UNIT 4 VOLCANOES WHAT IS A MAGMA Magma

UNIT - 4 VOLCANOES

WHAT IS A MAGMA Ø Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth. Ø In some instances, it solidifies within the crust to form plutonic rocks. In others, it erupts onto the Earth’s surface to form volcanic rocks

MAGMA BEHAVIOUR Ø The magma cools as it enters shallower and cooler levels of the Earth. Ø Second, pressure drops because the weight of overlying rock decreases. Ø Cooling tends to solidify the magma, but decreasing pressure tends to keep it liquid.

TYPES OF MAGMA Magma Type Basaltic Andesitic Rhyolitic/ Granitic Solidified Rock Chemical Composition Temperature Viscosity Gas Content 45 -55 Si. O 2 %, high in Fe, Mg, Ca, low in K, Na 1000 - 1200 o. C Low Andesite 55 -65 Si. O 2 %, intermediate in Fe, Mg, Ca, Na, K 800 - 1000 o. C Intermediate Rhyolite 65 -75 Sio 2 %, low in Fe, Mg, Ca, high in K, Na. 650 - 800 o. C High Basalt • Higher Si. O 2 (silica) content magmas have higher viscosity than lower Si. O 2 content magmas (viscosity increases with increasing Si. O 2 concentration in the magma). • Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).

PLUTONS Ø In most cases, granitic magma solidifies within the Earth’s crust to form a pluton. Ø A batholith is a pluton exposed over more than 100 square kilometers of the Earth’s surface. Ø A stock is similar to a batholith but is exposed over less than 100 square kilometers.

PLUTONS BATHOLITH AND STOCK

PLUTONS Ø A dike is a tabular, or sheet like, intrusive rock that forms when magma oozes into a fracture Dikes cut across sedimentary layers or other features in country rock and range from less than a centimeter to more than a kilometer thick. Ø Magma that oozes between layers of country rock forms a sheet like rock parallel to the layering, called a sill.

PLUTONS DIKE SILL

VOLCANIC ROCKS AND VOLCANOES Ø The material erupted from volcanoes creates a wide variety of rocks and landforms, including lava plateaus and several types of volcanoes. Ø Lava is fluid magma that flows onto the Earth’s surface. Lava generally comes on to the earth’s surface through volcanoes. Ø A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface

VOLCANIC ROCKS AND VOLCANOES LAVA

VOLCANIC ROCKS AND VOLCANOES VOLCANO

TYPES OF LAVA Ø A’a: Pronounced “ah-ah”, this is a basaltic lava that doesn’t flow very quickly. These types of lava erupt at temperatures above 1000 to 1100 degrees C

TYPES OF LAVA Ø Pahoehoe: Pronounced “pa-ho-ho”, this type of lava is much thinner and less viscous than a’a. It can flow down the slopes of a volcano in vast rivers. Pahoehoe erupts at temperatures of 1100 to 1200 degree C

TYPES OF LAVA Ø Pillow Lava: Pillow lava is typically found erupting from underwater volcano vents.

STRUCTURES IN VOLCANIC ROCKS Ø When lava cools, escaping gases such as water and carbon dioxide form bubbles in the lava. Ø If the lava solidifies before the gas escapes, the bubbles are preserved as holes called vesicles

STRUCTURES IN VOLCANIC ROCKS Ø Hot lava shrinks as it cools and solidifies. The shrinkage pulls the rock apart, forming cracks that grow as the rock continues to cool. Such cracks, called columnar joints

PYROCLASTIC ROCKS � If a volcano erupts explosively, it may eject both liquid magma and solid rock fragments. A rock formed from particles of magma that were hurled into the air from a volcano is called a pyroclastic rock � The smallest particles is called volcanic ash

PYROCLASTIC ROCKS Ø Cinders vary in size from 2 to 64 millimeters

PYROCLASTIC ROCKS Ø Particles greater than 64 mm in diameter are called volcanic bombs

FISSURE ERUPTIONS AND LAVA PLATEAUS Ø The gentlest type of volcanic eruption occurs when magma comes out from the cracks in the land surface called fissures and flows over the land like water. Ø Basaltic magma commonly erupts in this manner because of its low viscosity

FISSURE ERUPTIONS AND LAVA PLATEAUS Ø Some times fissures extend for tens or hundreds of kilometers and pour thousands of cubic kilometers of lava onto the Earth’s surface. Ø A fissure eruption of this type creates a flood basalt, which covers the landscape like a flood

FISSURE ERUPTIONS AND LAVA PLATEAUS Ø Many such eruptions may occur in rapid succession and to create a lava plateau covering thousands of square kilometers

VOLCANOES Ø If lava is too viscous to spread out as a flood, it builds a hill or mountain called a volcano.

VOLCANOES vent Ø Volcanoes differ widely in shape, structure, and size. Ø Lava and rock fragments commonly erupt from an opening called a vent. cone Ø The vent joins the crater which is a bowl shaped depression present at the top of the volcano. conduit magma chamber

VOLCANO TYPES BASED ON ACTIVITY Ø An active volcano is one that is erupting or is expected to erupt Ø A dormant volcano is one that is not now erupting but has erupted in the past and will probably do so again Ø An extinct volcano is one that is expected never to erupt again

TYPES OF VOLCANOES

TYPES OF VOLCANOES BASLAT PLATEAU SHIELD VOLCANO

TYPES OF VOLCANOES COMPOSITE VOLCANO CINDER CONE CALDERA

DISTRIBUTION OF WORLD VOLCANOES

PLATE TECTONICS AND VOLCANIC ACTIVITY Ø Most active volcanoes are associated with plate boundaries. Ø Active areas of volcanism are found along mid-ocean ridges where seafloor spreading is occurring (divergent plate boundaries), Ø in the vicinity of ocean trenches where one plate is being subducted beneath another (convergent plate boundaries), and Ø In the interiors of plates themselves (intraplate volcanism). Rising plumes of hot mantle rock are the source of most intraplate volcanism.

ACTIVE VOLCANOES AND PLATE TECTONICS

VOLCANO MONITORING AND HAZARD MITIGATION

VOLCANIC FATALITIES Ø 92, 000 Tambora, Indonesia 1815 Ø 36, 000 Krakatau, Indonesia 1883 Ø 29, 000 Mt Pelee, Martinique 1902 Ø 15, 000 Mt Unzen, Japan 1792 Courtesy of www. swisseduc. ch But, volcanoes cause fewer fatalities than earthquakes, hurricanes and famine.

VOLCANIC HAZARDS Ø Ø Ø Courtesy of www. swisseduc. ch Pyroclastic flow Lahars/Mud flows Pyroclastic fall Lava flow Noxious Gas Earthquakes

PYROCLASTIC FLOW Ø For example, eruption of Vesuvius in 79 AD destroyed the city of Pompeii

POMPEII (79 AD) On August 24, 79 AD Mount Vesuvius literally 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.

POMPEII (79 AD) Pyroclastic flows of poisonous gas and hot volcanic debris engulfed the cities of Pompeii, Herculaneum and Stabiae suffocating the inhabitants and burying the buildings.

POMPEII (79 AD) The cities remained buried and undiscovered for almost 1700 years until excavation began in 1748. These excavations continue today and provide insight into life during the Roman Empire.

HOW DO PYROCLASTIC FLOWS CAUSE DEVASTATION?

PYROCLASTIC FLOW - DIRECT IMPACT Courtesy of www. swisseduc. ch

PYROCLASTIC FLOW - BURIAL

PYROCLASTIC FLOW - BURNS

PYROCLASTIC FLOW - LAHARS Ø Ø Hot volcanic activity can melt snow and ice Melt water picks up rock and debris Forms fast flowing, high energy torrents Destroys all in its path

PYROCLASTIC FALL Ø Ash load • Collapses roofs • Brings down power lines • Kills plants • Contaminates water supplies • Respiratory hazard for humans and animals

LAVA FLOW Ø It is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous.

So…. How do we minimize the risk of active volcanoes?

VOLCANO MONITORING Volcano Observatories are set up on all active volcanoes that threaten the human population. These are designed to monitor and potentially to predict the eruptive behaviour of the volcano in question.

VOLCANO MONITORING Seismicity � Deformation � Gas Output � � (on volcano and remote sensing techniques) These three things are the most important precursors to an eruption.

SEISMIC ACTIVITY Ø Earthquake activity commonly precedes an eruption • • • Result of magma pushing up towards the surface Increase volume of material in the volcano shatters the rock This causes earthquakes

SEISMIC ACTIVITY • Earthquake activity is measured by Seismographs – Seismographs are stationed on the flanks of the volcano – These record the frequency, duration and intensity of the earthquakes and report it back to the volcano observatory.

DEFORMATION MONITORING � “Tiltmeters” are used to measure the deformation of the volcano � The tiltmeters measure changes in slope as small as one part per million. A slope change of one part per million is equivalent to raising the end of a board one kilometer long only one millimeter!

DEFORMATION MONITORING � Tilltmeters can tell you when new material enters the magma chamber. A B Note the presence of earthquakes in relation to the deformation. Often it is a combination of events that fore -warns of an eruption.

GAS MONITORING � Commonly gas output from a volcano increases or changes composition before an eruption. � As magma rises to the surface it releases (exsolves) much of its gas content. � This can be measured

GAS MONITORING � Gas samples are collected from fumaroles and active vents. � Gas levels may also be monitored by remote sensing techniques

IN SUMMARY. . Ø Volcanoes are extremely hazardous. Ø However, the volcano can be studied, monitored and understood. Ø Each volcano is different, and offers a unique set of dangers Ø Plans may be emplaced to help control potential damage.