The scientific results of the mapping phase of the mission will be reported by Magellan scientists in two ways: at press conferences (held at JPL), which are currently planned to begin in September 1990 and which will continue periodically throughout the 8-month mapping cycle, and eventually in articles published in science journals and popular science magazines.
The science results will focus primarily on the nature of the surface features of Venus and will necessarily include some terminology that is unfamiliar to those of us not educated in the field of geology. We hope the following list of basic geological terms will help decode some of the jargon. Within the definition of an entry, items set in boldface type are entries in this Glossary.
aa A basaltic lava flow with a rough, jagged surface. abyssal hill A low, rounded submarine hill, with a relief of about 150 meters (492 feet), common in deep ocean basins. angle of repose The slope at which unconsolidated material remains stable. anticline A convex, upward-folded rock structure, with older rocks in the core and limbs that dip away from the fold axis. arachnoids Spider-and-cobweblike features; 100-kilometer- (62-mile-) diameter circular structures on Venus, with a central volcanic feature surrounded by a complex network of lineaments. asthenosphere A worldwide layer below the lithosphere, composed of partially molten or liquid rock where convection may take place. basalt Fine-grained igneous rock (rich in mafic minerals) that has erupted onto the surface. basement The oldest rocks in a given area. bedrock Continuous solid rock that underlies regolith and is exposed at outcrops. breccia Coarse-grained rock composed of angular fragments of preexisting rock. caldera A large volcanic depression at the summit of a volcano, caused by collapse or explosion. cinder cone A conical hill formed of volcanic cinders. compensation A mechanism by which segments of the crust rise or (isostasy) sink to equilibrium positions, depending on the mass and density of the rocks above and below a certain depth called the depth of compensation. convection A mechanism of heat transfer from the interior to the exterior of a medium, in which hot material rises, because of its lower density, and cooler material sinks. convergence zone A band along which moving plates collide and area is lost either by shortening and crustal thickening or by subduction and destruction of crust. Convergence zones are sites of earthquakes, volcanism, trenches, and mountain building. corona A 170- to 1,000-kilometer- (106- to 621-mile-) diameter circular-to-elongate Venusian feature surrounded by multiple concentric ridges, thought to be formed by hot spots. crater An abrupt circular depression formed by extrusion of volcanic material and its deposition in a surrounding rim, or by explosive ejection of material on meteorite impact. cross-cutting The principle that a rock is younger than any rock relationship across which it cuts. principle crust The outermost layer of the lithosphere. crustal spreading A mechanism by which new crust is created at ridges in divergence zones and adjacent plates plates move apart to make room. degradation A general lowering of the surface by processes of erosion. differentiated A planet where heavier materials have sunken to planet the center and lighter materials have accumulated in the crust. dike A roughly planar body of intrusive igneous rock. dip The angle that a surface makes with the horizontal, measured perpendicular to the strike. discontinuity A physical interruption in sequence or distribution of strata (layers of rock. divergence zone A belt along which plates move apart and new crust and lithosphere are created. Divergence zones are sites of midocean ridges, earthquakes, and volcanism. ductile Capable of considerable deformation or change without breaking. dune An elongate mound of sand formed by wind or water. en echelon A steplike arrangement of features. endogenic Of or relating to a geologic process originating within a planet. ejecta Material thrown out of a volcano or impact crater. eolian Related to wind deposits and associated effects. fault A fracture or zone of fractures in a planet's crust, accompanied by displacement of the opposing sides. Faults are classified according to the direction of relative movement: (1) normal A hanging wall has moved down relative to a footwall. (2) reverse A hanging wall has moved up relative to a footwall. (3) thrust A low-angle reverse fault where the dip of the fault plane is below 45 degrees. (4) strike-slip Movement is parallel to the strike of the fault. (5) transform A special type of strike-slip fault forming the boundary between two moving lithospheric plates, usually along an offset segment of the oceanic ridge. flood basalt Extensive, high-volume basaltic lava flows erupted from fissures. fluvial Relating to a river or rivers. fold The product of the deformation of planar rock bodies. footwall A block beneath a dipping fault surface. fracture zone A zone of long, linear fractures expressed topographically by ridges and troughs; the surface expression of a transform fault. graben A depressed, elongate crustal block bounded by normal faults along its sides and produced by extensional forces. granite Coarse-grained intrusive or plutonic igneous rock composed mostly of quartz and feldspar. gravitational A process by which rocks behave ductilely and flow relaxation on relatively short geologic time scales (hundreds of millions of years), resulting in the lowering of topographic relief. greenhouse The heating of the atmosphere by the absorption of effect infrared energy reemitted by a planet as it receives light energy in the visible band from the Sun. hanging wall A block above a dipping fault surface. horst An uplifted, elongate crustal block bounded by reverse faults along its sides. hot spot A persistent volcanic center thought to be the surface expression of a rising hot mantle plume. igneous rock Rock solidified from a molten state. ignimbrite Igneous rock formed by widespread deposition and welding of ash flows. intrusion An igneous rock body that, when in a molten state, forced its way into the surrounding rock. lineament A linear feature that may depict crustal structure. lithosphere The relatively strong outer layer of a planet that includes the crust and part of the upper mantle. mafic Relating to rock or magma comparatively rich in iron and magnesium silicates. magma Molten rock material (liquids and gases). mantle The main bulk of a planet between the crust and the core; on Earth, the mantle ranges from about 40 to 2,900 kilometers (25 to 180 miles) below the surface. mare A dark, low-lying lunar plain, filled to some depth with volcanic rocks. melange A formation consisting of a heterogeneous mixture of rock materials intermingled and consolidated by tremendous deformational pressure. meteorite A stony or metallic object from interplanetary space that impacts a planetary surface. multiringed A large impact crater containing a series basin of concentric ridges and depressions (e.g., the Orientale Basin on the Moon). orogeny The process of mountain building. pahoehoe A basaltic lava flow with a smooth, undulating surface. partial melting The process by which minerals with low melting points liquify within a rock body as the result of an increase in temperature and/or a decrease in pressure, while other minerals in the rock body are still solid. plate A broad segment of the lithosphere (the rigid upper mantle plus the crust) that floats on the underlying asthenosphere and moves independently of other plates. plate tectonics The theory and study of plate formation, movement, interaction, and destruction. This theory attempts to explain volcanism, seismic activity (earthquakes), mountain building, and paleomagnetic data in terms of plate motions. plume (hot spot) A rising, buoyant mass of hot, partially molten mantle material that rises to the base of the lithosphere. pluton A large igneous rock intrusion formed at depth in the crust. pyroclast Fragmental material ejected by a volcanic eruption. regolith Any solid material lying on top of bedrock, including soil and rock fragments. relative age/ The age of a rock or event compared with those of relative dating other rocks or events without reference to years; a geologic determination based on superposition and cross-cutting relationships. relief The maximum regional difference in elevation. rheology The physical properties that govern the flow characteristics of solid material. rift A valley formed at a divergence zone or other area of extension. scarp A cliff or steep slope of some extent that may form a marked topographic boundary. shield volcano A broad volcanic cone with gentle slopes constructed of successive nonviscous, mostly basaltic, lava flows. silicic Relating to rock or magma comparatively rich in aluminum and potassium silicates. spreading See crustal spreading. center strike The horizontal direction of a structural surface. subduction The process of one lithospheric plate descending beneath another. superposition The principle that, except in extremely deformed principle rock, a rocky unit that overlies another rocky unit is always younger. syncline A concave folded rock structure with younger rocks in the core and limbs that dip toward the fold axis. talus A deposit of large, angular rock fragments of eroded bedrock at the base of a cliff or steep slope. tectonic Pertaining to structural and deformational features in a planet'ss crust and to the forces that produce such features. terrain A physical region or feature. terrane A region where a particular rock or rock group predominates. tesserae Complex, deformed terrain on Venus consisting of at least two sets of intersecting ridges and troughs. topography The shape and form of the surface of a planet. trough A long linear depression. vent An opening or fissure in a planet's surface through which volcanic material erupts. viscosity A measure of resistance to flow. volcanic rock Rock formed by eruption in a planet's surface.
F. Press and R. Siever, Earth, San Francisco, W. H. Freeman and Company, 1978.
W. K. Hamblin, The Earth's Dynamic Systems, Minneapolis, Burgess Publishing Company, 1985.
American Geological Institute, Dictionary of Geological Terms, New York, Anchor Press, 1976.
Chapter 17 - Acknowledgement
The Magellan's Venus Explorer Guide