

converge: to move together.

convergent plate movement: the movement of tectonic plates toward each other. animation

core: the central part of the Earth. Studies show that the diameter of the core is about 3500 km (2170 miles), more than half of the Earth's total diameter of 6370 km (3950 miles). The core is divided into two parts, the outer core and the inner core. Scientists believe that the outer core is about 2250 km (1400 miles) think and is composed of melted iron and nickel. The inner core is solid iron and nickel, and it is smaller: about 1220 km (750 miles) thick. diagram

crust: the outermost layer of the Earth. The crust is like a thin rock and soil “skin” covering the planet. Its thickness ranges from about 5 to 75 km (3 to 46 miles). The continental crust is made of rocks similar to granite and basalt. The oceanic crust is mostly basalt. diagram

diverge: to move apart.

divergent plate movement: the movement of tectonic plates away from each other. animation

earthquake: an abrupt shaking of the ground occurring when forces inside the earth's crust cause a sudden movement in the rock. Earthquakes can be the result of sudden release of stress along a fracture in the Earth's crust, volcanic activity, or human activity such as an explosion. Most earthquakes occur in the outer 720 km (445 miles) of the crust, where rocks under stress are likely to break, rather than bend or flow.

epicenter: the exact location on the Earth's surface directly above the place where the earthquake is focused. The epicenter is the point at which the crust shakes and sends out shock waves. animation

fault: a fracture or zone of fractures leading to displacement of rock layers in the Earth's crust. Pressure builds up on either side of the fault until the rock snaps and movement takes place. Most faults are underground, but some are visible at the surface. diagram

fossil: the remains of an ancient plant or animal, preserved in rock. A fossil may be a remnant of the plant or animal itself, or it may a track or burrow left by an animal. Some fossils have been found that were preserved in ice, tar, or hardened sap.

granite: a coarse-grained light-colored igneous rock composed of quartz, two types of feldspar, and mica. photo

heterogeneous: consisting of parts that are not related or alike.

homogeneous: consisting of parts that are alike or very similar; uniform.

hot spot: an area in the middle of a tectonic plate where magma rises from the mantle and erupts at the surface. Sometimes volcanoes are formed above a hot spot. diagram

igneous: the group of rocks that is formed by the hardening of molten rock. Some examples are: granite, andesite, basalt, feldspar, and obsidian. photo 1, photo 2

lava: magma (molten rock) that has erupted onto the surface of the Earth. Lava usually refers to the liquid rock flowing from a volcano, as well as to the solidified rock formed when lava cools. See magma. photo 1, photo 2

lithosphere: the outer layer of the earth, including the crust and the uppermost part of the mantle. The lithosphere is made up of six large tectonic plates and several smaller ones. Its thickness ranges from less than 50 km (30 miles) beneath the oceans to 120 km (70 miles) at the continents. See tectonic plates. diagram

lithospheric plates: See tectonic plates.

magma: molten rock beneath the surface of the Earth. When magma emerges on the surface of the Earth, it is called lava. See lava. diagram

mantle: the zone of the Earth between the crust and the core. The mantle is about 2900 km (1740 miles) thick and consists of dense igneous rock. diagram

mid-oceanic ridge: a chain of mountains on the ocean floor where oceanic crust is created by magma rising from between two tectonic plates. diagram 1, diagram 2

mountain-building: most mountains were formed by tectonic forces. When two continental plates collide, the rocks buckle, forming mountains and valleys. Mountains are also formed by volcanoes. diagram

pillow lavas: sack-like formations of lava formed under water. When molten lava breaks through on the ocean floor, or flows into the ocean, it is quickly cooled by the seawater, and a crust forms around the lump. Pressure builds until lava breaks through the new crust and oozes out like toothpaste to form another pillow. As the lava flow advances, pillow lavas continue to stack up on each other. Eventually they solidify all the way through. photo 1, photo 2, photo 3

plate boundaries: the edges of individual tectonic plates. See tectonic plates. map

plate tectonics theory: the theory that the Earth's crust is broken up into individual plates that gradually move about in relation to each other. The edges of the plates move against each other, causing geologic activity such as mountain-building, earthquakes, and volcanoes. See tectonic plates.

Ring of Fire: an arc located along the edge of the Pacific Ocean, stretching from New Zealand to the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the western edge of North and South America. The Ring of Fire is noted for its frequent earthquakes and volcanic eruptions. More than half of the world's volcanoes that rise above sea level are located in the Ring of Fire, and about 90% of the world's earthquakes take place there. map

rock: a naturally occurring mixture of stony material forming part of the Earth's crust. Rocks may contain a single mineral or be composed of many minerals. There are three main types of rock: sedimentary, igneous, and metamorphic. Sedimentary rocks are formed from sediments deposited on the surface of the Earth. Igneous rocks come from melted rock deep in the earth. Metamorphic rocks begin as sedimentary or igneous rocks, but they are changed by the effects of high temperature and pressure.

sediment: grains of minerals, fragmented rocks, or organic material that can be transported and deposited by water, ice, or wind.

seismic: pertaining to shock waves of energy that travel through the earth as the result of an earthquake or explosion. Seismic waves are measured by a seismograph and studied by seismologists.

seismics: the study of man-made waves.

seismology: the study of earthquakes.

subduction: a process in which two tectonic plates collide, and one descends beneath the other. An oceanic plate is denser than a continental plate, so the oceanic plate tips down and slides beneath the continental plate, forming a deep ocean trench. The area where two tectonic plates collide and subduction takes place is the subduction zone. animation

tectonic plates: (also known as lithospheric plates) huge, irregular slabs of solid rock that are pieces of the earth's lithosphere. There are seven large tectonic plates and many smaller ones. A plate may be oceanic, continental, or both. Australia is located entirely on one plate, as is the Antarctic, but the other continents have plate boundaries running through them. map

theory: an explanation of some aspect of the natural world based upon observable facts, studies, principles, and propositions.

transform plate movement: the movement of two tectonic plates sliding past each other. The areas where this movement takes place are called transform boundaries. In some places, transform boundaries are marked by formation of features such as valleys or streambeds where the rock has been fragmented by the sliding motion. animation

tsunami: a large destructive ocean wave, or a series of waves, resulting from sudden vertical movement of the Earth’s crust.

volcano: an opening in the surface of the Earth through which magma, gases, and ash erupt. A mountain created by the accumulation of hardened magma is also called a volcano. Most volcanoes are cone-shaped, but some are less steep. These are called shield volcanoes. photo of volcano, photo of shield volcano, photo of fissure