Glossary - Geology of Oregon - page 245

whereas those paralleling stratification are
called sills. Huge masses without definite shape
are called batholiths.
Island arc -Curved chain of islands, often volcanic in
origin, generally convex to the open ocean and
bordered by a deep submarine trench.
Isostasy -Theoretical balance of all large portions of the
earth's crust as though they were floating on a
denser underlying layer; thus areas of less
dense crustal material rise topographically
above areas of more dense material.
Joint -A fracture without appreciable movement parallel
to the plane.
Lahars -A mudflow of volcanically derived debris.
Laterite -Asoil profile formed under tropical conditions
where decomposition has been extreme leaving
very little of the parent rock.
Lava -Molten rock that is extruded upon the earth's
surface. Even after cooling it may be
referred to as a lava flow.
Limb -One side or flank of a fold.
Limestone -A bedded sedimentary deposit consisting
largely of calcium carbonate (CaC03). It may
include fragments of shells composed of
calcium carbonate.
Lineament -A regular large-scale continuation of linear
features visible at the earth's surface.
Lithification -Induration or hardening of soft sediments
to compact rock by cementation and pressure.
Loess -A homogeneous, nonstratified, unindurated
deposit consisting largely of silt. Loess has a
rude vertical parting in many places. Most
loess is thought to be eolian or wind-blown in
origin.
Maar -A crater formed by violent explosion not
accompanied by igneous extrusion. It is
commonly occupied by a small circular lake. A
relatively shallow, flat-floored explosion
crater, the walls of which consist largely or
entirely of loose fragments of the country rock
and only partly of essential, magmalic ejecta.
Maars are apparently the result of a single
violent volcanic explosion, probably from steam
generated when groundwater comes in contact
with hot rock.
Mafic -Dark-colored igneous rocks high in magnesium
and iron and low in silica. Peridotites and
serpentinite are referred to as ultramafic rocks
and are even lower in silica content.
Magma -Molten silicate material in the crust or mantle
from which igneous rocks are derived.
Mantle -Thick layer between the overlying crust and the
underlying core composed of ultrabasic
(ultramafic) rock of a composition much like
peridotite or serpentinite.
Marine dcposits -Any rock laid down in the ocean of
which sedimentary rocks are the most common,
but it also includes submarine volcanic
extrusive rocks.
Matterhorn -A sharply pointed alpine peak formed by
glacial erosion when 3 or more glaciers
meet by headward erosion.
Meander -One of a series of somewhat regular and
looplike bends in the course of a stream,
developed when the steam is flowing at grade

through lateral shifting of its course toward
the convex sides of the original curves.
Melange -Mixture of irregular blocks of rock of diverse
origin and size commonly formed by shearing
during tectonic movements.
Metamorphic rocks -Rocks altered by heat and pressure
causing recrystallization and loss of
original characteristics. Includes those rocks
which have formed in the solid state in
response to pronounced changes in pressure,
temperature, and chemical environment, below
the zone of weathering and cementation.
Mineral -A naturally occurring solid having a definite
atomic structure and chemical composition. It
may have characteristic color, cleavage, luster
or hardness.
Moraine -Debris left by a glacier and composed of
unsorted material called till. Descriptive terms
such as end or terminal moraine, medial
(middle) moraine, lateral (side) moraine
denote relative position of the glacial debris to
the glacier.
Mudflow -Downslope movement of saturated weathered
rock and soil. Sometimes applied to wet
volcanic ash that moved down slope.
Obsidian -Volcanic glass formed by quick cooling of
high-silica lava.
Olistolith -Olistostromes-Large chaotic rocks or rock
masses that have broken from a marine and
coastal headland and slid whole downslope into
deep water.
Ophiolites -An ocean crust sequence of rocks character-
ized by basal ultramafics overlain by gabbro,
dikes, pillow lavas, and deep-sea clay and
chert sediments.
Ore -Minerals or aggregrates of minerals (usually
metalliferous) that may become ores with
changing economic conditions.
Orogeny -Mountain building associated with uplift,

folding and faulting.
Outcrop -An exposure of bedrock often in roadcuts,
along streams or in cliffs.
Outwash -Glacial-fluvial material carried by streams
from the glaciers.
Overthrust -Rock shoved over another rock mass along
a low angle (thrust) fault. Some thrusts have
been traced for miles.
Pahoehoe -A Hawaiian term for lava flows with a ropy,
somewhat smooth surface, which can be
contrasted with the rough blocky surface of aa
flows.
Peneplain -A land surface worn down by erosion to a
nearly flat or broadly undulating plain. The
surface may be uplifted to form a plateau and
become subjected to renewed erosion.
Peridotite -A coarse-grained ultramafic rock consisting
of olivine and pyroxene with accessory minerals
It is thought to make up much of the mantle,
and when altered is called serpentinite.
Period -A major, worldwide, geologic time unit
corresponding to a system such as the
Cambrian Period.
Phyllite -A medium-grade metamorphic rock with planar
structure (foliation).
Pillow basalt -Volcanic extrusion of lava underwater