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.
Sources:
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
