A1261683006qKzse - PowerPoint PPT Presentation

Title: A1261683006qKzse


1
Volcanoes
2
Magma
Simple put, a volcano occurs when magma erupts
onto the surface (land or under the
ocean) Magmas originate in the upper mantle at
depths between 50 and 250 kilometers (30 to 150
miles)
3
Source of Magma
Magmas can reach the surface in one of three
plate tectonic settings Divergent plate
boundaries Subduction zones Hot spots
4
Composition of Magma
A magmas chemical composition influences its
physical properties, which determines how it will
erupt (quietly or violently) and the kind of
volcanic structure it will form The three major
compositional variables of magma are the
proportions of silica (SiO2) iron
(Fe) magnesium (Mg)
5
Composition of Magma
A rock formed from a magma rich in iron and
magnesium is described as mafic A rock formed
from a magma rich in silica is described as
felsic There is a gradation from mafic to felsic
as the proportion of iron/magnesium decreases and
the proportion of silica increases
6
Magma and Lava
In fact, the mantle is extremely rich in iron and
magnesium (so the mantle is mafic) Magmas
erupting along the mid-oceanic ridges are mafic
in composition and create basaltic lavas For a
magma to erupt on land, the magma will have to
pass through the crust which is rich in silicate
minerals These silicate minerals are melted and
become part of the magma, which becomes felsic
7
Major Types of Lava
Basaltic lavas Magma has mafic composition,
typically erupts at 1000o to 1200o C, flowing
basaltic lavas have low-viscosity, cools to form
basalt Andesitic lavas Intermediate in
composition and viscosity between mafic and
felsic magmas, cools to form andesite Rhyolitic
lavas Felsic composition, typically erupts at
800o to 1200o C, high-viscosity, cools to form
rhyolite
8
Major Types of Lava
Basaltic lavas tend to be a dark color, because
most iron/magnesium minerals are dark Rhyolitic
lavas tend to be lighter in color, because most
silica minerals are lighter in color
Basalt Andesite
Rhyolite
9
Flowing Lava
The silica-poor mafic magmas that produce the
basalt of the ocean floors have low
viscosity Low viscosity means that the lava
flows easily In contrast, silica-rich felsic
magmas have high viscosity and flow very poorly
(like cold syrup)
10
Examples of Lava Flow
Pahoehoe Aa Pillow basalts Vesicular basalt
We will look at each type
11
Aa and Pahoehoe
Aa a relatively low viscosity basaltic lava
characterized by a sharp, jagged, blocky
texture Pahoehoe a very low viscosity basaltic
lava characterized by a smooth, ropy texture
12
Aa and Pahoehoe

• A single downhill basaltic lava flow commonly has
  the features of pahoehoe near the source, where
  the lava is still hot and fluid

While Aa usually develops farther downstream
where the cooling lava has developed a thicker
outer layer
13
Pillow Basalt

• When a basaltic magma erupts under the ocean, it
  cools very rapidly and forms a pillow-shaped
  lava that has a glassy texture
• This exposed pillow lava was formed under the sea
  and uplifted by tectonic forces

14
Vesicular Basalt
Magma contain dissolved gases which become
trapped in the basalt when the lava cools The
pitted surface is referred to as vesicular
15
Pyroclastic Material
Pyroclastic material is volcanic ejecta violently
blown out of the volcano into the atmosphere
during an eruption It can be molten, partially
molten or solid
16
Pyroclastic Material
The molten pyroclasts can cool and solidify as
they fly through the atmosphere Smaller
particles can be blown 20-25 kilometers up into
the stratosphere
17
Pyroclasts

• Pyroclasts are classified by the size of the
  particle
• Volcanic ash is very fine dust which is
  ash-like in consistency
• Cinder is composed of gravel-sized pieces of
  ejecta and usually feels rough to the touch
• Volcanic bombs are large fragments of ejected
  magma which become rounded and cool (solidify) as
  they fly thru the atmosphere

18
Volcanic Bomb
19
Gases
Volcanoes can also eject great quantities of
gases and steam which can be mixed with the
pyroclasts
20
Gases
Sulfur dioxide (SO2) Hydrogen sulfide (H2S)
Carbon dioxide (CO2) Hydrogen Chloride (HCl)
Hydrogen Fluoride (HF)
21
Fumaroles
Fumaroles are volcanic vents emitting steam and
other gases, some charged with dissolved
minerals, such as yellow sulfur
22
Volcanic Tuff
Rocks created from smaller particles (such as
ash) are called volcanic tuffs
23
Volcanic Breccia
Volcanic breccia is composed of large pieces of
volcanic ejecta solidified into rock
24
Types of Volcanoes
Shield Volcano
Volcanic Dome
Cinder Cones
Composite Volcano
Caldera
25
Shield Volcano
Formed mainly of basaltic lavas Gentle slopes
averaging 2-10 degrees Can be huge, up to 120 km
wide! Long duration of activity, lasting tens
of thousands of years Eruptions usually
non-violent Long lava flows
26
Shield Volcano
27
Shield Volcanoes
The entire island of Hawaii was created by a
series of shield volcanoes The cratered top of
Mauna Loa is seen covered with snow
28
Volcanic Dome
Composed of rhyolitic and andesitic lavas (felsic
magma) Lava oozes out onto the surface like
thick toothpaste Grows slowly Steep-sided and
small, rarely more than a couple of hundred
meters wide Typically forms inside an already
existing crater
29
Volcanic Dome
30
Volcanic Dome
Novarupta Dome, Alaska
Mount St. Helens
31
Cinder Cones

• A volcano formed of cinders and other basaltic
  pyroclastic material built up around the volcanic
  vent
• Steep sides, with slopes of 30 degrees
• Relatively small, averaging one kilometer in
  diameter
• Short-lived, typically a single event

32
Cinder Cones
33
Cinder Cones
Cerro Negro Cinder Cone, Nicaragua Amboy Cinder
Cone, California
34
Composite Volcano
An active volcano can erupt with different types
of material during its life Some eruptions may
spew out pyroclastic deposits, another eruption
may consist of andesitic lava flows Slopes are
intermediate in steepness Relatively large,
easily 10-15 kilometers in diameter Intermittent
eruptions over long time span, lasting thousands
of years Eruptions often highly explosive
35
Composite Volcano
36
Mt Fujiyama, Japan
Composite Volcano
Raga/The Stock Market
37
Mount St. Helens
Mount St. Helens in Washington is a composite
volcano On March 16, 1980 the first of a series
of minor earthquakes occurred under the
volcano This marked the beginning of a new
eruption cycle It had been 350 years since the
last eruption
38
Mount St. Helens
On May 18, 1980, at 832 am, a magnitude 5.1
earthquake occurred 1 mile directly under the
volcano 10 seconds later, the north side of the
volcano began to collapse followed immediately by
an explosion that blew the top 400 meters (1300
feet) off of the peak
39
Mount St. Helens
...24 square miles of valley was filled by a
debris avalanche, 250 square miles of recreation,
timber, and private lands were damaged by a
lateral blast, and an estimated 200 million cubic
yards of material was deposited directly by
lahars (volcanic mudflows) into the river
channels Fifty-seven people were killed
40
Mount St. Helens Ash Fall
41
Mount St. Helens
Mount St. Helens is now a volcanic national
monument and remains active
42
Caldera

• A large depression (typically several km wide)
  formed by collapse of a volcano into a partially
  drained magma chamber
• The collapse may eject a tremendous amount of
  pyroclastic material into the atmosphere and
  cover very large areas of land with ash and
  debris
• Caldera may have younger domes within it

43
Caldera
44
Crater Lake Caldera
Stages in the development of Crater Lake The
volcano that existed before the caldera was named
Mount Mazama
45
Crater Lake Caldera
46
Crater Lake Caldera
47
Crater Lake Caldera
48
Crater Lake Caldera
49
Crater Lake Caldera
50 cubic kilometers of material was blown off the
top of the mountain
50
Fissure Eruptions
A volcanic eruption originating along an elongate
fissure rather than a central vent
51
Mid-Oceanic Ridges
Fissure volcanic activity can occur any where
along the 60,000 kilometer-long mid-oceanic ridges
52
Fissure Eruptions
Laki Fissure in Iceland formed in 1783, resulting
in a lava flow that covered 13 square kilometers
53
Flood Basalts
Thick, widespread accumulations of basalt that
cover a large area Typically fed by fissures An
excellent example is the Columbia River Plateau
in Washington and Oregon
54
Columbia Plateau Flood Basalts
Successive flows of flood basalt over a time
period of 10 to 15 million years built up a
plateau 1.8 kilometers (6,000 feet) thick and
covers an area of 160,000 square kilometers
(63,000 square miles)
55
Large Igneous Provinces
Areas of extensive lava flows are found both on
land and under the oceans
56
Deccan Plateau in India
At the end of the Cretaceous period, 65 million
years ago, volcanic eruptions created the Deccan
Plateau that covers most of southern India
57
Deccan Plateau in India
After several thousand years, the combined
thickness of the lava flows were as much as 2000
meters (6500 feet) and covered an area of over
1,500,000 square kilometers
58
Deccan Plateau in India
Such staggering quantities of gas were released
into the Earths atmosphere by these continuous
eruptions, that the chemical composition of the
entire atmosphere was changed world-wide This may
have played a major role in the extinction of the
dinosaurs at the end of the Cretaceous
59
Diatreme
When a volcano goes dormant, the magma and
breccia in the volcanic vent will cool and
solidify into rock If the volcanic cone is
eroded away, this plug, called a diatreme, will
be exposed
60
Diatreme
Shiprock, New Mexico
Fig. 6.12
61
Hydrothermal Activity
Hydrothermal activity refers to the below ground
circulation of water through hot volcanic rocks
and magma The heated water can carry dissolved
minerals and gases, which can react with
surrounding minerals and rocks The heated water
can reach the surface at hot springs, geysers,
fumaroles and mud pots
Fig. 6.12
62
Hot Springs and Geysers
Fig. 6.12
63
Phreatic Explosion
When water mixes with magma below the surface,
you can get a tremendous steam explosion Nisin
o-sima Volcano in the Pacific Ocean
64
Oceanic Volcanic Hot Spots
65
Continental Volcanic Hot Spots
Yellowstone Caldera Chain formed as the North
America Plate moved to the west southwest
66
Continental Volcanic Hot Spots
The ages of the Yellowstone Calderas formed over
the hot spot range from 16 million years ago to
the present
67
Continental Volcanic Hot Spots
630,000 years ago, the Yellowstone hot spot was
the site of a tremendous volcanic eruption 1000
cubic kilometers of pryoclastic material was
blown into the atmosphere Most of the United
States was covered with ash
68
Continental Volcanic Hot Spots
The caldera is 80 kilometers long and 55
kilometers wide (50 by 35 miles) Or roughly the
size of Knox County
69
Volcanic Hazards
70
How Bad Can It Be?

• 1815 Indonesia 92,000 killed
• 1822 Indonesia 5,500 killed
• 1826 Indonesia 3,000 killed
• 1883 Indonesia 36,417 killed
• 1902 Martinique 29,025 killed
• 1902 Guatemala 6,000 killed
• 1919 Indonesia 5,110 killed
• 1951 New Guinea 2,942 killed
• 1982 Mexico 1,700 killed
• 1985 Columbia 23,000 killed

71
How Bad Can It Be?
Cumulative deaths due to volcanic eruptions over
the past 500 years
72
How Bad Can It Be?
Causes of volcanic-related deaths Over the last
2,000 years
73
Lava Engulfing Buildings
74
Lahar
Lahar is a fast moving volcanic mud flows Formed
by the sudden mixing of large volumes of
pyroclastic material, such as ash, with water
For example, when volcanic activity melts a
glacier on a mountain top, or because of heavy
rain, or the draining of a lake in a crater
75
Lahar
Lahars can flow very fast, at speed faster than
100 kilometers per hour (60 miles per hour) They
can flood large areas and flow for long distances
in river channels They kill by drowning people in
mud that is too thick and heavy to swim in
http//www.geo.mtu.edu/volcanoes/hazards/primer/im
ages/mpegs/lahar.front.mpg
76
Nuée Ardente
A poetic-sounding French word introduced in 1904
to describe the terrible pyroclastic flow that
destroyed the entire city of St. Pierre on the
island of Martinique in 1902 It translates to
glowing incandescence Nuée Ardente (pyroclastic
flows) are a common and devastating result of
some volcanic eruptions
77
Nuée Ardente
Nuée ardente are extremely fast moving fluidized
bodies of very hot gas, ash and rock that hug the
ground as they flow down the volcanic
slope Temperatures can be as high as 800oC When
viewed at night or in low light, a nuée ardente
may appear to glow red
78
Nuée Ardente
Mayon Volcano, Philippines, 1984
79
1902 Martinique
Martinique is an island formed by volcanoes in
the Caribbean It was a French colony The
dormant Mount Pelée volcano began a new series of
eruptions on April, 25, 1902
80
1902 Martinique
With each passing day, the eruptions became
worse The French governor refused to evacuate the
city of St. Pierre and surrounding smaller cities
at the base of the volcano
On Wednesday, May 7, 1902, the governor with his
family and entourage visited St. Pierre There was
a gala ball that night
81
1902 Martinique
The next day at 802 am, the city of St. Pierre
was destroyed by a pyroclastic flow that roared
down the slopes of Mount Pelée at of 670
kilometers per hour (415 mph)
82
1902 Martinique
Over 29,000 people are burned alive in
seconds The entire town was completely destroyed
83
1902 Martinique
Dont worry! A French government review panel
concluded that the island governor was correct in
not evacuating the city
84
Santorini
Santorini is a circular group of islands in the
Aegean Sea The largest island is called
Thera The central lagoon is a volcanic caldera
12 by 7 kilometers (8 by 4 miles) in size
85
Santorini
About 3500 years ago there was a tremendous
volcanic eruption 30 cubic kilometers of the
island was blown as high as 36 kilometers into
the atmosphere This is believed to be the
largest volcanic explosion in recorded human
history
86
Atlantis?
The eruption, ash fall and tsunamis are believed
to have destroyed the Minoan civilization at the
height of its wealth and power This may be the
source of Platos story of Atlantis
87
Santorini
The volcano is still active as recent as 1950