ugrc 144 science and technology in our lives/geohazards · volcanic eruption to occur. –volcanic...

College of Education

School of Continuing and Distance Education2014/2015 – 2016/2017

UGRC 144

Science and Technology in Our

Lives/Geohazards

Session 5 – Magma and Volcanism

Lecturer: Dr. Patrick Asamoah SakyiDepartment of Earth Science, UG

Contact Information: [email protected]

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


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


MAGMA FORMATION Topic One


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.


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


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


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


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


Magma Formation – Environment of Formation


• Environments include; - continental rift zones- subduction zones - mid-oceanic ridges- hotspots (mantle plumes)

LAVA, VOLCANO AND VOLCANISMTopic Two


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 compositionsdepending 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.



• 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.



• Anatomy of a “Typical” Volcano



• 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



• If magma is extruded onto the surface of the earth, it becomes a lava and forms Extrusive/Volcanic igneous rocks.


VOLCANIC MATERIALTopic Three


Volcanic Material

• Three basic kinds of materials may eruptfrom a volcano. They are:

- Lava

- Rock Fragments and

- Gas

When lava comes to the surface, it is red hotand may have temperatures of more than1,100oC.


Volcanic Material

• Lava Flows –

– Basaltic lavas are more fluid


• Pahoehoe Lava (smooth, unbroken lava) - has a smooth and undulating surface

• Aa lava (stony rough lava)- characterized by a rough or rubble surface composed of broken lava blocks.

• Pillow lava contains characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion

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.


Volcanic Material

• Volcanic dust consists of particles 0.25 mm indiameter and can be carried great distances. Somescientists believe that large quantities of volcanic dustcan affect the climate by reducing the amount ofsunlight that reaches the earth.

• Volcanic Ash is made up of fragments less than 0.5mm. Sometimes volcanic ash combines with water ina stream or lake and forms a boiling mudflow.Mudflows may rich speeds of 100 kilometres per hourand can be highly destructive.


Volcanic Material

• Volcanic bombs are large fragments. The largestones may measure more than one metre across.Small volcanic bombs are generally called cinders.

• Gas pours out of volcanoes in large quantitiesduring most eruptions. Volcanic gas carries a largeamount of volcanic dust and this combination ofgas and dust looks like black smoke.


FACTORS AFFECTING VOLCANIC ERUPTIONS

Topic Four


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



• 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)



• 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)



• 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.



• 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

CLASSIFICATION OF VOLCANOESTopic Five


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.

TYPES OF VOLCANOESTopic Six


Types of Volcanoes


• Volcanoes can be classified by their shape and sizes;

– Shield Volcanoes

– Cinder Cones

– Calderas

– Stratovolcano (Composite Volcano)

– Lava Dome

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.

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 bowl-shaped crater at the summit

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.

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 emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption.

• If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic structure above it

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.

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.

END


ugrc 144 science and technology in our lives/geohazards · volcanic eruption to occur. –volcanic...

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