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astronomy the study of the moon, stars, and other objects in space axis an imaginary line that passes through Earth's center and the north and south poles, about which Earth rotates revolution the movement of an object around another object orbit the path of an object as it revolves around another object in space latitude a measurement of distance from the equator, expressed in degrees north or south solstice The two days of the year on which the noon sun is directly overhead at either 23.5 degrees South or 23.5 degrees North equinox the two days of the year on which neither hemisphere is tilted toward or away from the sun vernal equinox the day of the year that marks the beginning of spring in the northern hemisphere autumnal equinox The day of the year that marks the beginning of fall in the Northern hemisphere phase one of the different shapes of the moon as seen from Earth solar eclipse the blocking of sunlight to earth that occurs when the moon is directly between the sun and earth umbra the darkest part of a shadow penumbra the part of a shadow surrounding the darkest part lunar eclipse the blocking of sunlight to the moon that occurs when Earth is directly between the sun and the moon tide The rise and fall of the level of water in the ocean gravity attractive force between two objects that depends on the masses of the objects and the distance between them spring tide a tide with the greatest difference between low and high tides neap tide a tide with the least difference between low and high tides nuclear fusion the process by which hydrogen atoms join together to form helium, releasing energy core The central part of the sun, where nuclear fusion takes place photosphere the inner layer of the suns atmosphere chromosphere The middle layer of the sun's atmosphere corona outer layer of the sun's atmosphere solar wind a stream of electrically charged particles produced by the sun's corona sunspots areas of gas on the sun's surface that are cooler than the gases around them prominences A loop of gas that protrudes from the sun's surface, linking parts of sunspot regions solar flares an explosion of hydrogen gas from the sun's surface that occurs when the loops in sunspot regions suddenly connects continental drift the hypothesis that the continents slowly move across Earth's surface fossil a trace of an ancient organism that has been preserved in rock mid-ocean ridge an undersea mountain chain where new ocean floor is produced; a divergent plate boundary sonar a device that determines the distance of an object under water by recording echoes of sound waves deep-ocean trench a deep valley along the ocean floor beneath which oceanic crust slowly sinks toward the mantle subduction the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary sea-floor spreading the process by which molten material adds new oceanic crust to the ocean floor focus The point beneath Earth's surface where rock breaks under stress and causes an earthquake epicenter the point on Earth's surface directly above an earthquake's focus seismograph a device that records ground movements caused by seismic waves as they move through Earth magnitude The measurement of an Earthquake's strength based on seismic waves and movement along faults Mercalli scale A scale that rates earthquakes according to their intensity and how much damage they cause Richter Scale a scale that rates seismic waves as measured by a particular type of mechanical seismograph moment magnitude scale a scale that rates earthquakes by estimating the total energy released by an earthquake seismic waves vibrations that travel through Earth carrying the energy released during an earthquake P waves A type of seismic wave that compresses and expands the ground S waves A type of seismic wave that moves the ground up and down or side to side surface waves a type of seismic wave that forms when P waves and S waves reach Earth's surface rotation the spinning motion of a planet on its axis eclipse the partial or total blocking of one object in space by another Pangaea the name of the single landmass that broke apart 200 million years ago and gave rise to today's continents geologist a scientist who studies the forces that make and shape planet earth rock the material that forms Earth's hard surface geology study of the planet earth constructive force a force that builds up mountains and landmasses on Earth's surface destructive force A force that slowly wears away mountains and other features on the surface of Earth continent a great landmass surrounded by oceans seismic wave A vibration that travels through Earth carrying the energy released during an earthquake. pressure the amount of force pushing on a surface or area crust the layer of rock that forms Earth's outer surface basalt A dark, dense, igneous rock with a fine texture, found in oceanic crust granite A usually light colored rock that is found in continental crust mantle the layer of hot, solid material between Earth's crust and core. lithosphere a rigid layer made up of the uppermost part of the mantle and the crust asthenosphere The soft layer of the mantle on which the lithosphere floats. outer core a layer of molten iron and nickel that surrounds the inner core of the Earth inner core a dense sphere of solid iron and nickel at the center of Earth heat transfer the movement of energy from a warmer object to a cooler object radiation the transfer of energy through empty space conduction the transfer heat by direct contact of particles of matter convection the transfer of heat by movements of a heated fluid density the amount of mass in a given space; mass per unit volume convection current the movement of a heated fluid, caused by differences in temperature, that transfers heat from one part of the fluid to another. earthquake the shaking that results from the movement of rock beneath Earth's surface stress A force that acts on rock to change its shape or volume. shearing stress that pushes a mass of rock in opposite directions tension Stress that stretches rock so that it becomes thinner in the middle. compression Stress that squeezes rock until it folds or breaks. deformation A change in the volume or shape of Earth's crust. fault a break in Earth's crust where masses of rock slip past each other strike-slip fault a type of fault where rocks on either side move past each other sideways with little up or down motion normal fault a type of fault where the hanging wall slides downward; caused by tension in the crust hanging wall The block of rock that forms the upper half of a fault. footwall the block of rock that forms the lower half of a fault reverse fault A type of fault where the hanging wall slides upward fault-block mountain a mountain that forms where a normal fault uplifts a block of rock folds a bend in rock that forms where part of Earth's crust is compressed anticline an upward fold in rock formed by compression of Earth's crust syncline a downward fold in rock formed by compression in earth's crust plateau a large area of flat land elevated high above sea level geologist a scientist who studies the forces that make and shape planet earth rock the material that forms Earth's hard surface geology study of the planet earth constructive force a force that builds up mountains and landmasses on Earth's surface destructive force A force that slowly wears away mountains and other features on the surface of Earth continent a great landmass surrounded by oceans seismic wave A vibration that travels through Earth carrying the energy released during an earthquake. pressure the amount of force pushing on a surface or area crust the layer of rock that forms Earth's outer surface basalt A dark, dense, igneous rock with a fine texture, found in oceanic crust granite A usually light colored rock that is found in continental crust mantle the layer of hot, solid material between Earth's crust and core. lithosphere a rigid layer made up of the uppermost part of the mantle and the crust asthenosphere The soft layer of the mantle on which the lithosphere floats. outer core a layer of molten iron and nickel that surrounds the inner core of the Earth inner core a dense sphere of solid iron and nickel at the center of Earth heat transfer the movement of energy from a warmer object to a cooler object radiation the transfer of energy through empty space conduction the transfer heat by direct contact of particles of matter convection the transfer of heat by movements of a heated fluid density the amount of mass in a given space; mass per unit volume convection current the movement of a heated fluid, caused by differences in temperature, that transfers heat from one part of the fluid to another. sea-floor spreading the process by which molten material adds new oceanic crust to the ocean floor deep-ocean trenches a deep valley along the ocean floor through which oceanic crust slowly sinks towards the mantle subduction the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary plates a section of the lithosphere that slowly moves over the asthenosphere, carrying pieces of continental and oceanic crust scientific theory a well-tested concept that explains a wide range of observations plate tectonics the theory that pieces of Earth's lithosphere are in constant motion, driven by convection currents in the mantle transform boundaries a boundary where two plates slide past one another in opposite directions divergent boundaries area where two plates move away from each other rift valley a deep valley that forms where two plates move apart convergent boundaries places where two tectonic plates move toward each other