Asthenosphere: The viscous and mechanically weak region of the Earth’s upper mantle just below the lithosphere.
Basalt: A common magnesium- and iron-rich igneous rock.
Continental Crust: The continental crust is typically ~20-30 miles (~30-50 km) thick and is mostly made of slightly less dense rocks, such as granite, than those of the oceanic crust.
Convection: The transfer of heat by the motion of heated parts. In the Earth’s mantle, density-driven flow moves hot, thermally-expanded and buoyant material upwards, and cool, thermally-contracted material downwards.
Convergent Boundary: An actively deforming region where two (or more) tectonic plates move toward one another and collide. As a result of pressure, friction, and plate material melting in the mantle, earthquakes and volcanoes are common near convergent boundaries. When two plates move towards one another, they form either a subduction zone or a continental collision.
Core: The innermost part of the Earth that is primarily composed of an iron-nickel alloy. The outer portion is liquid, while the inner core is solid. Its outer boundary lies ~1800 miles (~2900 km) beneath the Earth’s surface. The inner core-outer core boundary is located ~3200 miles (~5150 km) beneath the Earth’s surface.
Core-mantle boundary: Lies between the Earth’s mantle and its liquid iron-nickel core at ~1800 miles (~2900 km) depth beneath the Earth’s surface. The boundary is observed via a sharp change in seismic wave velocities at that depth.
Crust: The outermost solid shell of a rocky planet, which is chemically distinct from the underlying mantle.
Density: Mass per unit volume. The density of a material varies with temperature and pressure. Increasing the pressure on an object decreases the volume of the object and therefore increases its density. Increasing the temperature of a substance generally decreases its density by increasing the volume of that substance.
Divergent Boundary: A boundary between two tectonic plates moving away from each other. Most active divergent plate boundaries occur between oceanic plates and exist as mid-ocean ridges. Over millions of years, tectonic plates move many hundreds of kilometers away from both sides of a divergent plate boundary. Because of this, rocks closest to a boundary are younger than rocks further away on the same plate.
Earthquake: An earthquake occurs when two tectonic plates slip past each other resulting in a sudden release of energy that creates seismic waves. Earthquakes are measured using observations from seismometers. All else being equal, the shallower an earthquake, the more damage to structures it causes.
Elasticity: A measure of how a material reversibly deforms under stress.
Epicenter: the point on the surface of Earth directly above the point where an earthquake originates (focus) or where the fault begins to rupture. Usually this is also the area of greatest damage.
Fault: A planar fracture in a rock, across which there has been significant movement. Large faults within the Earth’s crust result from the action of plate tectonic forces. Energy release associated with rapid movement on active faults is the cause of most earthquakes.
Focus: Where an earthquake originates.
Granite: A common type of igneous rock, which is granular and crystalline in texture consisting mainly of quartz and feldspar minerals. Granite is usually found in continental crust.
Hotspots: Volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. Well known examples include Hawai’i or Yellowstone.
Igneous: Igneous rock is formed through the cooling and solidification of magma or lava. The magma is derived from melting of rocks in either the mantle or crust. Typically, one or more processes cause melting: an increase in temperature, a decrease in pressure, or a change in composition.
Inner Core: The innermost, hottest part of the Earth and as detected by seismological studies, is primarily solid and about 760 miles (~1220 km) in radius. It is believed to be made of an iron-nickel alloy.
Island Arc: A chain of volcanic islands, whose alignment is arc-shaped, and situated parallel and close to the boundary between two, converging tectonic plates.
Lava: Refers both to molten rock erupted by a volcano and the resulting rock after solidification and cooling.
Lithosphere: The rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.
Magma: Molten or semi molten rocks found beneath the surface of the Earth. Besides molten rock, magma may also contain suspended crystals, dissolved gas and sometimes gas bubbles. Magma often collects in magma chambers that may feed a volcano or turn into a pluton. Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as lava, and as explosive ejecta.
Magnetic Field: The Earth’s magnetic field extends from the Earth’s core outward. It is approximately the field of a magnetic dipole (like a bar magnet at the center of the Earth) tilted at an angle of 11 degrees with respect to the rotational axis. However, unlike the field of a bar magnet which doesn’t change with time, Earth’s field changes over time because it is generated by the motion of fluid iron alloys in the Earth’s outer core, called the geodynamo. At random intervals (averaging several hundred thousand years) the Earth’s magnetic field reverses (the north and south magnetic poles change places with each other). These reversals leave a record in rocks that allow geologists to calculate past motions of continents and ocean floors as a result of plate tectonics.
Magnetic Stripes: The past record of geomagnetic field reversals was observed by the magnetic “stripe” anomalies on the ocean floor. Minerals in the “stripes” record the prevailing geomagnetic field direction at the time of their formation. The “stripes” on one side of the mid-ocean ridge are the mirror image of those on the other side and the magnetic variation in successive bands of ocean floor parallel with mid-ocean ridges is important evidence supporting the theory of seafloor spreading, central to plate tectonics.
Mantle: Earth’s mantle is a rocky shell about 1800 mi (~2900 km) thick and constitutes about 84% of Earth’s volume. It is predominantly solid although past episodes of melting and volcanism at the shallow depths produced a thin crust.
Mantle Plume: A hypothetical thermal upwelling of abnormally hot rock that starts at the core-mantle boundary and rises through the Earth’s mantle. Some of these volcanoes lie far from tectonic plate boundaries (e.g., Hawai’i), whereas others represent unusually large-volume volcanism on plate boundaries (e.g., Iceland). The currently active volcanic centers are known as hot spots.
Mid-ocean ridges: An underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is responsible for seafloor spreading and the growth of new oceanic crust. A mid-ocean ridge marks the boundary between two tectonic plates, and is a divergent plate boundary.
Oceanic Crust: The oceanic crust is ~3-6 miles (~5-10 km) thick and is made of primarily of dense rocks such as basalt.
Outer Core: The outer core of the Earth is a liquid layer about 1408 miles (~2266 km) thick primarily made of an iron-nickel alloy which lies above the Earth’s solid inner core and below its mantle. Its outer boundary lies ~1800 miles (~2890 km) beneath Earth’s surface.
Pangea: A supercontinent that formed ~300 million years ago and began to rift ~200 million years ago, before the component continents were separated into their current configurations.
Plate Tectonics: A scientific theory that describes the large-scale motions of Earth’s lithosphere. The theory builds on the concepts of continental drift, developed during the first decades of the 20th century. The geoscientific community accepted it after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.
Pluton: A body of intrusive igneous rock that crystallized from magma slowly cooling within the crust.
Primordial Heat: The heat or energy that resulted from the formation of the Earth.
Radioactive decay: The process by which the nucleus of an unstable atom loses energy by emitting ionizing particles. A decay, or loss of energy, results when an atom with one type of nucleus, called the parent radionuclide, transforms to an atom with a nucleus in a different state, or to a different nucleus containing different numbers of nucleons, called the daughter radionuclide.
Ring of Fire: An area where a large number of earthquakes and volcanic eruptions occur surrounding the Pacific Ocean. In an ~25,000 mile (~40,000 km) horseshoe shape, it is associated with a nearly continuous series of trenches, island arcs, and volcanism. The Ring of Fire is home to more than 75% of the world’s active and dormant volcanoes.
Seafloor Spreading: Process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
Seismic waves: Waves of energy that travel through the Earth, and are a result of an earthquake, explosion, or a volcano. The speed of the waves depends on density and elasticity of the rock it travels through.
Seismometer: Instruments that measure ground motion generated by earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of the earthquakes.
Strike-slip fault: A usually nearly vertical fault that moves either left or right or laterally with very little vertical motion. A special class of strike-slip faults is the transform fault, which form plate boundaries.
Subduction: Process that takes place at convergent boundaries by which one tectonic plate sinks under another tectonic plate, as the plates converge. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately 2 to 8 centimeters per year (about the rate fingernails grow).
Subduction zone: Where two tectonic plates move towards one another and one sinks under the other.
Supercontinent: A landmass composed of more than one continents or cratons.
Tectonic plate: Pieces of the Earth’s crust and uppermost mantle, together referred to as the lithosphere. The plates range in thickness and composition. Tectonic plates composed of oceanic crust are ~3-6 miles (5-10 km) thick and of basaltic composition, whereas tectonic plates composed of continental crust are ~18-28 miles (~30-45 km) thick and less dense in composition.
Transform Boundary: Also known as conservative plate boundary as these faults neither create nor destroy lithosphere. Transform faults end abruptly and are connected on both ends to other faults, ridges, or subduction zones. While most transform faults are hidden in the deep oceans where they form a series of short zig-zags accommodating seafloor spreading, they can also be found on land at the margins of tectonic plates.
Trench: Created by the tectonic plate motion at subduction zones. An example is the Mariana Trench, which is deep ~6 miles (~10 km), linear and formed by subduction.
Tsunami: A series of water waves caused by the displacement of a large volume of water, caused by disturbances such as earthquakes, volcanic eruptions, underwater explosions, landslides, glacier calvings and meteorite impacts. Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. Wave heights of tens of meters can be generated by large events.
Volcano: An opening in a planet’s crust, which allows hot magma, volcanic ash and gases to escape from below the surface. Volcanoes are generally found at divergent or convergent boundaries. Volcanoes can also form where there is stretching and thinning of the crust in the interiors of plates. Volcanism away from plate boundaries has also been explained as mantle plumes, often called “hotspots“, such as Hawaii.