Component-I(A) - Personal Details Role Name Affiliation Principal Investigator Prof. Masood Ahsan Siddiqui Department of Geography, Jam Jamia Millia Islamia, New Delhi Paper Coordinator, if any Dr. Syed Zaheen Alam Dayal Singh College, Delhi University Content Writer/Author Dr. Syed Zaheen Alam Dayal Singh College, (CW) Delhi University Content Reviewer (CR) Language Editor (LE) Component-I (B) - Description of Module Items Description of Module Subject Name Geography Paper Name Geomorphology Module Name/Title Volcanoes. Module Id GEO-11 Pre-requisites Objectives Keywords VOLCANOES Sayed Zaheen Alam, Assistant Professor, Department of Geography, Dyal Singh College, University of Delhi, New Delhi. A volcano is an opening or rupture in the Earth's surface or crust, which allows hot molten magma, ash and gases to escape from deep below the surface. The term volcano in general refers to the landform built up from the accumulation of lava and pyroclastic debris. Therefore they are very different from other mountains; since they are not formed by folding and crumpling or by uplift and erosion. Volcanoes are spectacular windows to the Earth's internal parts. These natural windows allow us to catch a glimpse of the inaccessible depths of the surface of the Earth. Volcanic activity directly provides an explanation that the earth’s interior part must be experiencing high pressure and temperature. There are more than 500 active volcanoes on Earth’s surface, including well-known examples such as Mt. Fuji, Mt. St. Helens, and Mauna Loa. Skeleton of a volcano Commonly a cone shaped volcano constitutes a vent, a pipe, a crater, and a cone. The volcanic vent is an opening at the Earth's surface through which volcanic materials are erupted at the surface of the earth. The shape of the vent may be roughly circular conduits or elongated fissures. The pipe is a passageway in the volcano in which the magma rises through to the surface during an eruption. The crater is a steep bowl-shaped depression at the top of the volcano out of which volcanic materials like, ash, lava, and other pyroclastic materials are released. The molten material beneath the surface of the Earth is called magma. Once it starts moving towards the crust or it reaches the surface, here it is referred to as lava. Figure: Skeleton of a typical volcano. Did You Know? The word "volcano" comes from the little island of Vulcano in the Mediterranean Sea of Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the copy of Vulcan - the blacksmith of the Roman gods. Why Do Volcanoes Erupt? For volcanologist it is still a puzzling question that why do volcanoes erupts. Various hypotheses have been proposed. It is assumed that in the beginning Earth was in molten state. Then gradually it radiated its heat and cooled down. Earthquake waves suggests that the earth’s mantle Figure: Process of Volcanic Eruption. and core is hot which melts the rocks and creates the magma. The hot magma comes in contact with water the later becomes too hot and generate steam pressure. The high pressure inside the magma chamber caped by surrounding rock act as strong lid (like the lid of the pressure cooker) which keeps the gas dissolved within the liquid magma. The weaker areas within the crust allow magma and gases to penetrate the rocks and may come out in the form volcanic eruption. For example, when you shake and open a tightly sealed cold drink bottle then the pressure inside the bottle instantly decreases. Once that happens, the gas comes out quickly of its dissolved state and forms bubbles. A volcano may erupt in a similar way, though obviously with much more power and explosion. Before the eruption volcanoes may also give early warning signals by producing a rushing, a hissing and a coughing and puffing sounds. However it is still difficult to predict the volcanic eruption. Nevertheless scientists in order to predict the volcanic eruption monitor seismic activity in volcanic are, the geochemical conditions, change in temperature, topographical change and change in gas composition. The major human adjustment to volcanic activity is evacuation. Education and awareness plays an important role in informing people about the hazards of volcanoes. Did You Know?` Sometimes high-viscosity magma comes out from the pipe like toothpaste being squeezed out of a tube. The volcanologists sometimes use the term “toothpaste lava” to denote this eruption. Distribution of volcanoes and volcanic activity Most of the world’s volcanoes and volcanic activity can be sighted along the plate boundaries. The distribution can be classified into one of the following tectonic settings: (1) Subduction Zones in the Circum Pacific Belt The zones where one plate goes down under the other due to density difference are the sites of most of the world’s active and explosive volcanoes. The oceanic plate having higher density is subducted under the continental crust. The subducted slab melts under the increasing pressure and temperature to produce magma which comes out through andesitic chain of volcanoes. The volcanoes are mainly situated on the continental side of the trenches. The figure portrays that the so called “Pacific Ring of Fire” is the collection of volcanoes bordering the Pacific Ocean. This zone is infact a ring of subduction zones. It includes some of the deadliest volcanoes known, such as Pinatubo and Mt. St. Helens. The figure demonstrates that it starts from the Andean region of South America and extends northwards through Central America, Mexico, Western U.S.A. and Canada to Alaska. From Alaska it extends through Aleutian Islands towards the islands off the eastern coast of Asia and passes through Kamchatka, the Kurile Islands, Japan, the Philippines and further south to the New Guinea, Solomon Islands, New Zealand and Antarctica. The volcanic belt of the Indian Ocean which passes through Andamans, Sumatra, Java, Bali, Sunda and Burma meets the Pacific belt near the Malacca Island. Figure: Pacific Ring of Fire. (2) Divergence zones: volcanoes of the Mid Atlantic Ridge and over the Continents In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. For example the Mid-Atlantic Ridge separates the North and South American Plate from the Eurasian and African Plate. The figure demonstrates that, this pulling apart is causing "sea-floor spreading" as new volcanic material is added to the oceanic plates. The spreading sites are the common sites of basaltic lava eruption. On the whole, sea-floor spreading is basically volcanic, but it is a very slow and regular process, without the explosive outbursts of the volcanoes on land. Magma rises through the cracks and leaks out onto the ocean floor like a long, thin, undersea volcano. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. This process along the divergent boundary has created longest topographic feature in the form of Mid oceanic ridges under the Oceans of the world Most of this activity is out of sight under the oceans which is less hazardous to people. Figure: Volcanoes of the Mid Atlantic Ridge in divergent zones. The map shows that over the continents, the divergence zones with fissure type of volcanic eruptions are represented by the East African Rift Valley Zone. This belt extends from Ethiopia to Tanzania. The Kilimanjaro in Tanzania is a well known example of this belt. Figure: East African Rift Valley Zone. (3) Intra-plate oceanic volcanism (Hawaiian chain and other oceanic volcanic seamounts) Intra-plate oceanic volcanism can be represented by a single oceanic volcano, or lines of volcanoes such as the Hawaiian-Emperor seamount chains. They are also popular as hotspots and are located within the tectonic plates instead of plate margins. The map demonstrate that the Hawaiian volcanoes are located well within the Pacific plate rather than near a plate boundary. Figure: Location of Hawaiian volcanoes within the Pacific plate. (4) Mid Continental belt and Volcanoes in the Mediterranean region: This belt is extended from the Mediterranean Alps to the Himalayan Region. Most often visited active volcanoes are found in this belt. Vesuvius and Stromboli are well known example of this belt. Mount Etna in Sicily is Europe’s largest volcano. Its frequent eruptions often attract visitors. Figure: Etna in Sicily, Europe’s largest volcano. Products of volcanic eruption The products of volcanic eruption may be in the form of gas, solid and liquid. They are as follows: Gases It is assumed that interactions between the gases contained in molten magma increase pressure and heat within the ascending lavas which results in explosive activity. Therefore, the most explosive eruptions are probably the consequence of gas-rich magmas. Major part of gases consists of hydrogen (H), carbon (C), oxygen (O), sulfur (S) and nitrogen (N). The hydrogen is largely present as water (H2O). Volcanoes also emit methane (CH4) and ammonia (NH3) but in trace amounts. They also have an important effect on the regional and global environment and may contribute greenhouse gases to the atmosphere. Do You Know? Volcanologists also use typical gas mask near volcanoes to safe their own life from the hazardous effect of volcanic gases. Solid When fragments are blown out by explosive eruptions, soild materials are ejected, known generally as pyroclastic materials or pyroclasts (Pyro = Fire Clast = Fragments). The term tephra covers all the pyroclastic debris that collects through vertical airfall. The finest of these is the dust which can remain suspended in the air for long time and can travel long distances. The size of pyroclasts may be less than 2 millimeters to pieces that weigh several tons.