VOLCANOES
VOLCANOES
Mt. Arenal –
Costa Rica
It’s all in the magma…
Mafic or Felsic ?
• Mafic – rich in ferromagnesian minerals,
flows easily (low viscosity), gas releases
easier. (common in non-explosive)
• Felsic – high silica content, highly viscous,
commonly plugs up creating explosive
volcanism
Composite (Strato) Volcano
• A volcano that is
composed of
alternating layers of
lava and pyroclastic
material
• Steep sided slopes
• Silicic to
intermediate magma
= EXPLOSIVE
• Example: Cascade
Mountains.
Mt. St. Helens – a typical composite
volcano (prior to eruption)
After Eruption…
Sequence of events
prior to Mt. St.
Helens eruption in
May of 1980.
Volcanic
Dome
•Extremely viscous lava
•Very steep sided
•Often forms within crater
of erupted volcano
•Dome inside Mt. St.
Helens has recently been
very active.
St Helens Lava Dome
Shield Volcanoes
•Broad, low angle slopes
•Composed primarily of mafic lava
•Generally cover large areas
•Produced by non-explosive eruptions of large
volumes of lava
•Mauna Loa on Hawaii is a good example
Fluid Mafic Magma Forms Lava Tubes
24 km high
Olympus Mons
Caldera
650 km
Cinder Cone
•Built from
ejected lava
(mainly cindersized) fragments
•Steep slope
angle
•Rather small
size
•Frequently
occur in groups
Near Mt. Edziza,
Northern B.C.
Manu loa, Hawaii
Cinder Cones tend to
be semi – explosive.
Fissure Eruptions
•Fluid
basaltic
lava
extruded
from
crustal
fractures
called
fissures
The Gorge Amphitheater – George, Washington
Flood Basalts
Ben Harper
Caldera
•Calderas (form by collapse of evacuated magma
chamber)
•Steep-walled depressions at the summit
•Size generally exceeds 1 km in diameter
Caldera of Mt Mazama now filled by Crater Lake
Ash Plume
Lava Flows
Aa
•`A`a (pronounced "ah-ah")
has a rough rubbly surface
composed of broken lava
blocks called clinkers.
• Typical aa lava is very
viscous, either because of
high silica or because the
temperature is close to rocks
melting point
A typical Aa lava flow
A typical aa flow
Pahoehoe
•Pahoehoe is basaltic lava that has a smooth, hummocky, or
ropy surface that typically develops when the lava is very
fluid.
•This is most likely if the lava has a basaltic composition.
•It is also most likely when the lava's temperature is well
above the melting point
Pillow Lavas
Formed from lava erupting
underwater.
•mounds of elongate lava
"pillows" formed by repeated
oozing and quenching of the
hot basalt.
•First, a flexible glassy crust
forms around the newly
extruded lava, forming an
expanded pillow.
• Next, pressure builds until
the crust breaks and new
basalt extrudes like
toothpaste, forming another
pillow.
Pillow lava movie
Pillow Lava
Deposit
Columnar jointing/basalts
The type of jointing that breaks rock, typically basalt, into
columnar prisms. Usually the joints form a more or less
distinct hexagonal pattern. Forms from contractional cooling.
Devils Postpile sheer
wall face is 60 feet
high
Aerial view of
Devils Postpile
showing
columns'
geometry
Columnar
jointing/
basalts
Pyroclastics
•When lavas are viscous and
charged with gas, they are likely
to erupt explosively rather than
forming lava flows.
•The result is a cloud of
pyroclastic debris that may
shoot several kilometers into the
air and then settle over a wide
area.
•Fine-grained materials, known
as ash (<2mm), may travel great
distances. Cinders (2 – 64 mm)
and lapilli the size of pebbles
land closer to the volcano. Large
particles are bombs ( > 64 mm).
Ash
Cinders
Lapilli
Bombs
Nuee
Ardente
pyroclastic
flow
A glowing cloud or Nuée Ardente is a gas generated
eruptive phenomenon consisting of two parts.
1) glowing avalanche (lower denser part)
2) Lighter fraction of volcanic gases, ash, and dust which
cauliflower upwards.
Nuee Ardente
off of Mt. St.
Helens,
30/61 1980
Nuee Ardente /Pyroclastic flow
The 1902 Eruptions of Mt. Pelée
The city of St. Pierre, before the
1902 eruptions.
The city of St. Pierre, after the
1902 eruptions, June 1904.
•Lahar - 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.
Lahars
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.
Lahars from Mt. St. Helens
Deadly Lahars from Nevado del Ruiz,
Colombia
November 13, 1985
Within four hours of the beginning of the eruption, lahars had
traveled 100 km and left behind a wake of destruction: more than
23,000 people killed, about 5,000 injured, and more than 5,000
homes destroyed
Phreatic Eruptions
Phreatic eruptions are
steam-driven explosions
that occur when water
beneath the ground or on
the surface is heated by
magma, lava, hot rocks, or
new volcanic deposits
generating an explosion of
steam, water, ash, blocks,
and bombs.
Mount St. Helens, Washington
A Final Summary
Assignment
• Use Text – Chapter 4 to fill in and complete
your notes package. Remember to include:
Caldera, Phreatic eruptions, columnar
basalt, lava tubes, pillow lava.
• Test your understanding by doing the
chapter 4 fill in blank worksheet. Do it
without the text first, and then look for the
ones you don’t know.
Volcanism and Tectonic setting
• In general subduction
zones will result in felsic
magma (and lava) creating
explosive volcanism
• Melting of basaltic slab,
sediments, and water.
• The addition of water
(gas) will increase melting
of crust.
• Assimilation of silica rich
crust turns magma into an
intermediate compostion.