I. Introduction to volcanoes
A. What causes formation of volcanoes?
1. The Earth's Interior
2. The Theory of Continental
Drift
3. Sea Floor Spreading and Subduction
B. Inside a Volcano
C. Types of Volcanoes
D. Active volcanoes
around the world (hotspots)
E. Volcanoes on other
planets and the moon
II. Volcanic Eruptions
A. What causes an eruption?
B. What are the effects of volcanic
eruptions?
1.
Volcanic eruptions and the earth's climate
2. Volcanic eruptions and the earth's atmosphere
a. Sulfur
b. Carbon dioxide and water vapor
c. Volcanoes and ozone depletion
3. Positive
effects of volcanic eruptions
C. Historic volcanic
eruptions and their effects
Introduction to Volcanoes
When the earth was formed, the early earth heated up, the center became molten, and convection currents developed. The lighter compounds rose towards the surface forming the brittle crust. Together with the top part of the mantle, it formed the hard slabs known as lithosphere. The continents are embedded in these slabs. The lithosphere is divided into oceanic and continental crusts.
The oceanic crust (sima), the floor of the deep oceans, is about 7 km thick, and made of relatively dense rocks like basalt. The continental crust (sial) is much thicker, averaging 33 km, and is made of relatively light material such as granite.
The denser materials, like iron (Fe) sank to create the core, which is partially solid. Temperatures are extremely high, at about 3000 C.
Between the core and the crust, the intermediate zones form the mantle, which is mainly solid rock. However, there is also a layer of molten rock called magma nearer the core. Temperatures are high, at about 2000 C.
2. The Theory of Continental Drift
Alfred Wegener, the scientist who proposed the theory, believed that the continents once formed a single land mass, which he called Pangaea (Greek for "all land"). This land mass broke into different pieces due to the weaknesses in the earth's crust, and drifted apart over millions of years until they arrived at where they are now.
The convection currents in the mantle allow
the plates to move.
Similarly, the magma nearer the core expands
and rises.
The magma that has risen spreads out beneath
the plates.
--As the magma spreads out, the plates move away from each other. This is the process of sea-floor spreading.
Sea-floor spreading is the process in which the ocean
floor is extended when two plates move apart. As the plates move
apart, the rocks break and form a crack between the plates. Magma
rises through the cracks and seeps out onto the ocean floor like a long,
thin, undersea volcano.
When magma meets the water, it cools and solidifies,
adding to the edges of the sideways-moving plates. Magma piles along
the crack, and a long chain of mountains, called an oceanic ridge forms
gradually on the ocean floor. The boundaries where the plates move
apart are 'constructive' because new crust is being formed and added to
the ocean floor. The ocean floor gradually extends, increasing the
size of the plates.. As these plates get bigger, others become smaller
as they melt back into the Earth in the process called subduction.
--When the magma sinks, the plates are dragged towards each other. This is the process of subduction. The repeated heating and rising of the magma sets up continuous convection currents in the mantle, causing the plates to move.
Subduction is the process in which one plate is pushed downward beneath another plate into the underlying mantle as plates move towards each other. The denser plate will slide under the thicker, less dense plate. Faulting, the process in which rocks break and move or are displaced along the fractures, occurs. The subducted plate usually moves in jerks, resulting in earthquakes. The area where the subduction occurs is the subduction zone. A long, narrow, deep depression called an oceanic trench, forms.
The jerking movement, as well as the friction
between the plates, causes much heat. This heat and radioactive decay,
causes the subducted plate to melt. Magma is produced by the melting
plate. It rises through fractures in the crust and reaches the surface
to form volcanoes. In the end, the heat may be so intense that large
areas of the crust are melted, forming granites just below the surface.
The boundary of such plate movements is 'destructive' because part of the
plate melts during subduction and is destroyed.
Most volcanoes are formed
from the interaction of the plates or
plate tectonics.
The column of a volcano is
brought about by the build-up of
materials (fragments, dust, lava) that is expelled from the center of the
earth. The earth's mantle melts to form magma. As the
amount of magma increases, it ascends to the surface.
Repetitive eruptions of lava
and solid matter results in the
formation of the walls (cone) of the volcano. Hence the cone may
be a
result of millions of years of volcanic activity.
Each volcano has:
a summit crater – this is the mouth of
the volcano, where the lava exists
a magma chamber - where the lava wells
up underground
a central vent - leads from the magma
chamber to the summit crater.
The biggest variation in volcano structure is the edifice,
the structure surrounding the central vent. The edifice is built up by
the volcanic material spewed out when the volcano erupts. Consequently,
its composition, shape and structure are all determined by the nature of
the volcanic material and the nature of the eruption.
C. Different types of volcanoes
Stratovolcanoes have the most destructive history, erupt hundreds of years apart. Fairly symmetrical edifice.
Scoria cone volcanoes : Small, most common. Steep slopes, wide summit crater. Most have only one eruption event.
D. Active volcanoes around the world (hot spots)
Hot spots are places within the mantle or oceanic lithosphere where rocks melt to generate magma. When a hot spot is located in the oceanic lithosphere, shield volcanoes are built. These are constructed on the deep ocean floor and may become high enough to rise above sea level as volcanic islands. The Hawaiian hot spot, for example, has been active at least 70 million years, producing a volcanic chain (of shield volcanoes) that extends 3,750 miles (6000 km) across the northwest Pacific Ocean.
The Yellowstone hot spot has been active for at least 15 million years, producing a chain of calderas and other volcanic features along the Snake River Plain (US) that extends 400 miles (650 km) westward from northwest Wyoming to the Idaho-Oregon border.
Where a hot spot lies beneath a continental plate the hot spot may generate huge amounts of lava that accumulate layer upon layer, and can cover thousands of square kilometers. These accumulations are flood basalts.
E. Volcanoes on other planets and the moon
Volcanoes are not restricted to the planet Earth.
Manned and unmanned planetary explorations, beginning in
the late 1960's, have revealed graphic evidence
of past volcanism and its products on the Moon, Mars, Venus and other
planetary bodies. Many pounds of volcanic rocks
were collected by astronauts during the various Apollo lunar landing
missions. Only a small amount of these samples
have been subjected to exhaustive study by scientists. The bulk of the
material is stored under controlled-environment
conditions at NASA's Lunar Receiving Laboratory in Houston, Tex., for
future study by scientists.
A volcano on Mars
A
volcanic plume on one of Jupiter's moons (Io)
II. Volcanic eruptions
An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the volcano's vents. When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma. The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.
B.
Effects of volcanic eruptions
1. Volcanic eruptions and the earth's climate
Volcanic eruptions can alter the climate of the earth for both short
and long periods of time.
Volcanoes affect the climate through the gases and dust particles thrown
into the atmosphere during eruptions.
2.
Volcanic eruptions and the earth's atmosphere
a. Sulfur
When sulfur dioxide
gas is released, it reacts chemically with sunlight, oxygen, dust particles,
and water in the air to form a mixture of sulfate sulfuric acid and other
oxidized sulfur species.
Together, this produces a hazy gas and aerosol mixture atmospheric
condition known as volcanic smog or "vog.“ Vog pollutes air quality and
causes increased acidity in the rainwater, destroying buildings,
contaminating drinking water, and harming wildlife.
b. Water and carbon dioxide
Volcanoes also release large amounts of water and carbon dioxide. These
gases absorb infrared radiation emitted by the ground and hold it in the
atmosphere. This causes the air below to get warmer. Water and carbon dioxide
from volcanoes do not have a significant immediate effect on climate because
amounts released are very small compared to amount already present in the
atmosphere. Human activities release more than 150 times the amount of
CO2 emitted by volcanoes
However, over long periods of time (thousands or millions of years),
multiple eruptions of giant volcanoes can raise the carbon dioxide levels
enough to cause significant global warming.
c. Interaction between humans and volcanic activity
Aerosols released by volcanoes are combining with bromine and chlorine gases released by man-made CFC’s to destroy the ozone layer.
3. Positive effects of volcanic eruptions
Not all volcanic phenomena are destructive. The
oceans, atmosphere, and continents owe their
origin and evolution largely to volcanic
processes throughout geologic time.
Volcanic gases are the source of all the water (and most of the atmosphere)
that we have today
Volcanic ash contains minerals which produce fertile soil
Volcanoes can provide refuges for rare plants and animals from the
ravages of humans and livestock.
C.
Historic volcanic eruptions and their effects
World climate seems to have been affected by the
eruptions of Krakatoa in 1883 and of Agung
Volcano in Bali in 1963 in the form of a lowering
of average world temperature by about 0.5 C over
the few years following these eruptions. Although
world temperature data was poorly recorded in the
early 1800s, the eruption of Tambora in April 1815
was followed in 1816 in North America and
Europe by what was called "the year without a
summer." Other large volcanic eruptions such as
Katmai in Alaska in 1912, however, appear to have
produced no cooling effect. Records of average
world temperature over the past several decades
often show changes of 0.1-0.3 C from year to year
unrelated to any known volcanic eruptions, so it is
difficult to establish clearly whether or not volcanic
eruptions have a major impact on climate.
Intro to Global Change class web site: http://ess.geology.ufl.edu
Volcano World: http://volcano.und.nodak.edu/
USGS Fact Sheet: http://water.usgs.gov/wid/index-hazards.html
"Earth's Interior & Plate Tectonics" http://www.hawastsoc.org/solar/eng/earthint.htm
NASA Fact Sheet: "Volcanoes and Global Cooling" :
http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/earthsci/volcano.htm