Volcanoes Past Paper BSED SCI 2A PDF

BSED SCI 2A LESTERSIGIEMO KENTCAPIRIG IVANREBUSTO VOLCANOES & VOLCANIC HAZARDS OUTLINE i. Nature of Volcanic Eruptions ii. Materials Extruded During an Eruption iii. Anatomy of a Volcano iv. Shield Volcanoes v. Cinder Volcanoes vi. Composite Volcanoes vii. Volcanic Haza...

BSED SCI 2A LESTERSIGIEMO KENTCAPIRIG IVANREBUSTO VOLCANOES & VOLCANIC HAZARDS OUTLINE i. Nature of Volcanic Eruptions ii. Materials Extruded During an Eruption iii. Anatomy of a Volcano iv. Shield Volcanoes v. Cinder Volcanoes vi. Composite Volcanoes vii. Volcanic Hazards viii. Other Volcanic Landforms ix. Plate Tectonics and Volcanic Activity x. Monitoring Volcanic Activity NATURE OF VOLCANIC ERUPTIONS Volcanic activity is commonly perceived as a process that produces a picturesque, cone-shaped structure that periodically erupts in a violent manner. However, many eruptions are not explosive due to various factors. Q: What are these factors? FACTORS AFFECTING VISCOSITY The source material for volcanic eruptions is magma, molten rock that usually contains some solid crystalline material and varying amounts of dissolved gas. Erupted magma is called lava. Factors that affect the behavior of magma include: Temperature Composition Amount of Dissolved Gases VISCOSITY The property of resistance to flow in any material with fluid properties (viscos = sticky) More viscous, more resistance to flow Less Viscous More Viscous FACTORS AFFECTING VISCOSITY TEMPERATURE - The measure of the average kinetic energy of the particles in a substance. - Indicates how hot or cold a substance is. - It strongly influences the mobility/viscosity of lava (as lava cools and begins to congeal, its viscosity increases, and eventually the flow halts) Congeal – To solidify or thicken from a fluid or semi- fluid state. ↑ Temperature ↓ Viscosity FACTORS AFFECTING VISCOSITY COMPOSITION - Recall that a major difference among various igneous rocks is their silica (SiO2) content. - Magmas that produce mafic rocks (igneous rocks that contain a high proportion of mafic minerals) such as basalt contain about 50% silica - Magmas that produce felsic rocks like granite contain more than 70% silica - Intermediate rock types like andesite and diorite contain about 60% silica. Silica Content Felsic > Intermediate > Mafic ↑ Silica Content ↑ Viscosity  ↑ Silica Content ↑ Viscosity - Felsic (rhyolitic) lavas are very viscous and tend to form comparatively short, thick flows. - Mafic (basaltic) lavas are fluid and have been known to travel 300 kilometers or more before congealing. FACTORS AFFECTING VISCOSITY DISSOLVED GASES - Gaseous components in magma, mainly dissolved water vapor, also affect the mobility of magma. - Dissolved water vapor tends to increase fluidity because it reduces formation of long silicate chains by breaking silicon-oxygen bonds. - It follows, therefore, that the loss of gases renders magma (lava) more viscous. Gases also give magmas their explosive character. ↑ Dissolved Gases ↓ Viscosity QUIESCENT VS EXPLOSIVE ERUPTIONS QUIESCENT HAWAIIAN- TYPE ERUPTIONS Eruptions that involve very fluid basaltic lavas, such as the eruptions of Kilauea on Hawaii’s Big Island, are often triggered by the arrival of a new batch of molten rock rising into a near-surface magma chamber. Geologists can often detect such an event because the summit of the volcano begins to inflate and rise months or even years before an eruption. The injection of a fresh supply of hot molten rock heats and remobilizes the semi-liquid magma chamber. In addition, swelling of the magma chamber fractures the rock above, allowing the fluid magma to move upward along the newly formed openings, often generating outpourings of lava for weeks, months, or possibly years. The eruption of Kilauea that began in 1983 has been ongoing ever since. EXPLOSIVE ERUPTIONS All magmas contain some water vapor and other gases that are kept in solution by the immense pressure of the overlying rock. As magma rises (or the rocks confining the magma fail), the confining pressure drops, causing the dissolved gases to separate from the melt and form large numbers of tiny bubbles. This is analogous to opening a can of soda, where the carbon dioxide dissolved in the soda quickly forms bubbles that rise and escape. When fluid basaltic magmas erupt, the pressurized gases readily escape. At temperatures that often exceed 1100°C (2000°F), these gases can quickly expand to occupy hundreds of times their original volumes. Occasionally, these expanding gases propel incandescent lava hundreds of meters into the air, producing lava fountains. At the other extreme, highly viscous magmas expel fragmented lava at nearly supersonic speeds, creating buoyant plumes consisting mainly of volcanic ash and gases called eruption columns. Eruption columns can rise perhaps 40 kilometers (25 miles) into the atmosphere. Because silica-rich magmas are sticky (viscous), a significant portion of the gaseous materials remain dissolved until the magma nears Earth’s surface, at which time tiny bubbles begin to form and grow. When the pressure exerted by the expanding magma body exceeds the strength of the overlying rock, fracturing occurs. As magma moves up the fractures, a further drop in confining pressure causes even more gas bubbles to form and grow. This chain reaction may generate an explosive event in which magma is literally blown into fragments (ash and pumice) that are carried to great heights by the hot gases. TYPES OF ERUPTIONS MATERIALS EXTRUDED DURING AN ERUPTION Volcanoes erupt lava, large volumes of gas, and pyroclastic materials (broken rock, lava “bombs,” and ash.) Q: What are these materials? LAVA FLOWS The vast majority of lava on Earth—more than 90 percent of the total volume—is estimated to be basaltic (mafic) in composition. Andesitic lavas and other lavas of intermediate composition account for most of the rest, while rhyolitic (felsic) flows make up as little as 1 percent of the total. AA & PAHOEHOE AA FLOWS The first, called aa (pronounced “ah-ah”) flows, have surfaces of rough jagged blocks with dangerously sharp edges and spiny projections. Crossing a hardened aa flow can be a trying and miserable experience. PAHOEHOE FLOWS The second type, pahoehoe (pronounced “pah- hoy-hoy”) flows, exhibit smooth surfaces that sometimes resemble twisted braids of ropes. Pahoehoe means “on which one can walk.” Although both lava types can erupt from the same volcano, pahoehoe lavas are hotter and more fluid than aa flows. In addition, pahoehoe lavas can change into aa lava flows, although the reverse (aa to pahoehoe) does not occur. LAVAS BLOCK LAVAS In contrast to fluid basaltic magmas that can travel many kilometers, viscous andesitic and rhyolitic magmas tend to generate relatively short prominent flows—a few hundred meters to a few kilometers long. Their upper surface consists largely of massive, detached blocks—hence the name block lava. Although similar to aa flows, these lavas consist of blocks with slightly curved, smooth surfaces rather than the rough, sharp, spiny surfaces typical of aa flows. LAVAS PILLOW LAVAS Recall that much of Earth’s volcanic output occurs along oceanic ridges (divergent plate boundaries). When outpourings of lava occur on the ocean floor, the flow’s outer skin quickly freezes (solidifies) to form basaltic glass. However, the interior lava is able to move forward by breaking through the hardened sur face. This process occurs over and over, as molten basalt is extruded—like toothpaste from a tightly squeezed tube. The result is a lava flow composed of numerous tube-like structures called pillow lavas, stacked one atop the other. Pillow lavas are useful when reconstructing geologic history because their presence indicates that the lava flow formed below the surface of a water body. GASES Magmas contain varying amounts of dissolved gases, called volatiles, held in the molten rock by confining pressure, just as carbon dioxide is held in cans and bottles of soft drinks. As with soft drinks, as soon as the pressure is reduced, the gases begin to escape. Obtaining gas samples from an erupting volcano is difficult and dangerous, so geologists usually must estimate the amount of gas originally contained in the magma. The gaseous portion of most magmas makes up 1 to 6 percent of the total weight, with most of this in the form of water vapor. Although the percentage may be small, the actual quantity of emitted gas can exceed thousands of tons per day. The composition of volcanic gases is important because these gases contribute significantly to our planet’s atmosphere. Volcanoes are also natural sources of air pollution; some emit large quantities of sulfur dioxide (SO2), which readily combines with atmospheric gases to form toxic sulfuric acid and other sulfate compounds. PYROCLASTIC MATERIALS When volcanoes erupt energetically, they eject pulverized rock, lava, and glass fragments from the vent. The particles produced are called pyroclastic materials (pyro = fire, clast = fragment) and are also referred to as tephra (Greek word meaning “ash”). These fragments range in size from very fine dust and sand sized volcanic ash (less than 2 millimeters) to pieces that weigh several tons. PYROCLASTIC MATERIALS Ash and dust are produced when gas-rich viscous magma erupts explosively. As magma moves up in the vent, the gases rapidly expand, generating a melt that resembles the froth that flows from a bottle of champagne. As the hot gases expand explosively, the froth is blown into very fine glassy fragments. When the hot ash falls, the glassy shards often fuse to form a rock called welded tuff. PYROCLASTIC MATERIALS Somewhat larger pyroclasts that range in size from small beads to walnuts are known as lapilli (“little stones”). These ejecta are commonly called cinders (2–64 millimeters [0.08–2.5 inches]). Particles larger than 64 millimeters (2.5 inches) in diameter are called blocks when they are made of hardened lava and bombs when they are ejected as incandescent lava. Because bombs are semi-molten upon ejection, they often take on a streamlined shape as they hurtle through the air. Because of their size, bombs and blocks usually fall near the vent; however, they are occasionally propelled great distances. Some materials are also identified by their texture and composition. In particular, scoria is the name applied to vesicular ejecta produced from basaltic magma. These black to reddish-brown fragments are generally found in the size range of lapilli and resemble cinders and clinkers produced by furnaces used to smelt iron. Pumice is usually lighter in color and less dense than scoria, and many pumice fragments have so many vesicles that they are light enough to float. ANATOMY OF A VOLCANO The popular image of a volcano is a solitary, graceful, snowcapped cone, such as Mount Hood in Oregon or Japan’s Fujiyama. These picturesque, conical mountains are produced by volcanic activity that occurred intermittently over thousands, or even hundreds of thousands, of years. However, many volcanoes do not fit this image. Q: What are the common features of a volcano? Volcanic activity frequently begins when a fissure (crack) develops in Earth’s crust as magma moves force fully toward the surface. As the gas-rich magma moves up through a fissure, its path is usually localized into a somewhat circular conduit that terminates at a surface opening called a vent. The cone-shaped structure we call a volcanic cone is often created by successive eruptions of lava, pyroclastic material, or frequently a combination of both, often separated by long periods of inactivity. Located at the summit of most volcanic cones is a somewhat funnel-shaped depression, called a crater (crater = a bowl). Volcanoes built primarily of pyroclastic materials typically have craters that form by gradual accumulation of volcanic debris on the surrounding rim. Other craters form during explosive eruptions, as the rapidly ejected particles erode the crater walls. Craters also form when the summit area of a volcano collapses following an eruption. Some volcanoes have very large circular depressions, called calderas, that have diameters greater than 1 kilometer (0.6 mile) and in rare cases can exceed 50 kilometers (30 miles). During early stages of growth, most volcanic discharges come from a central summit vent. As a volcano matures, material also tends to be emitted from fissures that develop along the flanks (sides) or at the base of the volcano. Continued activity from a flank eruption may produce one or more small parasitic cones (parasitus = one who eats at the table of another). Italy’s Mount Etna, for example, has more than 200 secondary vents, some of which have built parasitic cones. Many of these vents, however, emit only gases and are appropriately called fumaroles (fumus = smoke). SHIELD VOLCANOES Shield Volcanoes are broad volcanoes with gentle slopes and are shaped somewhat like a warrior’s shield lying flat on the Earth. Shield volcanoes have a convex shape as they are flatter near the summit. Shield volcanoes feature a gentle (usually 2° to 3°) slope that gradually steepens with elevation (reaching approximately 10°) before flattening near the summit, forming an overall upwardly convex shape. Shield Volcanoes go through 4 stages: PRESHIELD: First, the shield volcano starts with low volume and infrequent eruptions. SHIELD: Next, the volcano accumulates 95% of its mass through continuous eruptions. POSTSHIELD: Then, eruptions become slightly more explosive. REJUVENATION: Finally, the shield volcano can become active again by erupting small volumes of lava infrequently. If it doesn’t go through this phase, it slowly erodes away over time. MAONA LOA A basaltic shield volcano that rises almost 9 km from the ocean floor in Hawaii, the largest active volcano in the world CINDER CONES Cinder cones (scoria cones) are the most common type of volcano in the world. They may look like an idealized depiction of a volcano as they are steep, conical hills that usually have a prominent crater at the top. Cinder cones are typically built in a single eruptive period that lasts a few months and that may include the eruption of fluid lava flows from vents located along the base. Cinder cones are usually less than 1,000 feet tall (330 m). COMPOSITE VOLCANOES A composite volcano, also known as a stratovolcano is a cone-shaped volcano built from several layers of lava, pumice, ash, and tephra. Due to its viscous lava, a composite volcano tends to form tall peaks rather than rounded cones. Composite volcanos may rise as high as 8,000 feet above its bases. Most of the composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. MT. FUJI MT. PINATUBO Erupted in 1991 (for the first time in 600 years) and caused widespread devastation. Composite volcanoes account for nearly 60 percent of the volcanoes on Earth. MT. MAYON is the most active volcano in the Philippines, erupting over 52 times in the past 500 years VOLCANIC HAZARDS PYROCLASTIC FLOWS Consist of hot gases infused with incandescent ash and larger lava fragments. Also referred to as nuée ardentes (glowing avalanches), these fiery flows can race down steep volcanic slopes at speeds exceeding 100 kilometers (60 miles) per hour. There are two components: Low-density cloud of hot expanding gases containing fine ash particles and a ground-hugging portion composed of pumice. Other vesicular pyroclastic material Pyroclastic flows are propelled by the force of gravity and tend to move in a manner similar to snow avalanches. SURGES Low density clouds, powerful hot blasts that carry small amounts of ash separate from the main body of a pyroclastic flow. can be deadly but seldom have sufficient force to destroy buildings in their paths. Ex: The Destruction of St. Pierre in 1902, The Destruction of Pompeii LAHAR Large composite cones may generate a type of fluid mudflow known by its Indonesian name, lahar. These destructive flows occur when volcanic debris becomes saturated with water and rapidly moves down steep volcanic slopes, generally following stream valleys. OTHER VOLCANIC HAZARDS Volcano-Related Tsunamis: One hazard associated with volcanoes is their ability to generate tsunamis. Although usually caused by strong earthquakes, these destructive sea waves sometimes result from powerful volcanic explosions or the sudden collapse of flanks of volcanoes into the ocean. Volcanic Ash and Aviation: Volcanic ash consists of fine particles that can be abrasive and damaging. When ingested by aircraft engines, these particles can melt at high temperatures and form solid deposits on turbine blades, leading to engine failure or flame-out. Volcanic Gases and Respiratory Health: When sulfur dioxide is inhaled, it reacts with moisture in the lungs to produce sulfuric acid, a deadly toxin. Effects of Volcanic Ash and Gases on Weather and Climate: Volcanic eruptions can eject dust-sized particles of volcanic ash and sulfur dioxide gas high into the atmosphere. The ash particles reflect sunlight back to space, producing temporary atmospheric cooling. OTHER VOLCANIC LANDFORMS CALDERAS Recall that calderas are large steep-sided depressions with diameters exceeding 1 kilometer (0.6 miles) that have a somewhat circular form. bowl-shaped depression that forms when a volcano erupts and subsequently collapses into itself. This collapse occurs after the magma chamber beneath the volcano is emptied during a significant explosive eruption. The loss of support from the magma chamber causes the ground above to subside, creating the caldera. less than 1 kilometer across are called collapse pits, or craters. Crater Lake–Type Calderas are a specific type of caldera that resembles a crater but is formed through different geological processes. Craters are formed by the outward explosion of volcanic material, ejecting ash and lava, whereas calderas result from the inward collapse of a volcano after the magma chamber is emptied. Hawaiian-Type Calderas, unlike Crater Lake–type calderas, form gradually because of the loss of lava from a shallow magma chamber underlying the volcano’s summit. Hawaiian-type calderas develop from a magma chamber that is gradually drained by the outpouring of lava during eruptions. Unlike explosive calderas that result from violent eruptions, these calderas form more gradually as the magma chamber empties. Yellowstone-Type Calderas, often referred to as supervolcanoes, are large volcanic depressions formed by the collapse of land following a massive explosive eruption. The Yellowstone Caldera, located in Yellowstone National Park, is the most well-known example of this type. RESURGENT DOME Formed within calderas as a result of underground volcanic activity. They are characterized by the uplift or swelling of the caldera floor, which occurs due to the movement of magma beneath the surface. FISSURE The greatest volume of volcanic material is extruded from fractures in Earth’s crust called fissures (fissure = to split). Rather than build cones, fissure eruptions usually emit fluid basaltic lavas that blanket wide areas In some locations, extraordinary amounts of lava have been extruded along fissures in a relatively short time, geographically speaking. These voluminous accumulations are commonly referred to as basalt plateaus because most have a basaltic composition and tend to be rather flat and broad. LAVA DOMES As the thick lava is “squeezed” out of a vent, it often produces a dome- shaped mass called a lava dome. Lava domes are usually only a few tens of meters high, and they come in a variety of shapes that range from pancake-like flows to steep-sided plugs that were pushed upward like. Volcanic Neck: Formed when magma solidifies within the vent of a volcano. After a volcano becomes inactive, the softer surrounding material erodes away, leaving behind the more resistant igneous rock that comprises the neck. Volcanic Pipe: A volcanic pipe, often synonymous with a volcanic neck, refers specifically to the central conduit through which Volcanic Neck magma rises to the surface. It can also refer to the solidified magma within this conduit. PLATE TECTONICS AND VOLCANIC ACTIVITY CONVERGENT PLATE VOLCANISM When oceanic lithosphere descends beneath a continent, magma generated in the mantle rises to form a continental volcanic arc. v INTRAPLATE VOLCANISM When a large mantle plume ascends beneath continental crust, vast outpourings of fluid basaltic lava like those that formed the Deccan Plateau may be generated. DIVERGENT PLATE VOLCANISM When tectonic v forces pull a continental block apart, the lithosphere v stretches and thins, allowing molten rock to rise from the mantle. v MONITORING VOLCANIC ACTIVITY Volcano monitoring has two primary goals. It provides basic scientific data that helps geologists understand the structure and dynamics of a specific volcano as well as volcanoes in general. Equally important, volcanic monitoring is critical for hazard assessment because millions of people live and work on or near volcanoes. The three most noticeable changes in a volcanic landscape caused by the migration of magma are: 1 Changes in the pattern of earthquakes produced by the movement of magma 2 Magma entering a near-surface magma chamber, which leads to inflation of the volcano 3 Changes in the amount and/or composition of gases released from a volcano Almost one-third of all volcanoes that have erupted in historic times are now monitored using seismographs, instruments that detect earthquake tremors. In general, a sharp increase in seismic unrest followed by a period of relative quiet has been shown to be a precursor of many volcanic eruptions. However, some large volcanic structures have exhibited lengthy periods of seismic unrest. For example, in 1981, a strong increase in seismicity was recorded for Rabaul caldera in New Guinea. The activity lasted 13 years and culminated with an eruption in 1994. The development of remote-sensing devices has greatly enhanced our ability to monitor volcanoes. These instruments and techniques are particularly useful for monitoring eruptions in progress. Photographic images and infrared (heat) sensors can detect lava flows and volcanic columns rising from a volcano. Furthermore, satellites can detect ground deformation as well as monitor sulfur dioxide emissions. Because accessibility to many volcanoes is limited, remote-sensing devices, including lasers, Global Positioning System (GPS) devices, and Earth-orbiting satellites, are often used to determine potential swelling of a volcano. Volcanologists also frequently monitor volcanoes in an effort to detect changes in the quantity and/or composition of the gases being released. The overriding goal of all monitoring is to discover precursors that may warn of an impending or imminent eruption. Volcanologists accomplish this by first diagnosing the current condition of a volcano and using the baseline data to predict its future behavior. A volcanologist must observe a volcano over an extended period of time in order to recognize significant changes from its “resting state.” Unfortunately, accurately predicting the timing and the potential hazard of a volcanic eruption still eludes scientists. This is perhaps best demonstrated by the February 2015 eruption of Sinabung volcano in Sumatra, Indonesia. This deadly eruption claimed at least 16 lives and occurred just days after authorities gave the “all- clear” for residents to return to their homes on Sinabung’s slopes.