Two aspects of volcanism are
relevant to the study of
1. There are a number of volcanic
features that in themselves
constitute unique landforms -
various types of volcanoes and
craters being the most obvious
2. Igneous rocks, whether intrusive
or extrusive, in layers or in masses,
form distinctive components of local
geology, which can contribute to Caprock mesa
landscape development via
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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….
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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.
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are produced by the
eruption of viscous
magma and are
steep (30-40o) and
usually small (<
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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.
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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
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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.
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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
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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.
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Igneous Rocks As Geological
Components Plutons. Most igneous rocks are
Igneous rocks contribute to local INTRUSIVE, in other words they are
geology in 3 ways: created below the surface forming masses of
1. Plutons, 2. Lava Flows rock collectively known as PLUTONS.
3. Pyroclastic Flow Deposits
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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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LACCOLITHS are smaller dome-shaped masses intruded between pre-
existing rock layers. Often the country rock is deformed into a dome by the
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Exposed laccoliths may also form resistant uplands..
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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.
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A SILL is the horizontal equivalent of a dyke, intruded horizontally between
layers of pre-existing rock.
Salisbury Craigs, Edinburgh.
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Palisades, New York.
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Multiple sills, Big Bend.
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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.
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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.
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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...
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to resistant volcanic debris flow conglomerates.
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often, these pyroclastics are interbedded with lava flows..
Flow lines – a common feature of
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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.
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Lahar deposits infilling a valley.
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Depth of lahar indicated on tree.
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