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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Harry Williams, Geomorphology 1VOLCANIC STRUCTURESIntroductionTwo aspects of volcanism are relevant to the study of geomorphology:1. There are a number of volcanic features that in themselves constitute unique landforms - various types of volcanoes and craters being the most obvious examples.2. Igneous rocks, whether intrusive or extrusive, in layers or in masses, form distinctive components of local geology, which can contribute to landscape development via differential erosion. Caprock mesaHarry Williams, Geomorphology 2Volcanic Cones and Craters. The style of a volcanic eruption and the nature of the volcanic features it produces depends primarily on the characteristics of the magma. 1. GRANITIC magma (usually formed in subduction zones) is very viscous and does not flow easily; near the surface it solidifies quickly, often causing blockages of the vent and explosive eruptions….Harry Williams, Geomorphology 3resulting in pyroclastic deposits. These deposits consist of volcanic fragments ranging in size from fine ash to large boulders. Successive eruptions result in an accumulation of volcanic rock surrounding the vent, producing a VOLCANIC CONE.Harry Williams, Geomorphology 4CINDER CONES: are produced by the eruption of viscous magma and are composed almost entirely of pyroclastic deposits. These angular fragments form steep (30-40o) and usually small (< 1000') cones.Harry Williams, Geomorphology 5Harry Williams, Geomorphology 6Harry Williams, Geomorphology 7Mount CapulinHarry Williams, Geomorphology 8Juan Torres MesaHarry Williams, Geomorphology 9COMPOSITE CONES (or STRATOVOLCANOES): consist of alternating layers of viscous lava flows and pyroclastic deposits in a relatively steep-sided cone, commonly reaching over 10 000' in height.Harry Williams, Geomorphology 10Mount St. Helens (pre-1980 eruption)Harry Williams, Geomorphology 11Most volcanic cones have craters at the top, due to erosion or collapse of rocks surrounding the vent during eruptions; however, CALDERAS are much larger volcanic craters caused by particularly large and explosive eruptions.Harry Williams, Geomorphology 12mountainshillscliffs ridgesCRATER LAKE, OREGON.Harry Williams, Geomorphology 13canyonsvalleysbeachesdeltasThe caldera is formed by removal of material in the eruption and by collapse of the surface into the magma chamber. Crater Lake, Oregon, is the site of the former volcano Mt. Mazama which erupted explosively about 6800 years ago.Harry Williams, Geomorphology 142. Fluid BASALTIC magma (usually formed over hot spots) flows very readily and therefore does not usually erupt explosively and form pyroclastic deposits. Instead the lava tends to spread out forming large gently sloping cones or extensive layers.Harry Williams, Geomorphology 15The cones are SHIELD VOLCANOES, which form on the ocean floor above hot spots. The most famous example is Hawaii, consisting of 5 shield volcanoes joined together. The largest of these, Mauna Loa, rises 30,077' from the ocean floor.Harry Williams, Geomorphology 16Igneous Rocks As Geological ComponentsIgneous rocks contribute to local geology in 3 ways:1. Plutons, 2. Lava Flows3. Pyroclastic Flow DepositsPlutons. Most igneous rocks are INTRUSIVE, in other words they are created below the surface forming masses of rock collectively known as PLUTONS.Harry Williams, Geomorphology 17These contribute to geomorphology when they are exposed at the surface after the overlying rocks are worn away. Most igneous rock, such as granite, is usually of relatively high resistance - therefore many exposed plutons form high relief features due to differential erosion.Harry Williams, Geomorphology 18Harry Williams, Geomorphology 19LACCOLITHS are smaller dome-shaped masses intruded between pre-existing rock layers. Often the country rock is deformed into a dome by the intrusion.Harry Williams, Geomorphology 20Exposed laccoliths may also form resistant uplands..Harry Williams, Geomorphology 21A DYKE is a near-vertical layer intruded along a fracture in pre-existing rock. Often dykes radiate from volcanic necks, such as Ship Rock, N.M., which is a volcanic neck (plugged vent of a former volcano), from which the softer cone has eroded away.Harry Williams, Geomorphology 22A SILL is the horizontal equivalent of a dyke, intruded horizontally between layers of pre-existing rock. Salisbury Crags, Edinburgh.Harry Williams, Geomorphology 23Harry Williams, Geomorphology 24Palisades, New York.Harry Williams, Geomorphology 25Multiple sills, Big Bend.Harry Williams, Geomorphology 26Lava FlowsLava flows on the surface tend, for the most part, to be basalt, because it is fluid and capable of flowing over large areas, especially if erupted from a long fissure rather than a single vent.Fissure eruption.Harry Williams, Geomorphology 27Kamoamoa 2011Harry Williams, Geomorphology 28Repeated eruptions of basaltic lava forms FLOOD BASALTS, which can build up to great thicknesses and cover very large areas, such as the Columbia Plateau of the Pacific Northwest. Flood basalts can erode like horizontal strata, forming canyons with "stepped" sides.Harry Williams, Geomorphology 29Pyroclastic Flow DepositsPyroclastic (ash, dust, rocks) flows can form thick deposits over large areas. A wide range of rocks are included from compacted, welded ash (tuff – pronounced tough), which can be quite erodable...TUFFHarry Williams, Geomorphology 30to resistant volcanic debris flow conglomerates.Harry Williams, Geomorphology 31often, these pyroclastics are interbedded with lava flows..Flow lines – a common feature of lava flows.Harry Williams, Geomorphology 32If pyroclastics are mixed with water (eg. melting ice/snow), LAHARS can form (volcanic debris flows). All types of pyroclastic flow can fill in pre-existing valleys or form layers, which then become part of the geology and contribute to differential erosion.Harry Williams, Geomorphology 33Mount St. Helens (1980) lahar deposits infilling a valley.Harry Williams, Geomorphology 34Lahar deposits.Harry Williams, Geomorphology 35Depth of lahar indicated on tree.Harry Williams, Geomorphology 36Lahar


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