Skip Navigation: Avoid going through Home page links and jump straight to content

Chapter 16
Glossary of Geological Terms


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

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.

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
                  (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.

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 

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.

right_arrow.gif Chapter 17 - Acknowledgement
magellan_icon.gif The Magellan's Venus Explorer Guide