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VOLCANIC STRUCTURES Introduction

VOLCANIC STRUCTURES Introduction Two 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.

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VOLCANIC STRUCTURES Introduction

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  1. VOLCANIC STRUCTURES Introduction Two 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 mesa Harry Williams, Geomorphology

  2. Volcanic 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

  3. resulting 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

  4. CINDER 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

  5. Harry Williams, Geomorphology

  6. Harry Williams, Geomorphology

  7. Mount Capulin Harry Williams, Geomorphology

  8. Juan Torres Mesa Harry Williams, Geomorphology

  9. COMPOSITE 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

  10. Mount St. Helens (pre-1980 eruption) Harry Williams, Geomorphology

  11. Most 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

  12. CRATER LAKE, OREGON. hills mountains cliffs ridges Harry Williams, Geomorphology

  13. The 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. valleys canyons deltas beaches Harry Williams, Geomorphology

  14. 2. 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

  15. The 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

  16. Igneous Rocks As Geological Components Igneous rocks contribute to local geology in 3 ways: 1. Plutons, 2. Lava Flows 3. Pyroclastic Flow Deposits Plutons. 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

  17. These 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

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  19. LACCOLITHS 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

  20. Exposed laccoliths may also form resistant uplands.. Harry Williams, Geomorphology

  21. A 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

  22. A SILL is the horizontal equivalent of a dyke, intruded horizontally between layers of pre-existing rock. Salisbury Crags, Edinburgh. Harry Williams, Geomorphology

  23. Harry Williams, Geomorphology

  24. Palisades, New York. Harry Williams, Geomorphology

  25. Multiple sills, Big Bend. Harry Williams, Geomorphology

  26. Lava Flows Lava 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

  27. Kamoamoa 2011 Harry Williams, Geomorphology

  28. Repeated 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

  29. Pyroclastic Flow Deposits Pyroclastic (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... TUFF Harry Williams, Geomorphology

  30. to resistant volcanic debris flow conglomerates. Harry Williams, Geomorphology

  31. often, these pyroclastics are interbedded with lava flows.. Flow lines – a common feature of lava flows. Harry Williams, Geomorphology

  32. If 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

  33. Mount St. Helens (1980) lahar deposits infilling a valley. Harry Williams, Geomorphology

  34. Lahar deposits. Harry Williams, Geomorphology

  35. Depth of lahar indicated on tree. Harry Williams, Geomorphology

  36. Lahar damage. Harry Williams, Geomorphology

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