| 'A'a: | A Hawaiian term for basaltic lava
flows that are typically rough and jagged with a clinkery
surface. |
| Agglutinate: | Welded, ejected material
characterized by glassy material binding the pyroclasts (ejected
particles), or "sintered" glassy pyroclasts. |
| Ash: | Fine, 4.0 mm (.16 in) to 0.25mm (.01
in) in diameter, unconsolidated pyroclastic (ejected particles)
material. |
| Basalt: | A dark colored, hard, volcanic rock
containing 45% to 54% silica rich minerals (SiO2) which
results in low viscosity (resistance to flow). Basalt is erupted at
temperatures between 2012 °F and 2282 °F. Common minerals in
basalt include olivine, pyroxene, and plagioclase. |
| Blister: | A blister is formed by the swelling
of the crust that occurs as a result of the expansion of gas or vapor
beneath a flow; typically about 1 meter (3.3 ft) in diameter and
hollow. |
| Block: | A
solid pyroclastic fragment greater than 64 mm (2.5 in) and up to several
meters in diameter, that was ejected from a volcano during an explosive
eruption. |
| Bomb: | Pyroclastic fragments greater than
64mm (2.5 in) in diameter, that were molten or plastic at the time of
ejection. The shape of a bomb is determined by the viscosity of the
magma, velocity and length of flight, the rate at which the lava cooled,
the rate of expansion of gases, and the type of deformation that
occurred upon impact. |
| Breadcrust
bomb: | Breadcrust bombs are characterized by a crust that
cooled during flight and as gases within it continued to expand the
crust cracked much like bread rising in an oven. |
| Cinder: | Uncemented, glassy, vesicular (holes
created by escaping gas bubbles), pyroclastic material. Cinder can be
thought of as "volcanic froth". |
| Cinder
cone: | A steep, conical hill that is formed by the
accumulation of cinders, spatter, and other pyroclastic
material. |
| Cow-pie
bombs: | Cow-pie bombs, also known as cow-dung and pancake
bombs, form from very fluid lava that is still plastic when it lands
causing it to flatten upon impact; some still have a liquid core upon
impact. |
| Crater: | A circular depression in a volcano
that formed from a gradual accumulation of pyroclastic material around
the vent, an explosive eruption, or collapse. |
| Dike: | A
dike is tabular, intrusive, igneous rock that cuts across the layering
of adjacent rock, intrudes into an existing fracture, or creates a new
fracture by forcing its way through pre-existing rock. |
| Fault: | A
fracture in the Earth's crust where there has been displacement of one
side relative to the other. |
| Fissure/vent: | An elongate fracture or crack
at the surface from which molten rock and volcanic gases escape onto
surface. Eruptions from fissures typically dwindle to a central vent
after a period of hours or days. An opening or crack in the Earth's
crust from which material is ejected or flows. |
| Hornito: | A rootless spatter cone (fed by
lava from within an underlying lava tube) that has a steep sided,
inverted cone shape and is formed from an accumulation of pyroclastic
materials. |
| Inflation
structure: | A inflation structure occurs along a crack where
swelling of underlying lava causes one side to become uplifted relative
to the other, whether due to degassing or influx of more lava. |
| Kipuka: | A mound of older land, usually
covered by vegetation, that is surrounded by a younger lava
flow. |
| Lava: | Lava is magma that has erupted onto
the Earth's surface and or molten rock on the surface; also used to
refer to magma after it has solidified. |
| Lava
curb: | Lava curbs form when blobs of lava floating in a river
of lava accumulate on the edges of the flow and begin to build out. If
the curbs build out far enough on either side to connect to each other
and create a crust, they create a new lava tube roof. |
| Lava
flow: | A lava flow can be described as an outpouring of
molten rock onto the Earth's surface forming a river or sheet. |
| Lava fountains: | A vertical eruption of lava
from a vent or along a fissure. Lava fountains can reach a height of
2000 ft. |
| Lava
toe: | Small, bulbous extensions of lava that form at the
front of pahoehoe flows by breaking through crusts on the flow
front. |
| Lava
tube: | Lava tubes form when the surface flowing lava congeals
forming a crust. The lava underneath the solidified crust continues to
flow, now insulated from the cooling air. When the lava eruption
ceases, and if the tube drains, a large tubular cave may be left
behind. |
| Loess: | Unconsolidated, silt-sized particles
with accessory clay and sand particles that are deposited primarily by
the wind. Loess that has filtered down into cracks in the lava and
between the cinders provides the growth medium for vegetation. |
| Magma: | Magma is molten rock beneath the
Earth's surface. |
| Pahoehoe: | A Hawaiian term for a basaltic
lava flow that has a smooth, billowy, or ropy surface. |
| Pit
crater: | Also known as a volcanic sink, is a circular-shaped
depression with steep to vertical walls that formed by collapse of the
ground that results from the removal of support such as from the
withdrawal of the underlying magma. |
| Pressure
plateau: | A pressure plateau forms from a sill-like injection
of new lava beneath the crust of an earlier flow that has not completely
solidified. |
| Pressure
ridge: | Elongated uplift of the congealing crust of a lava
flow believed to be caused by the pressure of the underlying, still
flowing, lava. |
| Pumice: | Pumice is a light colored, frothy
volcanic rock having the composition of rhyolite. It is often buoyant
enough to float on water. |
| Pyroclastic: | Pyroclastic is a term that
refers to volcanic rock material that is formed by a volcanic explosion
or by ejection from a volcanic vent. |
| Rafted
block: | A volcanic fragment that was caught up in a lava flow
and detached from its source, such as a piece of crater-wall carried off
much like an iceberg. |
| Ribbon
bombs: | Ribbon bombs are strands of fluid lava ejected from a
vent that take the shape of thin twisted "ribbons". |
| Shelly
pahoehoe: | A type of pahoehoe lava with a surface that
consists of broken blisters and small open lava tubes. In the Craters
of the Moon lava field surface crusts are typically about 10 cm (3.9 in)
thick. |
| Shield
volcano: | A broad, gently sloping volcano that has a
flattened dome shape, not unlike that of a knights shield. Shield
volcanoes usually cover a large area and form from overlapping and
interfingering, low viscosity, lava flows. |
| Slabby
pahoehoe: | A type of pahoehoe with a surface that consists of
a jumbled arrangement of jagged plates, or slabs, of pahoehoe that were
rafted, sheared, tilted, upturned, overturned and heaped on each
other |
| Spatter: | An accumulation of very fluid
pyroclasts (ejected material). |
| Spatter
cone: | A spatter cone is a low, steep sided cone formed from
the accumulation of spatter ejected from a vent or fissure. |
| Spatter
rampart: | A broad, elongate embankment of spatter that is
built by a curtain of fire and forms along either side of a
fissure. |
| Spindle
bomb: | Volcanic bombs with a twisted shape; spindle bombs
form from blobs of fluid lava that often take on a smooth stoss side
(front side), a rougher lee side (backside) marked by ribs and fluting
caused by frictional resistance to air, and have prominent, usually
twisted, projections on either side that form as ribbon bombs
separate. |
| Spiny
pahoehoe: | A type of pahoehoe with a surface that consists of
elongate vesicles that formed from stretching of very viscous lava,
giving it a surface texture of small ridges or spines. |
| Squeeze
up: | A bulbous blob of viscous, molten lava that was forced,
by pressure, up through a fracture or opening in solidified
lava. |
| Tachylite: | A black, green or brown volcanic
glass that forms when basaltic magma is rapidly chilled. |
| Tension
fractures: | Tension fractures result from stresses that pull
rocks apart. |
| Tree
mold: | A tree mold or lava tree forms when lava flows around
a tree and chills, leaving behind a "mold" of the space occupied by the
tree, or impression of the charred wood. Tree molds can also be
horizontal if the tree was knocked down by the lava flow. |
| Tumulus: | A tumulus is a dome or mound shaped
structure on the crust of a lava flow caused by pressure from the
difference in rates of flow beneath the crust. Unlike a volcanic
blister a tumulus is a solid structure. |
| Vesicles: | A small cavity in a volcanic rock
that was formed by the expansion of escaping gas bubbles. |
| Volcanic rift
zone: | An elongate system of crustal fractures associated
with underlying dike complexes. |
| Volcano: | A vent in the Earth's surface
through which magma and associated gases and ash erupt. The structure
produced by ejected material. |
| Xenolith: | An inclusion of a foreign body of
rock in an igneous rock. |