Harry Williams, Geomorphology1 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, Geomorphology2 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, Geomorphology3 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, Geomorphology4 CINDER CONES: are produced by the eruption of viscous magma and are composed almost entirely of pyroclastic deposits. These angular fragments form steep (30-40 o) and usually small (< 1000') cones.

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7 Mount Capulin

Harry Williams, Geomorphology8 Juan Torres Mesa

Harry Williams, Geomorphology9 COMPOSITE CONES (or STRATOVOLCANOES): consist of alternating layers of viscous lava flows and pyroclastic deposits in a relatively steep- sided cone, commonly reaching over ' in height.

Harry Williams, Geomorphology10 Mount St. Helens (pre-1980 eruption)

Harry Williams, Geomorphology11 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, Geomorphology12 mountains hills cliffsridges CRATER LAKE, OREGON.

Harry Williams, Geomorphology13 canyons valleys beaches deltas 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.

Harry Williams, Geomorphology14 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, Geomorphology15 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, Geomorphology16 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, Geomorphology17 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.

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Harry Williams, Geomorphology19 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, Geomorphology20 Exposed laccoliths may also form resistant uplands..

Harry Williams, Geomorphology21 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, Geomorphology22 A SILL is the horizontal equivalent of a dyke, intruded horizontally between layers of pre-existing rock. Salisbury Crags, Edinburgh.

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Harry Williams, Geomorphology24 Palisades, New York.

Harry Williams, Geomorphology25 Multiple sills, Big Bend.

Harry Williams, Geomorphology26 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, Geomorphology27 Kamoamoa 2011

Harry Williams, Geomorphology28 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, Geomorphology29 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, Geomorphology30 to resistant volcanic debris flow conglomerates.

Harry Williams, Geomorphology31 often, these pyroclastics are interbedded with lava flows.. Flow lines – a common feature of lava flows.

Harry Williams, Geomorphology32 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, Geomorphology33 Mount St. Helens (1980) lahar deposits infilling a valley.

Harry Williams, Geomorphology34 Lahar deposits.

Harry Williams, Geomorphology35 Depth of lahar indicated on tree.

Harry Williams, Geomorphology36 Lahar damage.