ch 2 plate tectonics
Terms
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- Volcanic island arc, p 60
- Long, curved chain of oceanic islands associated with intense volcanic and seismic activity and orogenic (mountain-building) processes. Volcanic arcs form near subduction zones at convergent plate boundaries where oceanic crust converges on oceanic crust. Prime examples of this form of geologic feature include the Aleutian-Alaska Arc and the Kuril-Kamchatka Arc.
- Transform fault boundary, p 56
- In plate tectonics, a transform boundary (also known as transform fault boundary, transform plate boundary, transform plate margin or conservative plate boundary)occurs when tectonic plates slide and grind against each other along a transform fault. The relative motion of such plates is horizontal.
- Subduction zone, p 58
- Zone of deformation at convergent plate boundaries where oceanic crust is involved. Because one plate is usually lighter and thicker than the other plate, the thinner denser oceanic plate is driven down into the mantle. Subduction zones are areas of the world where some of the most intense earthquakes and volcanism occurs. Subduction zones are ocean-floor consuming zones.
- Spreading center, p 57
- A general term for an elongated region (above sea-level or below sea-level) where two plates are being forced away from each other. New crust is formed as molten material is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.
- Slab subduction, p 70
- The process in which one lithospheric plate (a slab) collides with and is forced under another plate and drawn downward into the Earth's mantle.
- Slab-pull, p 70
- A gravity-driven driving force for plate motion caused by the pulling and tugging of an oceanic plate, which occurs as a slab subducts. For this reason, gravity might be a driving force for plate motion because gravity, in part, causes a dense plate to subduct.
- Seafloor spreading, p 48
- Sea floor spreading is the process by which new ocean floor is formed by lava erupting from an underwater rift valley, pushing the newly-formed floor outward from the rift. This mechanism, which is a more accurate version of Alfred Wegener's original drift of continents that "plow" through the sea, was proposed by Harry Hess from Princeton University in the 1960s. This phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere.
- Rift (rift valley), p 57
- A place above sea level where the Earth's continental crust (part of the lithosphere)is being pulled apart. Typical features of a rift valley are a central linear downdropped fault segment (a graben), parallel extendional faulting, and rift-flank uplifts on either side. The axis of the rift area commonly contains volcanic rocks; Actiave volcanism is a part of many (but not all) active rift systems. Rifts are distinct from Mid-ocean ridges, where new oceanic crust is formed underwater.
- Ridge push, p 70
- A gravity-driven driving force for plate motion caused by the sliding down of the newly formed oceanic plate off of the elevated oceanic ridge.
- reverse polarity, p 49
- A state of magnetic polarity opposite from that developing presently in rocks forming underwater along mid-ocean ridges where the ocean floors are developing.
- Plate tectonics, p 70
- A theory in geology developed initially to explain the phenomenon of continental drift. The theory deals with the structure and movement of the rigid plates which make up earth’s outer shell. Several major plates and a few minor ones move and interact with one another to produce earthquakes, mountain ranges, volcanic activity and many other features.
- Plate, p 56
- One of about fifteen rigid sections of the Earth's surface; a section of the Earth's surface bordered by seismic activity (earthquakes and volcanoes).
- Partial melting, p 60
- Incomplete melting of a 'parent rock' to produce a molten rock (like magma) with a different chemical composition. This happens because different minerals melt at different temperatures. When partial melting occurs, the minerals with higher melting points remain solid while the minerals whose melting points have been reached dissolve into magma.
- Pangaea, p 37
- Pangaea (Greek for "all lands") is the supercontinent that existed during the Mesozoic era, before the process of plate tectonics separated the continents we recognize today. The name was proposed by Alfred Wegener in 1915. When the continents first came together to form Pangaea about 300 MYA, mountains were formed, and some of these ranges still exist, such as the Appalachians, the Atlas Mountains, and the Urals.
- Paleomagnetism, p 46
- Paleomagnetism refers to the orientation of the Earth's magnetic field as it is preserved in various magnetic iron bearing minerals throughout time. The study of paleomagnetism has demonstrated that the Earth's magnetic field has changed both in orientation and intensity over time.
- Oceanic ridge system, p 48
- An oceanic ridge is an underwater mountain range, usually formed by plate tectonics.
- Normal polarity
- A state of magnetic polarity similar to that developing presently in rocks forming underwater along mid-ocean ridges where the ocean floors are developing.
- Mantle plume, p 66
- A buoyant mass of hot mantle material rising to the base of the lithosphere. Mantle plumes commonly produce volcanic activity and structural deformation in the central part of lithospheric plates.
- Magnetic time scale, p 50
- Just as the geologic time scale records the changes in rock types (and signs of life preserved in the rocks), the magnetic time scale records changes in the magnetic properties of rocks over time. The magnetic time scale traces magnetic reversals over time.
- Magnetometer, p 50
- Instrument detecting changes in the earth's magnetic field. Used by archaeologists and geologists to detect and map historic features and artifacts both in the ground and underwater.
- Island arc, p 60
- A chain of volcanic islands. Island arcs are generally convex toward the open ocean. Island arcs can be volcanic or continental.
- Hot spot, p 66
- A volcanic center, 60 to 120 miles (100 to 200 km) across which persists for at least a few tens of million of years. Hot spots are thought to be the surface expression of a semi-permanent rising plume of hot mantle material. Hot spots are not linked to arcs. As well, they may not be associated with ocean ridges.
- Geomagnetic reversal
- A change in the Earth’s magnetic polarity. When a reversal occurs, the field flips from normal to reversed polarity, or vice versa.
- Fracture zone, p 62
- A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault. A zone of long, linear fractures on the ocean floor, expressed topographically by ridges and troughs. Fracture zones are the topographic expression of transform faults.
- Fossil magnetism (remnant magnetism), p 46
- Also called paleomagnetism, fossil magnetism is the preserved direction of the magnetic poles during the time that any particular rock unit formed. Normal and reversed magnetism is "frozen" in the rocks as iron crystals orient themselves in the cooling magma.
- Divergent boundary, p 56
- In plate tectonics, a divergent boundary (divergent fault boundary or divergent plate boundary) is a linear feature that exists between two tectonic plates where the plates are moving away from each other. While these areas can originally form in the middle of continents, they eventually become the cores of ocean basins. Divergent boundaries within continents initially produce rifts which produce rift valleys.
- Curie point, p 46
- The temperature above which a magnetic material will lose its magnetism. Discovered by the French scientist Pierre Curie in the 1890s.
- Convergent boundary, p 56
- In plate tectonics, a convergent boundary (convergent fault boundary, convergent plate boundary, or active margin) is where two tectonic plates slide towards each other and collide. Three outcomes are possible. Continental crusts colliding form mountain ranges similar to the Himalayas. Ocean crusts colliding form volcanic island arcs like The Aleutian and Tonga Islands. Oceanic-Continental crustal convergence forms a subduction zone with its associated continental island arcs like the Andes Mountains.
- Continental volcanic arc, p 60
- Mountains such as the Andes, which are produced by volcanic activity related to the subduction of oceanic lithosphere. The Cascade Range in Washington, Oregon, and California are also part of a continental volcanic arc.
- Continental drift
- The theory, first advanced by Alfred Wegener, that Earth's continents were originally one land mass. Pieces of the land mass split off and migrated to form the continents.
- asthenosphere, p 53
- a section of the mantle located below the lithosphere. It separates the crust from the rest of the mantle. Because the rock in this area is easy to deform, it's the zone upon which the crustal plates slip and move.