Download UGRC 144_Session 5

UGRC 144
Science and Technology in Our
Lives/Geohazards
Session 5 – Magma and Volcanism
Lecturer: Dr. Patrick Asamoah Sakyi
Department of Earth Science, UG
Contact Information: [email protected]
College of Education
School of Continuing and Distance Education
2014/2015 – 2016/2017
Session Overview
• This session introduces students to ;
– sequence of events that take place in order for
volcanic eruption to occur.
– volcanic processes and the materials involved, driving
forces of volcanism, the volcanic processes,
environments where magma is formed prior to its
ascent to the surface of the earth or to shallow levels.
– the composition of magma or lava and its effect on
the degree of eruption and the type of volcano
formed.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 2
Session Outline
The key topics to be covered in the session are as follows:
• Topic One – Magma Formation
• Topic Two – Lavas, Volcano and Volcanism
• Topic Three – Volcanic Material
• Topic Four– Factors Affecting Volcanic Eruptions
• Topic Five – Classification of Volcanoes
• Topic Six - Types of Volcanoes
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 3
Reading List
• Chapter 5 of Environmental Geology 4th Edition – by
Carla W Montgomery (1995)
• Unit 1, Section 5 of UGRC 140 II Geohazards –
Institute of Continuing and Distance Education
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 4
Topic One
MAGMA FORMATION
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 5
Magma Formation
• Partial melting of portions of the upper mantle
and crust occurs to form hot, molten material in
the interior of the Earth is called Magma.
• Magma is simply a melt, a liquid solution of
elements at high temperature that forms deep in
the Earth (50 - 200 kilometres or so)
• Once melt occurs, the magma tends to rise
toward the surface of the earth and because it is
less denser than solid rock.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 6
Magma Formation
• As magma rises, cooling begins.
• With a decrease in
temperature, the melt starts to
solidify and mineral grains begin
to grow.
• The heat required to generate
magma comes from within the
earth
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 7
Magma Formation
• As magma rises, it moves into harder, colder
rock of the lithosphere, where it slows down
and collects in larger bodies to form magma
chambers which are network of interconnected
channels
• As magma nears the surface, its dissolved gases
begin to come out of solution.
• The combination of buoyancy and degassing is
what powers volcanic eruptions
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 8
Magma Formation - Role of Heat &
Water
• Role of Heat
– The geothermal gradient - Earth’s natural
temperature increases with depth but is not
sufficient to melt rock in the lower crust and upper
mantle
– Additional heat is generated by;
- friction in subduction zones
- crustal rocks heated during subduction
- rising, hot mantle rocks
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 9
Magma Formation - Role of Heat &
Water
• Role of Water
– Causes rock to melt at a lower temperature
– Plays an important role in subducting ocean
plates
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 10
Magma Formation – Environment of
Formation
• Environments include;
- continental rift zones
- subduction zones
- mid-oceanic ridges
- hotspots (mantle plumes)
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 11
Topic Two
LAVA, VOLCANO AND VOLCANISM
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 12
Lavas, Volcano & Volcanism
• A volcano is essentially a fissure or vent
communicating with the interior of the earth from
which hot, molten rock (magma), rock fragments,
ashes, dust and gases erupt
• Volcanoes erupt lavas with varying compositions
depending on what part of their magma chambers
they are tapping.
• The composition of a given magma also depends on
the composition of the part of the mantle that is
melted.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 13
Lavas, Volcano & Volcanism
• Volcanism is the eruption of Lava from deep in
the Earth.
• Volcanoes represent the surface expressions of
subsurface igneous activity.
• The mountain or hill formed by the eruptive
lava/debris is also often called a volcano.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 14
Lavas, Volcano & Volcanism
• Anatomy of a “Typical” Volcano
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 15
Lavas, Volcano & Volcanism
• A crater is the depression at the summit of a
volcano or that which is produced by a
meteorite impact.
• A conduit, or pipe, carries gas-rich magma to
the surface.
• A vent is an opening in the Earth's crust where
molten lava and volcanic gases escape onto the
land surface or into the atmosphere
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 16
Lavas, Volcano & Volcanism
• If magma is extruded onto the surface of the
earth, it becomes a lava and forms
Extrusive/Volcanic igneous rocks.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 17
Topic Three
VOLCANIC MATERIAL
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 18
Volcanic Material
• Three basic kinds of materials may erupt
from a volcano. They are:
- Lava
- Rock Fragments and
- Gas
When lava comes to the surface, it is red hot
and may have temperatures of more than
1,100oC.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 19
Volcanic Material
• Lava Flows –
– Basaltic lavas are more fluid
• Pillow lava contains characteristic pillow-shaped structures
that are attributed to the extrusion of the lava under
water, or subaqueous extrusion
• Aa lava (stony rough lava)- characterized by a rough
or rubble surface composed of broken lava blocks.
Dr. Patrick A. Sakyi, Dept. of Earth Science
• Pahoehoe Lava (smooth, unbroken lava) - has
a smooth and undulating surface
Slide 20
Volcanic Material
Highly fluid lava flows rapidly down a volcano’s slopes.
Viscous lava flows more slowly.
• Rock fragments, generally called tephra are formed
from viscous magma. Tephra includes
- volcanic dust,
- volcanic ash and
- volcanic bombs.
• Pyroclastic is the name given to particles produced in
volcanic eruptions.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 21
Volcanic Material
• Volcanic dust consists of particles 0.25 mm in
diameter and can be carried great distances. Some
scientists believe that large quantities of volcanic dust
can affect the climate by reducing the amount of
sunlight that reaches the earth.
• Volcanic Ash is made up of fragments less than 0.5
mm. Sometimes volcanic ash combines with water in
a stream or lake and forms a boiling mudflow.
Mudflows may rich speeds of 100 kilometres per hour
and can be highly destructive.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 22
Volcanic Material
• Volcanic bombs are large fragments. The largest
ones may measure more than one metre across.
Small volcanic bombs are generally called cinders.
• Gas pours out of volcanoes in large quantities
during most eruptions. Volcanic gas carries a large
amount of volcanic dust and this combination of
gas and dust looks like black smoke.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 23
Topic Four
FACTORS AFFECTING VOLCANIC
ERUPTIONS
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 24
Factors Affecting Volcanic Eruptions
• Factors that determine the violence of an
eruption are;
– Composition of the magma
–Temperature of the magma
–Dissolved gases in the magma
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 25
Factors Affecting Volcanic Eruptions
• Viscosity of Magmas
– Viscosity describes a fluid’s resistance to flow.
– Viscosity depends primarily on the
composition of the magma and temperature.
• Factors affecting viscosity
–Temperature (hotter magmas are less viscous)
–Composition (silica content)
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 26
Factors Affecting Volcanic Eruptions
• Temperature
– Lower temperature magmas have higher viscosity than
higher temperature magmas (viscosity increases with
decreasing temperature of the magma).
• Composition (Silica Content)
– Viscosity increases with increasing SiO2 concentration in
the magma).
1. High silica - high viscosity (e.g., rhyolitic lava)
2. Low silica – low viscosity/more fluid (e.g., basaltic
lava)
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 27
Factors Affecting Volcanic Eruptions
• Composition of Dissolved Gases in
Magmas/Lavas
– Mostly H2O (water vapor) & some CO2 (carbon dioxide)
– Minor amounts of Sulfur (Hydrogen sulphide (H2S) & Sulphur
dioxide (SO2)), Chlorine, and Fluorine gases
– Gases give magmas their explosive character, because
volume of gas expands as pressure is reduced.
– Gases expand near the surface and provide the force to
extrude lava.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 28
Factors Affecting Volcanic Eruptions
• Composition of Dissolved Gases in
Magmas/Lavas
– The amount of gas in a magma is also related to the chemical
composition of the magma.
– Rhyolitic magmas usually have higher gas contents than
basaltic magmas.
– Violence of an eruption is related to how easily gases escape
from magma
• Gases escape easily from fluid magma
• Viscous magma produces a more violent eruption
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 29
Factors Affecting Volcanic Eruptions
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 30
Topic Five
CLASSIFICATION OF VOLCANOES
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 31
Classification of Volcanoes
• Volcanoes have been classified according to their
plate-tectonic settings. They are:
– Subduction-Zone Volcanoes: these occur along
convergent plate boundaries
– Rift Volcanoes: occur along separating plate
boundaries
– Hot-Spot Volcanoes: occur within plates.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 32
Topic Six
TYPES OF VOLCANOES
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 33
Types of Volcanoes
• Volcanoes can be classified by their shape and
sizes;
– Shield Volcanoes
– Cinder Cones
– Calderas
– Stratovolcano (Composite Volcano)
– Lava Dome
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 34
Types of Volcanoes
• Shield Volcano is a large, gently
sloping volcano characterized by
broader dome
• Shield volcanoes are formed
by lava flows of low
viscosity-lava that flows
easily
• Example is the Mauna Loa
Shield Volcano, Hawaii.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 35
Types of Volcanoes
• Cinder cone is small volcano
with a steep conical hill of
volcanic fragments that
accumulate around and
downwind from a volcanic vent.
• They are built primarily of
pyroclastic (rock fragments)
material ejected from a
single vent.
• Cinder cones have a bowlshaped crater at the summit
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 36
Types of Volcanoes
• Stratovolcanoes (Composite
Volcanoes) are
characterized by a steep
profile and periodic,
explosive eruptions.
• The lava that flows from
stratovolcanoes tends to be
viscous; it cools and hardens
before spreading far.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 37
Types of Volcanoes
• Caldera is usually a large
circular depression at the
summit of a volcano.
• It is usually formed by the
collapse of land following a
volcanic eruption.
• The collapse is triggered by the • If enough magma is ejected,
the emptied chamber is
emptying of the magma
unable to support the
chamber beneath the volcano,
weight of the volcanic
usually as the result of a large
structure above it
volcanic eruption.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 38
Types of Volcanoes
• Lava Dome is a roughly circular mound-shaped
protrusion resulting from the slow eruption of felsic lava
(usually rhyolite or dacite) from a volcano, or from
multiple lava episodes of different magma types.
• The characteristic dome shape is attributed to high
viscosity that prevents the lava from flowing very far.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 39
Summary
• This session touched on the formation of magma and
the roles played by heat and water.
• We also looked at the environment where magma
forms.
• Mention was also made of the factors that affect
volcanic eruptions and these include, temperature
and composition, as well as the gas component of
the magma/lava.
• Types of volcanoes and the anatomy of a volcano has
were also discussed.
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 40
END
Dr. Patrick A. Sakyi, Dept. of Earth Science
Slide 41