UGRC 144 Science and Technology in Our Lives/Geohazards

Transcription

1 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: pasakyi@ug.edu.gh College of Education School of Continuing and Distance Education 2014/ /2017

2 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

3 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

4 Reading List Chapter 5 of Environmental Geology 4 th 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

5 Topic One MAGMA FORMATION Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 5

6 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 ( 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

7 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

8 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

9 Magma Formation - Role of Heat & Role of Heat Water 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

10 Magma Formation - Role of Heat & Role of Water 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

11 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

12 Topic Two LAVA, VOLCANO AND VOLCANISM Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 12

13 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

14 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

15 Lavas, Volcano & Volcanism Anatomy of a Typical Volcano Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 15

16 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

17 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

18 Topic Three VOLCANIC MATERIAL Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 18

19 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 1,100 o C. Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 19

20 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 Slide 20 Pahoehoe Lava (smooth, unbroken lava) - has a smooth and undulating surface

21 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

22 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

23 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

24 Topic Four FACTORS AFFECTING VOLCANIC ERUPTIONS Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 24

25 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

26 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

27 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 SiO 2 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

28 Factors Affecting Volcanic Eruptions Composition of Dissolved Gases in Magmas/Lavas Mostly H 2 O (water vapor) & some CO 2 (carbon dioxide) Minor amounts of Sulfur (Hydrogen sulphide (H 2 S) & Sulphur dioxide (SO 2 )), 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

29 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

30 Factors Affecting Volcanic Eruptions Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 30

31 Topic Five CLASSIFICATION OF VOLCANOES Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 31

32 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

33 Topic Six TYPES OF VOLCANOES Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 33

34 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

35 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

36 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

37 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

38 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. Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 38 If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic structure above it

39 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

40 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

41 END Dr. Patrick A. Sakyi, Dept. of Earth Science Slide 41