Presentation on theme: "Volcanoes A volcano is a place on the Earth’s surface where magma breaks through. A volcano is called active if it erupts lava, rock, gas or ash, or if."— Presentation transcript:
1VolcanoesA volcano is a place on the Earth’s surface where magma breaks through.A volcano is called active if it erupts lava, rock, gas or ash, or if it shows seismic activity.A volcano is dormant when is not presently erupting but is considered likely to erupt in the future. (Foxworthy and Hill)An extinct volcano will never erupt again.
2The volcano includes the surrounding cone of erupted material. A volcano is a vent or 'chimney' that connects the magma chamber (subterranean cavity containing the gas-rich liquid magma) to the Earth’s surface.The volcano includes the surrounding cone of erupted material.ventconemagma chamberconduit
3In the Earth's mantle the temperature is hot enough (800-1 In the Earth's mantle the temperature is hot enough ( °C) to melt rock and form a thick, flowing substance called magma. Lighter than the solid surrounding rock, magma is buoyant much like a cork in water; being buoyant, it rises.Most magmas contain abundant gas , as the magma rises closer to the surface, the pressure decreases, which cause the gases dissolved in the magma to expand. This expansion propels the magma through openings fractures in the Earth’s surface creating a volcanic eruptionOnce magma is erupted, it is called lava.
4Volcanic eruptions do not occur randomly, but are mainly localized at plate boundaries. Some volcanoes, can be found in the interior of plates at areas called hot spots.Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring along spreading oceanic ridges.
5VOLCANIC ACTIVITYTwo styles of volcanic eruption: Explosive and EffusiveExplosive: where rapidly escaping gas bubbles (= vesicles) rip apart the magma, fragmenting it.Effusive: where the magma leaks out onto the surface passively as lava flows.
6Sometimes basaltic lava pours out quietly from long fissures and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaux. Lava plateaux can be seen in Iceland, India and USA. (Tilling, 1985)
7Mauna Kea (left) and Mauna Loa (right), Shield volcanoes are built almost entirely of fluid lava flows forming a broad, gently sloping cone, with a profile much like that a warrior's shield. They are built up slowly by highly fluid basaltic lava that spread widely over great distances. The Hawaiian Islands are composed of linear chains of these volcanoes, including Kilauea and Mauna Loa, two of the world's most active volcanoes.Mauna Kea (left) and Mauna Loa (right),
8Fuego (left) and Acatenango - Guatemala Some of the Earth's highest mountains are composite volcanoes; sometimes called stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombsFuego (left) and Acatenango - Guatemala
9In a "Strombolian"-type eruption huge clots of molten lava burst form the summit crater to form luminous arcs through the sky.StromboliStromboli is a volcan characterized by frequent activity and high viscosity lavas.
10A "Vulcanian"-type eruption is characterized by a dense cloud of ash-laden gas that explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone.
11In a "Pelean" eruption a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form a dense cloud that move down-slope at velocities as great as 100 miles per hour. Such eruptions can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mount Pelee on Martinique, West Indies.beforeafter
12“Plinian” eruption: on August 24, 79AD Vesuvius blew its top, erupting tonnes of molten ash, pumice and sulfuric gas miles into the atmosphere. Pyroclastic flows flowed over the city of Pompeii and surrounding areas.
13WHY DO COMPOSITE VOLCANOES TEND TO BE EXPLOSIVE AND SHIELD VOLCANOES NON-EXPLOSIVE?? CHEMISTRY (COMPOSITION) OF LAVALAVA TEMPERATURECOMPOSITEHigher silica content of lavas make them more viscous (thick).Lava temperatures are generally a few hundred degrees cooler than those of shield volcanoes, making the lavas “thicker” in consistency.SHIELDLower silica content of lavas make them more fluid (runny).Lava temperatures are quite hot, ~ 2200oF, making it easier for these lavas to “flow like water”.
14PRODUCTS OF EXPLOSIVE ERUPTIONS Pyroclastic materialRock fragments created by explosive eruptionsmagma explodes from volcano and solidifies in the airexisting rock is shattered by powerful eruptionsLapilliVolcanic bombsVolcanic blocksVolcanic ash
15Three products from an explosive eruption Ash fall: volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.Pyroclastic flow: a hot, fast-moving and high-density mixture of fine and coarse particles and gas formed during explosive eruptions or from the collapse of a lava domeBase surge: turbulent, low-density cloud of rock debris and water and (or) steam that moves over the ground surface at high speed.
16Deposits associated to ash fall are generally called tuffs, volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuffs commonly are composed of much shattered volcanic rock glass chilled magma blown into the air and then deposited regularly on the ground like a snow fall. If the deposits are made by coarse – grained materials they are called volcanic breccia.
17Pyroclastic flow consists in the lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material. The deposits are not layered and tend to fill ground hollows, they are generally called ignimbrites
18Direct measurements of pyroclastic flows are extremely dangerous!!!
19Explosive volcanic activity can also produces mud flows called lahar Hot lava can melt snow and ice on the top of the cone, so that melt water picks up ashes and rocks forming fast flowing, high energy torrents.
20Base surges are generated by phreatomagmatic eruption: a type of volcanic explosion that occurs when water comes in contact with hot rocks or ash near a volcanic vent, causing steam explosions .
21Effusive (lava) activity If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. Lava flows rarely kill people because they move slowly enough for people to get out of their way. Lava flows, however, can cause considerable destruction to buildings in their path.
22Because of basalt's low silica content, it has a low viscosity Because of basalt's low silica content, it has a low viscosity .Therefore, basaltic lava can flow quickly and easily move >20 kilometers from a vent.
23When lavas contain large amounts of gases, bubbles tend to escape while lava is flowing giving rise to highly irregular surfaces. Aa (a Hawaiian term), is lava that has a rough, spiny and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior. (Tilling, Heliker, and Wright, 1987)
24If magma has a low content of gas, lava flow quikly solidifies at his top, while inside the hot fluid keeps on running downhill. Pahoehoe is a Hawaiian term for basaltic lava that has a smooth, or ropy surface. A pahoehoe flow typically advances as a series of small lobes that continually break out from a cooled crust.
25Fluid lava erupted under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies. Typically ranging from less than a foot to several feet in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water.
26FEATURES ASSOCIATED WITH COMPOSITE AND SHIELD-TYPE VOLCANOES Sometimes basaltic lava flows give rise to some spectacular geological features such as the “Giant Causeway” in Northern Ireland
27A lava dome is a steep-sided mass of viscous lava extruded from a volcanic vent, often circular in plan view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. (Foxworthy and Hill, 1982)
28Cinder cones, also known as scoria cones, represent the smallest type of volcano, rarely exceeding 300 m in height. They are entirely composed of pyroclastic materials (volcanic ash). They are quite common and associated with both composite or shield volcanoes.Volcano Sudoeste and the smaller cinder cones at its base are part of the San Quintin volcanic field located in Mexico’s Baja Peninsula.
29When volcanoes explode or eject a significant volume of material onto the surface the cone may collapse into the voided reservoir, forming a steep, bowl-shaped depression called a Caldera. These can be quite large, km in diameter!Crater Lake, Oregon (10km) Aniachak Caldera, Alaska
30Diatreme is a general term for a volcanic vent or pipe drilled through enclosing rocks (usually flat-lying sedimentary rocks) by the explosive energy of gas-charged magmas. The diamond-bearing kimberlite pipes of South Africa are diatremes.