Download Diapositiva 1 - Russell-Moro

Volcanoes
A volcano is a place on the Earth’s surface
where magma breaks through.
A volcano is called active if it erupts lava,
rock, gas or ash, or if it shows seismic
activity.
A volcano is dormant when is not
presently erupting but is considered likely
to erupt in the future. (Foxworthy and Hill)
An extinct volcano will never erupt again.
vent
cone
conduit
A volcano is a
vent or 'chimney'
that connects the
magma chamber
(subterranean
cavity containing
the gas-rich liquid
magma) to the
Earth’s surface.
The
volcano
includes
the
surrounding cone
of
erupted
material.
magma
chamber
In the Earth's mantle the temperature
is hot enough (800-1.600 °C) to melt
rock and form a thick, flowing
substance called magma. Lighter than
the solid surrounding rock, magma is
buoyant much like a cork in water;
being buoyant, it rises.
Most magmas contain abundant
gas , as the magma rises closer to the
surface, the pressure decreases,
which cause the gases dissolved in
the magma to expand. This expansion
propels the magma through openings
fractures in the Earth’s surface
creating a volcanic eruption
Once magma is erupted, it is
called lava.
Volcanic eruptions do not occur randomly, but are mainly
localized at plate boundaries. Some volcanoes, can be
found in the interior of plates at areas called hot spots.
Although most of the active volcanoes we see on land
occur where plates collide, the greatest number of the
Earth's volcanoes are hidden from view, occurring along
spreading oceanic ridges.
VOLCANIC ACTIVITY
Two styles of volcanic eruption:
Explosive and Effusive
Explosive: where rapidly escaping gas
bubbles (= vesicles) rip apart the
magma, fragmenting it.
Effusive: where the magma leaks out
onto the surface passively as lava
flows.
Sometimes basaltic lava pours out quietly from
long fissures and floods the surrounding
countryside with lava flow upon lava flow,
forming broad plateaux. Lava plateaux can be
seen in Iceland, India and USA. (Tilling, 1985)
Shield volcanoes are built almost entirely of fluid
lava flows forming a broad, gently sloping cone,
with a profile much like that a warrior's shield. They
are built up slowly by highly fluid basaltic lava that
spread widely over great distances. The Hawaiian
Islands are composed of linear chains of these
volcanoes, including Kilauea and Mauna Loa, two
of the world's most active volcanoes.
Mauna Kea (left) and Mauna Loa (right),
Some of the Earth's highest mountains are
composite
volcanoes;
sometimes
called
stratovolcanoes. They are typically steep-sided,
symmetrical cones of large dimension built of
alternating layers of lava flows, volcanic ash,
cinders, blocks, and bombs
Fuego (left) and Acatenango - Guatemala
In a "Strombolian"-type eruption huge clots
of molten lava burst form the summit crater
to form luminous arcs through the sky.
Stromboli
Stromboli is a volcan characterized by frequent activity
and high viscosity lavas.
A "Vulcanian"-type eruption is characterized by
a dense cloud of ash-laden gas that explodes
from the crater and rises high above the peak.
Steaming ash forms a whitish cloud near the
upper level of the cone.
In a "Pelean" eruption a large quantity of gas, dust,
ash, and incandescent lava fragments are blown out
of a central crater, fall back, and form a dense cloud
that move down-slope at velocities as great as 100
miles per hour. Such eruptions can cause great
destruction and loss of life if it occurs in populated
areas, as demonstrated by the devastation of St.
Pierre during the 1902 eruption of Mount Pelee on
Martinique, West Indies.
before
after
“Plinian” eruption: on August 24, 79AD Vesuvius
blew its top, erupting tonnes of molten ash, pumice
and sulfuric gas miles into the atmosphere.
Pyroclastic flows flowed over the city of Pompeii
and surrounding areas.
WHY DO COMPOSITE VOLCANOES TEND TO
BE EXPLOSIVE AND SHIELD VOLCANOES
NON-EXPLOSIVE??
1) CHEMISTRY (COMPOSITION) OF LAVA
2) LAVA TEMPERATURE
COMPOSITE
SHIELD
-Lower silica content of lavas
make them more fluid
(runny).
- Lava temperatures are quite
hot, ~ 2200oF, making it
easier for these lavas to
“flow like water”.
- Higher silica content of
lavas make them more
viscous (thick).
- Lava temperatures are
generally a few hundred
degrees cooler than those of
shield volcanoes, making the
lavas “thicker” in
consistency.
PRODUCTS OF EXPLOSIVE ERUPTIONS
Pyroclastic material
• Rock fragments created by explosive eruptions
•magma explodes from volcano and solidifies in the air
•existing rock is shattered by powerful eruptions
Lapilli
Volcanic bombs
Volcanic blocks
Volcanic ash
• Three products from an explosive eruption
– Ash fall: volcanic ash that has fallen through
the air from an eruption cloud. A deposit so
formed is usually well sorted and layered.
– Pyroclastic flow: a hot, fast-moving and highdensity mixture of fine and coarse particles
and gas formed during explosive eruptions or
from the collapse of a lava dome
– Base surge: turbulent, low-density cloud of
rock debris and water and (or) steam that
moves over the ground surface at high speed.
Deposits associated to ash fall are generally called
tuffs, volcanic rock made up of rock and mineral
fragments in a volcanic ash matrix. Tuffs commonly
are composed of much shattered volcanic rock
glass chilled magma blown into the air and then
deposited regularly on the ground like a snow fall. If
the deposits are made by coarse – grained materials
they are called volcanic breccia.
Pyroclastic flow consists in the lateral flowage
of a turbulent mixture of hot gases and
unsorted pyroclastic material. The deposits are
not layered and tend to fill ground hollows, they
are generally called ignimbrites
Direct
measurements
of pyroclastic
flows are
extremely
dangerous!!!
Explosive volcanic activity can also produces
mud flows called lahar
Hot lava can melt snow and ice on the
top of the cone, so that melt water
picks up ashes and rocks forming fast
flowing, high energy torrents.
Base
surges
are
generated
by
phreatomagmatic eruption: a type of
volcanic explosion that occurs when water
comes in contact with hot rocks or ash near
a volcanic vent, causing steam explosions .
Effusive (lava) activity
If magma is thin and
runny, gases can escape
easily from it. When this
type of magma erupts, it
flows out of the volcano.
Lava flows rarely kill
people
because
they
move slowly enough for
people to get out of their
way.
Lava
flows,
however,
can
cause
considerable destruction
to buildings in their path.
Because of basalt's low silica content, it has
a low viscosity .Therefore, basaltic lava can
flow quickly and easily move >20 kilometers
from a vent.
When lavas contain large amounts of gases, bubbles
tend to escape while lava is flowing giving rise to
highly irregular surfaces. Aa (a Hawaiian term), is
lava that has a rough, spiny and generally clinkery
surface. In thick aa flows, the rubbly surface of loose
clinkers and blocks hides a massive, relatively dense
interior. (Tilling, Heliker, and Wright, 1987)
If magma has a low content of gas, lava flow quikly
solidifies at his top, while inside the hot fluid keeps on
running downhill. Pahoehoe is a Hawaiian term for
basaltic lava that has a smooth, or ropy surface. A
pahoehoe flow typically advances as a series of small
lobes that continually break out from a cooled crust.
Fluid lava erupted under water may form a special
structure called pillow lava. Such structures form when
molten lava breaks through the thin walls of underwater
tubes, squeezes out like toothpaste, and quickly
solidifies. Typically ranging from less than a foot to
several feet in diameter, each pillow has a glassy outer
skin formed by the rapid cooling of the lava by water.
FEATURES ASSOCIATED WITH COMPOSITE
AND SHIELD-TYPE VOLCANOES
Sometimes basaltic lava flows give rise to some
spectacular geological features such as the “Giant
Causeway” in Northern Ireland
A lava dome is a steep-sided mass of viscous lava
extruded from a volcanic vent, often circular in plan view
and spiny, rounded, or flat on top. Its surface is often
rough and blocky as a result of fragmentation of the
cooler, outer crust during growth of the dome.
(Foxworthy and Hill, 1982)
Cinder cones, also known as scoria cones, represent
the smallest type of volcano, rarely exceeding 300 m in
height. They are entirely composed of pyroclastic
materials (volcanic ash). They are quite common and
associated with both composite or shield volcanoes.
Volcano Sudoeste and the smaller cinder cones at its base are part of
the San Quintin volcanic field located in Mexico’s Baja Peninsula.
When volcanoes explode or eject a significant
volume of material onto the surface the cone may
collapse into the voided reservoir, forming a steep,
bowl-shaped depression called a Caldera. These can
be quite large, 1-100 km in diameter!
Crater Lake, Oregon (10km)
Aniachak Caldera, Alaska
Diatreme
is
a
general term for a
volcanic vent or
pipe drilled through
enclosing
rocks
(usually
flat-lying
sedimentary rocks)
by the explosive
energy
of
gascharged magmas.
The
diamondbearing kimberlite
pipes
of
South
Africa
are
diatremes.
References
• http:// egsc.usgs.gov /isb//pubs/teacherspackets/volcanoes
• http://www.nature.nps.gov/views/kcs/volca
nism
• http://geology.com/volcanoes/volcanichazards