Volcanoes Chapter 4 PDF
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Kuwait University
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This document provides general information about volcanoes. It describes the nature of magma, types of volcanoes and their features, destructive processes, and the effects on climate and the atmosphere.
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Volcanoes What is a volcano? Opening in the Earth’s surface that often forms a mountain built of lava and pyroclastics. Hi, I am A volcanologist Who study volcanoes? Volcanologists What is a Volcano? A volcano is a mount...
Volcanoes What is a volcano? Opening in the Earth’s surface that often forms a mountain built of lava and pyroclastics. Hi, I am A volcanologist Who study volcanoes? Volcanologists What is a Volcano? A volcano is a mountain that forms when magma reaches the surface of the Earth. Magma rises because it is less dense than the solid rock around it. Nature of Magma Magma: A body of molten rock found at depth, including any dissolved gases and crystals. It is the ancestral to all igneous rocks. Magma often collects in a magma chamber. Magma is under high pressure and sometimes emerges through volcanic vents in the form of flowing lava (molten rock as it exists above the Earth's surface) and pyroclastic ejecta. Nature of Magma Completely or partly molten material. Consist of three components: 1) a liquid component (melt, mobile Earth’s common elements; silica (SiO2), < Al, K, Ca, Na, Fe and Mg), 2) a solid component (silicate minerals that have already crystallized from the melt) and, 3) a gaseous phase (volatiles, water vapor (H2O), carbon dioxide (CO2) and sulfur oxide (SO2). Magma Sources Magma sources found at upper mantel at depths 50 - 250 km (where temp is high enough & pressure is low enough that rocks can melt wholly or partially). Volcanoes of the World Classification of volcanoes by activity Active: has erupted with in recent history. Dormant or “sleeping”: in active for the present but with the potential to become active again. Extinct or “dead”: very unlikely to erupt again Varieties in Magma Composition Volcanic rock type - Basalt (forms new sea floor at spreading ridges) and rich in + ferromagnesian (mafic). - Rhyolite: rich in silica (granite) (felsic) - Andesite: between (felsic + mafic) Picritic Magma Basaltic Magma Andesitic Magma Granitic Magma Originated from Originated from Interaction Originated from partial melting of partial melting of between mantle- partial melting of ultramafic rocks ultramafic rocks derived basaltic ultramafic rocks in the mantle in the mantle magma and more in the mantle silica-rich rocks in the crust SiO2 < 45% SiO2 < 50% SiO2 ~ 60% SiO2 > 70% Fe-Mg >8% up to Fe-Mg ~ 4% Fe-Mg ~ 3% Fe-Mg ~ 2% 32%MgO Temperature: up Temperature: ~ Temperature: ~ Temperature: ~ to 1500ºC 1200ºC 1000ºC 700ºC Viscosity: Viscosity: High Viscosity: Low to Viscosity: Low Intermediate very low Eruptive Eruptive Eruptive Eruptive behavior: behavior: behavior: Gentle behavior: Gentle Explosive Explosive What determines whether a volcano erupts violently or gently? Determined by: 1. Magma’s composition 2. Magma’s temperature 3. Amount of water and dissolved gasses it contains The above factors affect the viscosity (or mobility) of the magma. (Rhyolitic) magma (Andesitic) magma (Basaltic) magma (~70%) (~60%) (~50%) Violent eruption Intermediate eruption Gentle eruption Mafic lavas, quiescent volcanic eruption. Hawaii volcanoes Felsic lavas, violent volcanic eruption. Mount. St. Helen, USA Dissolved gases These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action. Trapped gases: Water (H2O) vapor and CO2 Dissolved gases increase the fluidity of magma. These gases are crucial in the generation of Earth’s atmosphere. Types of Volcanoes Types of Volcanoes Shield volcanoes: are created through the eruption of basaltic magmas, which flows easily as lavas and thus are capable of flowing over the landscape many kilometers from the central fissures or vents. Shield volcanoes are the largest volcanic features on earth – the largest shield volcano, Mauna Loa in the Hawaiian is an example. Islands of Hawaii Types of Volcanoes Cinder cones (tephra cones, scoria cones): are built from ejected lava fragments that take on the appearance of cinders or clinkers as they begin to harden while in flight. These fragments range in size from fine ash to bombs but consist mostly of pea- to walnut-size lapilli. They are the product of a single eruptive episode that sometimes last a few weeks and rarely exceeds a few years. Types of Volcanoes Composite cone (stratovolcano): Earth’s most picturesque volcanoes. It is a large, nearly symmetrical structure composed of both lava and pyroclastic deposits. Composite cones are the product of gas-rich magma having an andesitic composition (basaltic and rhyolitic magma are also common). Also, many composite cones generate explosive eruptions that eject huge quantities of pyroclastic material. Examples: Mount Fuji, Japan Fujiyama, Japan Mount Shasta, California Mount Mayon, Philippines Composite cone Mount Fuji, Japan Fissure Eruptions, Lava Domes,Volcanic Pipes and Volcanic Necks Fissure eruption (fissura = to split): Is a narrow linear fractures or cracks in the crust through which lava erupts, usually without any explosive activity. The fractures are few meters wide and may be many kilometers long. The Laki fissure in Iceland is 25 kilometers long. Lava domes: is a mound-shaped (bulbous mass) growth resulting from the eruption of high-silica lava (usually rhyolite and/or dacite) from a volcano. The high silica content makes the lava very viscous, so that it cannot flow very far from its vent before solidifying. Lava domes can be unstable and prone to collapse. When part of a lava collapses while it still contains molten rock and gases, it produces a pyroclastic flow, one of the most lethal forms of volcanic vent. Ultimately, many volcanic domes are destroyed by large explosive eruptions. Fissure eruption Laki fissure, Iceland Fissure eruption - eruption of magma out of crack in lithosphere (spreahding ridges). - in world spreading ridges length = 50,000 km , more spread of magma in few cm/yr Volcanic pipe Volcanic domes In case of rhyolitic and andesitic lavas (viscous) eruption More spread in low speed, not free, and in thick layer (like tooth past) Formation of volcanic dome. Lava domes Mt. St. Helen, USA Individual volcanoes Volcanoes not associated with plat boundaries (i.e., hot-spots) may be due to :- Existing radio active elements 1. more heat 2. more melting 3. goes back to magma. Hot spots lies in similar concentration of radio active elements in the outer core that leads to warm material and then rise from core/mantle boundary into magma. Volcanic Materials Lava Flow Materials and Pyroclastic Materials Pyroclastics When the built up gas pressure in rising magma is released Some of these magma freeze into solid pieces before falling to earth surface and it is called (pyroclastics). Materials extruded during an eruption Lava flows 1. Pāhoehoe flows (Hawaiian English meaning “smooth, unbroken lava”): Is a basaltic lava that has a smooth, billowy, undulating, or ropy surface. These surface features are due to the movement of very fluid lava under a congealing surface crust. 2. ‘A‘ā flows (Hawaiian English meaning “stony with rough lava”. But also to “burn” or “blaze”): Type of lava flow that has a surface of rough, jagged blocks with dangerously sharp edges and spiny projections. Pāhoehoe flows can turn into ‘A‘ā flows if it becomes turbulent due to meeting impediments or steep slopes. 3. Pillow lava: Basaltic lava that solidifies in an underwater environment and develops a structures that resembles a pile of pillows. Pāhoehoe flows Pāhoehoe flows ‘A‘ā flows Pāhoehoe flows and ‘A‘ā flows Pillow lavas Materials extruded during an eruption Pyroclastic materials (pyro = fire, clast = fragment): The volcanic rock during an eruption. Pyroclastics include: 1. Ash and dust: fragments less than 2 mm in diameter produced from gas laden viscous magma and blasted into the air during an explosive eruption. 2. Welded tuff (tuffaceous rocks): A pyroclastic deposit composed of particles fused together by the combination of heat and still contained in the deposits after it had come to rest and weight of overlying material. 3. Blocks: Fragments of lava or rock larger than 64 mm in size that are blasted into the air by volcanic explosions. Blocks are formed during the eruption in a solid state. Generally, blocks have an angular appearance, due to the fracturing of solid material during the eruption. 1. Ash and dust 2. Welded tuff 3. Blocks Materials extruded during an eruption Pyroclastic materials (pyro = fire, clast = fragment): The volcanic rock during an eruption. Pyroclastics include: 4. Volcanic bombs: Hot lava thrown out in twisted chunks that may change shape during flight from the volcano or on impact with the earth. Bombs are ejected during the eruption in a semi-solid, or partial molten, condition. 5. Lapilli (Little stone): Fragments of lava or rock (pyroclasts) between 2 and 64 mm in size (small beads to walnuts) that are blasted into the air during an eruption. 6. Scoria: A dark to reddish-colored, vesicular volcanic rock, usually mafic composition. Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles. Volcanic bombs 5. Lapilli 6. Scoria Materials extruded during an eruption Pyroclastic materials (pyro = fire, clast = fragment): The volcanic rock during an eruption. Pyroclastics include: 7. Pumice: A light-colored, frothy, vesicular rock, usually of intermediate and felsic composition, formed by the expansion of gas in erupting lava. Because of its numerous gas bubbles, pumice commonly floats on water. 8. Tephra: Solid material of all sizes ejected from a volcano into the atmosphere. Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. < 2 mm = Ash 2 - 46 mm = Lapilli > 64 mm = Blocks 7. Pumice Lahars - Combination of volcanic ash & water (volcanic mudflow) - Cause flood damages - Heat of the falling ashes melts snow & ice on mountain cause lahar. - Long-term effects: the mud deposited reduces the water – carrying capacity in the stream of these mud. Toxic gases - Water vapor+co2 release nontoxic but high temp. there gases maybe dangerous. - Carbon monoxide + sulfur gas + hydro- chloric acid are poisonous gases and might kill people. - Emitted gas may settle near ground (co2) and cause lack of O2. - Same gas release suddenly & has no order or color. Effects on climate & atmospheric chemistry - Intense explosive eruptions increase concentrations of volcanic dust into atmosphere for long time (up to months) - block out sun light and accordingly leads to temperature cooling. - Shooting sulfur-rich gas into atmosphere - Turn into clouds of sulfuric acid - cause acid rain - So aggravate ozone depletion 2 - Cause skin cancer when exposed to strong sun-light Destruction of Lava Lava eats everything on its way Destructive Power of Pyroclastics Pompeii Thank you First Exam Up to here