Volcanos Flashcards
Volcanos formed
Volcanoes are
formed at
openings where
magma and
associated
materials
emerge on the
Earth’s surface.
Once magma is
at the surface, it
is referred to as
lava. As lava-
derived
materials
accumulate,
they form
various volcanic
terrain
Where are Most of the Earth’s
volcanoes are found around
the rim of the Pacific Ocean
in an area called the “Ring of
Fire
Why do they form around the ring of fire
They are
boundaries between
pieces of the Earth’s crust
(plates). Specifically, they
are boundaries where the
adjacent plates are
interacting violently. The
movement and interaction
of these plates is call plate
tectonics.
Volcanoes at divergent boundaries
At a divergent boundary, two tectonic plates are
moving apart. This creates a gap/break in the
Earth’s crust, allowing existing magma from the
mantle to much more easily reach the surface.
There are actually two distinct types of divergent
boundaries - rift zones and spreading zones.
Rift Zones
areas where continental(land) crust pieces
are separating. As in spreading zones, the gap created
between the plates allows mantle magma to rise to the
surface. Continental crust is relatively thick and magma
has a longer journey to the surface and often slows or
solidifies before it emerges at the surface. Hence, rift
zones are considerably less likely than spreading zones
to have volcanoes.
Spreading zones
areas where two pieces of oceanic
crust are moving away from each other. Since oceanic crust
is relatively thin, the gap between the two pieces is shallow,
and mantle magma easily reaches the surface of the crust.
The most prominent spreading zone is the mid-Atlantic ridge.
Fortunately, as spreading zones are by definition at the
bottom of the ocean, they generally are not a hazard to
people. An exception is Iceland, where volcanoes of the
mid-Atlantic have grown large enough to reach the surface.
While Iceland benefits from a number of volcanic effects
such as hot springs, thermal energy, and spectacular
scenery, it also is subject to dangerous eruptions
Mafic magmas
Mafic magmas are relatively rich in magnesium and iron, relatively low in silica. These materials melt at
a higher temperature (1100-1200 degrees Celsius) and have a low viscosity (thickness), meaning that
these magmas flow easily.
Felsic magmas
Felsic magmas are relatively poor in magnesium and iron, and high in silica. These materials melt at a
lower temperature than mafic magmas (800-900 degrees Celsius) and have higher viscosity (thickness).
Felsic magmas flow slowly.
Cinder cones
Cinder cones are the
simplest and least hazardous
type of volcano. A mafic
type lava with a high gas
content will cause an
eruption called a fire
fountain as the hot magma
sprays into the air like a soda
pop explosion. If the
magma spray is fine
enough, individual droplets
may cool quickly and fall to
earth as cinders. As
eruptions proceed, these
cinders pile up as a cinder
cone. If the eruptions last a
very long time and include
lava flows (fire fountains and
effusive eruptions) the
volcano evolve into another
type of volcano, the
composite volcano
Composite volcanos
stratovolcanoes are
usually tall with steep
sides. They are made out of
sequential layers of lava flows,
volcanic ash, cinders, and
boulders.
A stratovolcano that goes
through a phase when it is
supplied by high viscosity
(thick) magma with a high gas
content may explode violently
and produce a variety of
volcanic hazards.
Sheild volcanos
Shield volcanoes are composed mostly of lava flows. These are typically low gas content, low
viscosity mafic lavas. Since the lava flows are “runny” (low viscosity) the lava flows spread out
and forms a broad, gently sloping shape that resembles a warrior’s battle shield. Shield
volcanoes build up slowly from thousands of lava flows that spread widely over great
distances, and cool as thin sheets.
Ash flow
ashfall is the most common and widespread hazard of
volcanoes.
Explosive eruptions emit a lava “spray,” which upon
cooling forms a substance called tephra.
Larger pieces (or conglomerates or smaller particles)
are relatively heavy and tend to be deposited near the
volcano. Over time, this material accumulates and
“grows” the volcano, perhaps forming a majestic
stratovolcano.
Pyroclastic flow
made up of a mixture of
hot ash and volcanic gases. They resemble
ash emissions, but are too heavy (or ejected
with to little force) to become airborne.
Rather, pyroclastic clouds flow down the side
of the volcano, often at very high speed (as
fast as 100 mph or more)
Pyroclastic flow are very hot (as the root pyro-
suggests) and also likely contain high
concentrations of toxic gases. These flows
are very dangerous and will burn, suffocate,
or bury anything they run over.
Lahar
Lahar is an Indonesian word for
a volcanic mudflow.
Lahars are composed of ash,
meltwater, rain, soil, and other
materials that form a thick mud-
like mixture
