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Chp 4: Volcanism
Volcanism – the process whereby lava and its contained gasses, and
pyroclastic materials are expelled upon the earth’s surface, or into the
earth’s atmosphere.
Volcano – a conical mountain formed around a vent where lava,
pyroclastic materials, and gasses have erupted. One purpose of volcanoes
is to help rid the interior of the earth of excess heat buildup.
Volcanoes and Religion –
Native Americans of the northwest tell of a titanic battle between the
volcano gods Skel and Llao accounting for the huge volcanic eruptions
that occurred ca. 6600 BP.
In Hawaii, Pele is the goddess residing in the crater of Kilauea responsible
for the eruptions and earthquakes there.
Ancient Greeks believed that the god Pluto or Vulcan was responsible
for eruptions there.
Table 4-1, p.89
Chp 4: Volcanism
Active Volcanoes –
There are approximately 550 active volcanoes on earth today.
(i.e. Mt. St. Helens, Mauna Loa, Kilauea, etc.)
At any one time there are about 12 volcanoes erupting somewhere on
earth.
Dormant Volcanoes –
There are numerous volcanoes that have erupted in the recent geologic
past and probably will erupt again in the future.
Mt. Vesuvius in Italy, Mt Pinatubo in the Philippines
Extinct Volcanoes –
These are volcanoes that have erupted in historic times but show no sign
of erupting again.
Mount Vesuvius, Italy looking SE
Fig. 4-CO, p.84
Pompeii, Italy:
-excavated city streets
-casts of remains of people
caught in eruption
Fig. 4-1, p.88
Niyaragongo crater in Africa
p.92a
Chp 4: Volcanism
Volcanic Gasses –
The gasses released from the magma as it moves upward.
In sialic magma, expansion if restricted due to the high viscosity
(high silica content) and gas pressure increases greatly causing explosions
releasing ash and other pyroclastics.
In mafic magmas, expansion occurs due to the low viscosity
(low silica content) allowing gasses to expand and escape easily…creating
a quieter eruption.
Composition of volcanic gasses –
50 – 80% of all volcanic gasses are water vapor, with lesser amounts of CO2, N2, and
sulfur gasses – sulfur dioxides and hydrogen sulfides.
Very small amounts of CO, H, and Cl are released.
The Blue Haze Famine – Iceland, 1783 – gasses (probably sulfur dioxide) escaped
from the Laki Fissure, causing 75% of livestock to die. The gas caused the overall
temperature to drop causing crop failures causing 25% of the human population to die.
Cameroon, Africa, 1986 – A cloud of volcanic CO2 was emitted from under Lake
Nyos (that sits atop a volcano) killing by asphyxiation ALL ANIMAL LIFE for miles
around, including 1746 humans.
Volcanic gases can be very dangerous
Fig. 4-3c, p.91
Chp 4: Volcanism
Lava Flows
Sialic (high viscosity flows) - tend to be thicker, “lobe-shaped flows
with distinct margins.
II. Mafic (low viscosity flows) – tend to be comparatively thinner,
fluid flows that are widespread.
III. Types of Lava Flows – (Hawaiian Terms)
1. Aa – a flow characterized by a surface consisting of rough,
angular, jagged blocks and fragments. This is due to a higher silica content than:
2. Pahoehoe – a flow characterized by a smooth, ropy surface,
almost like taffy. This is due to relatively lower silica content than Aa.
I.
Speed of Lava Flows
Not very quick…the fastest low viscosity in Hawaii ever measured had a speed of
9.5 kilometers/hour.
Faster speeds occur whenever the lava flow is insulated on all sides forming a lava tube
or conduit. After the eruption and the lava tube drains, sometimes the roof of the tube
collapses forming a skylight into the tube.
Pahoehoe lavanote the smooth surface
Fig. 4-5a, p.94
Pahoehoe lavanote the smooth surface,
almost taffy-like
Fig. 4-5b, p.94
Aa flow
Fig. 4-5c, p.94
Aa flow-Blocky lava blocks piled up
Fig. 4-5d, p.94
Lava tube or conduit
Fig. 4-4a, p.93
Lava skylight
Fig. 4-4b, p.93
Chp 4: Volcanism
IV. Pressure Ridges and Spatter Cones -
Pressure Ridges – As the surface of a flow begins to solidify, pressure
from the flow causes the surface to buckle, forming ripples of sorts called
pressure ridges.
Spatter Cones – Gasses escaping from the flow hurl globs of lava into the air. These
globs fall back to the surface and stick together forming small,
steep-sided spatter cones. These may rise several meters above the flow.
V. Columnar Jointing Common in flows that are relatively “thick” – several tens of meters to 100
meters thick.
They form because of differential cooling of the flow – the outer surfaces
“freeze” while the inside is still molten. This results in the vertical “splitting” of the flow
into roughly pentagonal prism-like columns – hence “columnar jointing”.
Examples include: Devil’s Postpile National Monument, California, Devil’s Tower,
Wyoming.
Pressure Ridges
Fig. 4-6a, p.95
‘Columnar jointing’:
Note the columns of
apparently vertically
stacked igneous rockshence the name….
As the lava cooled,
vertical fractures formed
and intersected to form
5 and 6 sided columns.
60 million year old lava
flow at Giant’s Causeway
Ireland
Fig. 4-7a, p.95
Columnar jointing’:
Note the columns of
apparently vertically
stacked igneous rockshence the name….this
time with a view from
top.
Staffa Is in Scotland85 km from previous
picture, but same age
basalts!!
Fig. 4-7b, p.95
VI. Pillow Lava –
Bulbous masses of basalt that resemble “pillows” result from mafic
magma cooling under water. This is VERY common at the submarin
spreading centers such as at the MOR (mid-oceanic ridge)
Anywhere pillow lava is found on the surface of the earth indicates tha
there was mafic, basaltic lava that cooled underwater.
Fig. 4-9, p.96
Chp 4: Volcanism
VIII. Distinguishing Flows from Sills in Cross-section Chilled Margin – The solidified outer edge of a lava flow or an
intrusive body such as a sill.
As a lava flow flows across the surface of the earth, the first to solidify
is the “margin that touches the relatively cold ground. In a sill, the
“chilled margin” encircles the intrusion since it “froze” at all contacts
with the country rock.
Gasses that are released in a flow tend to form “bubbles called “vesicles”.
In a flow, these vesicles accumulate at the top of the solidification much
like the foam of a beer poured into a glass. Vesicles usually are not as
abundant in a sill since it cooled intrusively.
Altered Country rock – Upon coming into contact with the surface of
the ground, a flow usually alters the country rock on the bottom surface
only, whereas a sill alters the country rock on all edges.
Chp 4: Volcanism
Flood Basalts –
Occur as high volume, low viscosity, mafic flows over a broad,
flat area, many times resulting in “flood basalts’ up to 100 meters
thick or greater.
Columnar jointing is common.
Lava “Plateaus” may be formed.
Flood Basalts –
Occur as high volume, low viscosity, mafic flows over a broad,
flat area, many times resulting in “flood basalts’ up to 100 meters
thick or greater.
Columnar jointing is common.
Lava “Plateaus” may be formed.
Columbia R basalts
Mt Pinatubo pyroclastic
flows
Fig. 4-18, p.110
a. Lava fills a valley
b. Adjacent layers erode much
easier than lava, leaving lava
flow now as a ridge.
c. Black rock at top of this ridge
is basalt flow that once filled
valley.
Fig. 4-8, p.96
X. Anatomy of a Volcano Supplying the volcano is a magma chamber deep beneath the surface vent where lava
begins to pour forth..
Magma reaches the volcano via a volcanic pipe or conduit.
The depression at the top of the volcano is the crater or main vent.
Along the side, there may be smaller
conduits forming side vents or side cones.
Long slits may occur along the sides
of the cone forming fissures or
fissure eruptions.
Fig. 4-19, p.111
Chp 4: Volcanism
XI.
Types of Volcanoes –
1. Shield Volcano Characteristics:
Usually mafic resulting in forming basaltic rocks
Low viscosity of the magma due to low silica content
Low slope angle of the cone – usually 6 – 12deg
Many fissure eruptions, lava tubes, conduits, etc.
Typical of the Hawaiian Islands or MOR volcanoes.
2. Cinder Cone Characteristics:
Quickly formed (sometimes overnight)
Symmetrical cone with relatively steep sides
Usually less than 300m high
Composed entirely of pyroclastic materials (i.e. cinders and ash)
Generally mafic to intermediate in composition
Erodes away easily and quickly
Shield volcano:
Consist of many layers of
thin basalt flows
Fig. 4-12, p.99
Cinder Cone
p.104
Cinder cone
Fig. 4-13a, p.102
Conical crater common at top of cinder cones
Fig. 4-13b, p.102
Chp 4: Volcanism
3. Stratovolcano or Composite Cone Characteristics:
Formed by an alternating series of lava flows followed by pyroclastic flow
Usually intermediate to sialic magmas.
Very explosive due to their high silica content and viscosity.
i.e. Mt. St. Helens
Many times after going extinct, the cone erodes away leaving the
volcanic neck or volcanic pipe exposed. This may contain the rock
Kimberlite associated with the formation of diamonds and other gems.
New magma uplift-lava dome- in center of volcano
Fig. 4-16b, p.105
Fig. 4-14, p.103
Chp 4: Volcanism
XII. Violent Magmas – Ash Flow Tuffs and Calderas
Characteristics of Violent Magmas:
a. “Dry”, granitic magmas
b. less than 10% H20
Will not solidify before reaching the surface
Volatiles separate from magma and form a frothy mixture of super
hot gasses and magma
Ensuing eruption is extremely violent
“Nuee ardente” super hot gas and ash flow clouds are produced
Many pyroclastics are produced – pumice, tuffaceous rocks, etc.
Calderas are formed. This occurs after the eruption, whereby the
top of the volcano falls back into itself creating a circular depression.
These create very good mining districts for precious metals.
i.e. Creede, Colorado.
Pyroclastic
material blown
into atmosphere
at Pinatubo
Fig. 4-10b, p.94
Ash flows that buried village
Fig. 4-15a, p.105
Icelandic pyroclastic eruption-Cinder cone
Fig. 4-13d, p.102
Caldera formation:
explosion followed
by collapse of center
(Crater Lake, Ore)
Fig. 4-11, p.98
XIII. Hot Spots -
Chp 4: Volcanism
Characteristics:
Bodies of magma that have risen near the surface creating a localized zone of melting
below the lithosphere.
This results in surface eruptions over long periods of time.
I.e. Hawaiian Islands, Yellowstone National Park
The Hawaiian Island chain is composed of islands dating from:
Hawaii (The Big Island) – 0.7 MYA to today
Maui – 0.8 – 1.3 MYA
Molokai – 1.3 – 1.8 MYA
Oahu – 2.3 – 3.3 MYA
Kauai – 3.8 – 5.6 MYA
These islands are also listed as to size: Hawaii the largest, and Kauai the smallest.
Hawaii is over the hot spot and is active. The other islands have either dormant or
extinct volcanoes. The Pacific Plate has been slowly moving to the northwest where
5.6 MYA Kauai was over the hotspot and was active while the other islands had not
even formed yet. There is a new volcanic seamount (an underwater volcanic mountain)
called Loihi is forming and is 940 meters below the surface. It will someday take the
place over the hotspot and become the largest island, while Hawaii erodes smaller as the
other islands have done
Hawaiian Islands: line of islands due to movement over Hotspot
Volcanoes typically occur along Convergent Margins-where 2 plates
Fig. 4-22, p.114
collide-composite volcanoes are very common
Volcanoes can be characterized by their explosiveness
Fig. 4-20, p.112
Volcanology Today:
monitor changes in
shape of volcanoes,
in order to determine
likelihood of any
eruptions.
Fig. 4-21, p.113
Chp 4: Volcanism- Summary
Volcanism – the process whereby lava and its contained gasses, and
pyroclastic materials are expelled upon the earth’s surface, or into the
earth’s atmosphere.
Volcano – a conical mountain formed around a vent where lave,
pyroclastic materials, and gasses have erupted. One purpose of volcanoes
is to help rid the interior of the earth of excess heat buildup.
Different types of Lava flows:
1. Aa: rough, blocky
2. Pahoehoe: smooth
3. Pressure ridges: buckling of lava flows into ridges (hard)
4. Columnar jointing: vertical stacking of columns
5. Pillow lavas: pillow like shape; submarine only
Chp 4: Volcanism- Summary
1. Chilled margins: edges of flow that cooled first
2. Vesicles: escaping gases as flow cools, large holes in rock
Flood Basalts: high volume flows over surface of Earth, stack into’
Layers that can be 100m or more thick (India)
3 types of Volcanoes:
-Cinder Cones: high relief, conical shape
-Shield volcanoes: low relief, built up by flows and intrusions
Stratovolcano or Composite: formed by an alternating series of
lava flows followed by pyroclastic flows
Ash Flows and Calderas: evidence for violent magmas (low H2O)
-Ash flows: mixture of gases and magma that erupt into air
-Caldera: remains of volcano that has erupted and blown top off
Chp 4: Volcanism- Summary
Plate Tectonics and Volcanoes:
Hot Spots: Bodies of magma that have risen near the surface
Creating a localized zone of melting below the lithosphere.
This results in surface eruptions over long periods of time.
‘Ring of Fire’ around Pacific: volcanoes related to edges of plates,
where one plate is subducted (sinks beneath) another…oceanic beneath
continental.
Composite volcanoes frequently occur where plate tectonic boundariessubduction zones especially-occur
Monitor volcano shape changes in order to predict eruptions….!!
Fig. 4-3a, p.91
Fig. 4-3b, p.91
Fig. 4-4c, p.93
Fig. 4-6b, p.95
Fig. 4-10a, p.97
p.101
Fig. 4-13c, p.102
Fig. 4-15b, p.105
Fig. 4-16a, p.105
Fig. 4-17a, p.106
Fig. 4-17b, p.106
Fig. 4-2, p.90