caldera cinder cone dacite debris avalanche dike dome dome lava fountain effusive eruption eruption cloud eruption size fault fissure fumarole gas geyser hornito hummock kipuka lahar lapilli lava lava cascade lava channel lava delta lava drapery lava flow lava fountain lava fountain dome lava lake lava spillway lava surge lava tube limu littoral cone maar magma mineral mud volcano obsidian pahoehoe pahoehoe blister pahoehoe coil pahoehoe entrail pahoehoe - ropy pahoehoe toe Pele's hair Pele's tears phreatic eruption pillow lava plinian eruption pit crater pumice pyroclastic flow reticulite rhyolite rift zone rock rootless shield scoria shield volcano spatter spatter cone and rampart spatter and cinder cone stratovolcano Strombolian eruption tephra tree mold tsunami tumulus vent volcanic dome volcano volcanic rocks volcanic gases Vulcanian eruption caldera cinder cone hornito maar mud volcano shield volcano spatter cone stratovolcano volcanic dome

1. Volcano Terms
With pictures

2. There are three main types of volcano.

3. Shield volcano
Volcanoes with broad, gentle slopes and built
by the eruption of fluid basalt lava are called
shield volcanoes. Basalt lava tends to build
enormous, low-angle cones because it flows
across the ground easily and can form lava
tubes that enable lava to flow tens of
kilometers from an erupting vent with very
little cooling. The largest volcanoes on Earth
are shield volcanoes. The name comes from
a perceived resemblance to the shape of a
warrior's shield.
Photograph by D. Little (date unknown). View
of the NNW flank of Mauna Loa Volcano
from the south side of Mauna Kea Volcano,
Hawai`i; both are shield volcanoes.

4. Mount Etna, Italy
This cinder cone on the flank of Mount Etna
is surrounded by a younger basaltic lava flow.
Cinder cones usually erupt lava flows, either
through a breach on one side of the crater or
from a vent located on a flank. Lava rarely
issues from the top (except as a fountain)
because the loose, non cemented cinders are
too weak to support the pressure exerted by
molten rock as it rises toward the surface
through the central vent.

5. Composite or
Stratovolcano
Steep, conical volcanoes built by the eruption of
viscous lava flows, tephra, and pyroclastic flows,
are called stratovolcanoes. Usually constructed
over a period of tens to hundreds of thousands
of years, stratovolcanoes may erupt a variety of
magma types, including basalt, andesite, dacite,
and rhyolite. All but basalt commonly generate
highly explosive eruptions. A stratovolcano
typically consists of many separate vents, some
of which may have erupted cinder cones and
domes on the volcano's flanks. A synonym is
composite cone.
Photo: Mount Mageik volcano viewed from the
Valley of Ten Thousand Smokes, Katmai National
Park and Preserve, Alaska. Mageik's broad
summit consists of at least four separate
structures built above different vents.
Photograph by R. McGimsey on 15 July 1990

6. Composite Volcanoes
("Stratovolcanoes")
Composite volcanoes are built by multiple eruptions,
sometimes recurring over hundreds of thousands of years,
sometimes over a few hundred. Andesite magma, the most
common but not the only magma type, tends to form
composite cones. Although andesitic composite cones are
built mostly of fragmental debris, some of the magma
intrudes fractures within the cones to form dike or sills. In
this way, multiple intrusive events build a structural
framework of dikes and sills that knits together the
voluminous accumulation of volcanic rubble. Such a
structure can stand higher than cones composed only of
fragmental material. Composite cones can grow to such
heights that their slopes become unstable and susceptible
to collapse from the pull of gravity.
Famous examples of composite cones are Mayon Volcano,
Philippines, Mount Fuji in Japan, and Mount Rainier,
Washington, U.S.A. Some composite volcanoes attain two to
three thousand meters in height above their bases. Most
composite volcanoes occur in chains and are separated by
several tens of kilometers. There are numerous composite
volcano chains on earth, notably around the Pacific rim,
known as the "Rim of Fire".
St. Augustine volcano, Alaska. Composite cone. Photograph
by Harry Glicken.

7. Volcanic ash
Volcanic ash consists of rock, mineral, and
volcanic glass fragments smaller than 2 mm
(0.1 inch) in diameter, which is slightly larger
than the size of a pinhead. Volcanic ash is not
the same as the soft fluffy ash that results
from burning wood, leaves, or paper. It is
hard, does not dissolve in water, and can be
extremely small--ash particles less than 0.025
mm (1/1,000th of an inch) in diameter are
common. Ash is extremely abrasive, similar
to finely crushed window glass, mildly
corrosive, and electrically conductive,
especially when wet.
Photograph by D.E. Wieprecht

8. Basalt
Basalt is a hard, black volcanic rock with less
than about 52 weight percent silica (SiO2).
Because of basalt's low silica content, it has a
low viscosity (resistance to flow). Therefore,
basaltic lava can flow quickly and easily move
>20 km from a vent. The low viscosity
typically allows volcanic gases to escape
without generating enormous eruption
columns. Basaltic lava fountains and fissure
eruptions, however, still form explosive
fountains hundreds of meters tall. Common
minerals in basalt include olivine, pyroxene,
and plagioclase. Basalt is erupted at
temperatures between 1100 to 1250° C.

9. Andesite
Andesite is a gray to black volcanic rock with
between about 52 and 63 weight percent silica
(SiO2). Andesites contain crystals composed
primarily of plagioclase feldspar and one or
more of the minerals pyroxene (clinopyroxene
and orthopyroxene) and lesser amounts of
hornblende. At the lower end of the silica range,
andesite lava may also contain olivine. Andesite
magma commonly erupts from stratovolcanoes
as thick lava flows, some reaching several km in
length. Andesite magma can also generate
strong explosive eruptions to form pyroclastic
flows and surges and enormous eruption
columns. Andesites erupt at temperatures
between 900 and 1100° C.
Close view of andesite lava flow Brokeoff
Volcano, California More about volcanic and
plutonic rocks.

10. Arching fountain
Instead of shooting lava vertically from a vent,
an arching lava fountain sends lava upward and
outward. Arching fountains form when the
shape or geometry of an erupting vent forces
the lava outward in a continuous stream of
airborne lava.
Photograph by J.D. Griggs on 25 February 1983.
Arching lava fountain 10-15 m tall on the east
rift zone of Kilauea Volcano, Hawai`i.

11. Block
A volcanic block is a solid rock fragment
greater than 64 mm in diameter that was
ejected from a volcano during an explosive
eruption. Blocks commonly consist of
solidified pieces of old lava flows that were
part of a volcano's cone. This block was
ejected into the air by an explosion caused
by the collapse of an active lava delta at
Kilauea Volcano, Hawai`i. New land built by
lava often slides into the ocean, which
enables seawater to mix with lava and hot
rocks. Such violent mixing may trigger steam
explosions that can blast hot rocks 10 to 50
cm in diameter more than 50 m inland.
Photograph by C. Heliker on January 26, 1988

12. Bomb
Volcanic bombs are lava fragments that were
ejected while viscous (partially molten) and
larger than 64 mm in diameter. Many acquire
rounded aerodynamic shapes during their
travel through the air. Volcanic bombs
include bread-crust bombs, ribbon bombs,
spindle bombs (with twisted ends),
spheroidal bombs, and “cow-dung”; bombs.
Photograph by J.P. Lockwood on July 10,
1982. These basaltic lava bombs were
erupted by Mauna Kea Volcano, Hawai`i.

13. Caldera
A caldera is a large, usually circular depression at
the summit of a volcano formed when magma is
withdrawn or erupted from a shallow underground
magma reservoir. The removal of large volumes of
magma may result in loss of structural support for
the overlying rock, thereby leading to collapse of
the ground and formation of a large depression.
Calderas are different from craters, which are
smaller, circular depressions created primarily by
explosive excavation of rock during eruptions
Photograph by M. Williams, National Park Service,
1977.

14. Cinder cone
A cinder cone is a steep, conical hill of
volcanic fragments that accumulate around
and downwind from a vent. The rock
fragments, often called cinders or scoria, are
glassy and contain numerous gas bubbles
“frozen” into place as magma exploded into
the air and then cooled quickly. Cinder cones
range in size from tens to hundreds of meters
tall.
This cinder cone (Pu`u ka Pele) was erupted
low on the southeast flank of Mauna Kea
Volcano. The cone is 95 m in height, and the
diameter of the crater at the top is 400 m.
Hualalai Volcano in background.
Photograph by J.P. Lockwood on 1 December
1975

15. Volcanic dome
Volcanic domes are rounded, steep-sided
mounds built by very viscous magma, usually
either dacite or rhyolite. Such magmas are
typically too viscous (resistant to flow) to move
far from the vent before cooling and
crystallizing. Domes may consist of one or more
individual lava flows. Volcanic domes are also
referred to as lava domes.
Volcanic dome atop Novarupta vent, Valley of
Ten Thousand Smokes, Katmai National Park and
Preserve, Alaska. The dome was erupted from
the same vent that expelled about 15 km3 of
magma in an enormous explosive eruption in
1912.
Photograph by T.P. Miller in June 1979

16. Debris avalanche
Debris avalanches are moving masses of rock,
soil and snow that occur when the flank of a
mountain or volcano collapses and slides
downslope. As the moving debris rushes down a
volcano and into river valleys, it incorporates
water, snow, trees, bridges, buildings, and
anything else in the way. Debris avalanches may
travel several kilometers before coming to rest,
or they may transform into more water-rich
lahars, which travel many tens of kilometers
downstream.
A debris avalanche rushes down the side of a
volcano to the valley floor. Many such debris
avalanches transform into lahars and travel tens
of kilometers from the volcano. Note horseshoe
shaped crater on volcano's side, which is the
scar created by the avalanche. Sketch and
animation by B. Myers

17. Lahars Triggered by
Melting Snow and Ice
Mount St. Helens, Washington
Dark pathways created by lahars streak the sides of Mount
St. Helens during its catastrophic eruption on May 18, 1980.
The lahars were triggered by the sudden melting of snow
and ice from hot volcanic rocks ejected by the initial
explosive activity and subsequent pyroclastic flows.
Many of the largest and most destructive historical lahars
accompanied eruptions from volcanoes mantled by a
substantial cover of snow and ice. Pyroclastic flows are the
most common volcanic events that generate lahars--even
relatively small pyroclastic flows can quickly melt large
quantities of snow and ice. The hot flowing rock debris
erodes and mixes with snow and ice to form water and
trigger snow avalanches on steep slopes. Lava flows moving
slowly across snow usually do not melt snow and ice rapidly
enough to form large lahars but the eruption of lava
beneath a glacier can result in substantial ponding of water,
which may lead to enormous outpourings of water.
Major, J.J, and Newhall, G.C., 1989, Snow and ice
perturbation during historical volcanic eruptions and the
formation of lahars and floods, Bulletin of Volcanology, v. 52,
p. 1-27.

18. Lahar
Lahar is an Indonesian word for a rapidly
flowing mixture of rock debris and water that
originates on the slopes of a volcano. Lahars
are also referred to as volcanic mudflows or
debris flows. They form in a variety of ways,
chiefly by the rapid melting of snow and ice
by pyroclastic flows, intense rainfall on loose
volcanic rock deposits, breakout of a lake
dammed by volcanic deposits, and as a
consequence of debris avalanches.
A small lahar triggered by rainfall rushes
down the Nima II River near the town of El
Palmar in Guatemala in the image shown
here. The lahar developed on the slopes of
Santiaguito volcano.
Photograph by J.N. Marso on 14 August 1989

19. Effusive eruption
An eruption dominated by the outpouring of lava onto the
ground is often referred to as an effusive eruption (as
opposed to the violent fragmentation of magma by
explosive eruptions). Lava flows generated by effusive
eruptions vary in shape, thickness, length, and width
depending on the type of lava erupted, discharge, slope of
the ground over which the lava travels, and duration of
eruption.
For example, basalt lava may become `a`a or pahohoe, and
flow in deep narrow channels or in thin wide sheets.
Andesite lava typically forms thick stubby flows, and dacite
lava often forms steep-sided mounds called lava domes.
Basalt lava erupts from Pu`u `O`o spatter and cinder cone at
Kilauea Volcano, Hawai`i. Lava spilling from the cone has
formed a series of `a`a lava channels and flows.
Photograph by J.D. Griggs on 31 January 1984

20. Eruption cloud
A cloud of tephra and gases that forms downwind of an
erupting volcano is called an eruption cloud. The vertical
pillar of tephra and gases rising directly above a vent is an
eruption column.
Eruption cloud is often used interchangeably with plume or
ash cloud.
Eruption clouds are often dark colored--brown to gray--but
they can also be white, very similar to weather clouds.
Eruption clouds may drift for thousands of kilometers
downwind and often become increasingly spread out over a
larger area with increasing distance from an erupting vent
(note fan-shaped eruption cloud in photographs at left).
Large eruption clouds can encircle the Earth within days.
Photographs taken by Space Shuttle astronauts about 24
hours after the start of the eruption of Rabaul Caldera. The
eruption column rose to at least 18 km above sea level
where the volcanic ash and gas were blown west to form a
fan-shaped eruption cloud.

21. Volcanic dike
Dikes are tabular or sheet-like bodies of
magma that cut through and across the
layering of adjacent rocks. They form when
magma rises into an existing fracture, or
creates a new crack by forcing its way
through existing rock, and then solidifies.
Hundreds of dikes can invade the cone and
inner core of a volcano, sometimes
preferentially along zones of structural
weakness.
This dike was exposed when a new pit crater
formed in about 1880 A.D. in the northeast
corner of the summit caldera of Mauna Loa
Volcano. The dike is about 1.5 m wide.
Photograph by J.P. Lockwood in March 1983

22. Fault
Faults are fractures or fracture zones in the
Earth's crust along which one side moves
with respect to the other. A fault scarp is a
cliff or steep slope that sometimes forms
along the fault at the surface. There are
many types of faults (for example, strike-slip,
normal, reverse, and thrust faults) ranging in
size from a few tens of meters to hundreds of
kilometers in dimension.
Aerial view toward the NE of the Pu`u
Kapukapu fault scarp (maximum height about
320 m) in the Hilina fault system, south flank
of Kilauea Volcano, Hawai`i. In Hawai`i, these
tall cliffs are called "pali's".
Photograph by D.A. Swanson on 24 June
1971

23. Fissure
In geology, a fissure is a fracture or crack in rock
along which there is a distinct separation;
fissures are often filled with mineral-bearing
materials. On volcanoes, a fissure is an elongate
fracture or crack at the surface from which lava
erupts. Fissure eruptions typically dwindle to a
central vent after a period of hours or days.
Occasionally, lava will flow back into the ground
by pouring into a crack or an open eruptive
fissure, a process called drainback; sometimes
lava will flow back into the same fissure from
which it erupted. Eruptive fissure on southeast
rim of Kilauea caldera, Hawai`i. This eruptive
fissure was active briefly during an eruption in
July 1974. Note prominent spatter ramparts on
right, and subdued rampart on left, built by the
ejection of lava along the fissure. The smooth
texture of the surface on the lip of the fissure
(lower right) is evidence that lava drained back
into the fissure toward the end of the 1974
eruption.
Photograph by S.R. Brantley on 14 August 1998

24. Fumarole
Fumaroles are vents from which volcanic gas
escapes into the atmosphere. Fumaroles may
occur along tiny cracks or long fissures, in
chaotic clusters or fields, and on the surfaces
of lava flows and thick deposits of pyroclastic
flows. They may persist for decades or
centuries if they are above a persistent heat
source or disappear within weeks to months
if they occur atop a fresh volcanic deposit
that quickly cools.
Close view of a fumarole on Kilauea Volcano.
Elemental sulfur vapor escaping from the
fumarole has cooled to form yellow-colored
crystals around its margins.
Photograph by R.L. Christiansen on 27 July
1973

25. Volcanic gas
Magma contains dissolved gases that are
released into the atmosphere during eruptions.
Gases are also released from magma that either
remains below ground (for example, as an
intrusion) or rises toward the surface. In such
cases, gases may escape continuously into the
atmosphere from the soil, volcanic vents,
fumaroles, and hydrothermal systems. The most
common gas released by magma is steam (H2O),
followed by CO2 (carbon dioxide), SO2 (sulfur
dioxide), (HCl) hydrogen chloride and other
compounds. Sulfur dioxide and other volcanic
gases rise from the Pu`u `O`o vent on Kilauea
Volcano, Hawai`i. During periods of sustained
eruption from Pu`u `O`o between 1986 and
2000, Kilauea emitted about 2,000 to 1,000
metric tons of irritating sulfur dioxide gas (SO2)
gas each day. Photograph by K.A. McGee on 19
September 1995

26. Geyser
Most geysers are hot springs that episodically
erupt fountains of scalding water and steam.
Such eruptions occur as a consequence of
groundwater being heated to its boiling
temperature in a confined space (for example, a
fracture or conduit). A slight decrease in
pressure or an increase in temperature will
cause some of the water to boil. The resulting
steam forces overlying water up through the
conduit and onto the ground. This loss of water
further reduces pressure within the conduit
system, and most of the remaining water
suddenly converts to steam and erupts at the
surface.
Castle Geyser erupts water and steam,
Yellowstone National Park, Wyoming.
Photograph by S.R. Brantley in September 1983

27. Hornito
A small rootless spatter cone that forms on the
surface of a basaltic lava flow (usually
pahoehoe) is called a hornito. A hornito
develops when lava is forced up through an
opening in the cooled surface of a flow and then
accumulates around the opening. Typically,
hornitos are steep sided and form conspicuous
pinnacles or stacks. They are "rootless" because
they are fed by lava from the underlying flow
instead of from a deeper magma conduit.
This hornito formed on the surface of a
pahoehoe flow during the Mauna Ulu eruption
on the east rift zone of Kilauea Volcano, Hawai`i.
Photograph by D.A. Swanson on 21 May 1970

28. Magma
Magma is molten or partially molten rock
beneath the Earth's surface. When magma
erupts onto the surface, it is called lava.
Magma typically consists of (1) a liquid
portion (often referred to as the melt); (2) a
solid portion made of minerals that
crystallized directly from the melt; (3) solid
rocks incorporated into the magma from
along the conduit or reservoir, called
xenoliths or inclusions; and (4) dissolved
gases.
This sketch shows a classic cartoon view of a
magma reservoir beneath a volcano and a
conduit leading up to a lava dome at the
surface. The arrow indicates direction of
magma movement from a deeper source.

29. USGS Photo Glossary
of volcanic terms
Appreciation to the USGS for providing the
photos and information used here to create a
glossary of terms.
courtesy of USGS Earthquake Hazards
Program
http://volcanoes.usgs.gov/images/pglossary/
index.php