harry williams, geomorphology1 volcanic structures introduction two aspects of volcanism are...

Harry Williams, Geomorphology 1

VOLCANIC 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 mesa


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….


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.


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.


Mount Capulin


Juan Torres Mesa


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.


Mount St. Helens (pre-1980 eruption)


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.


mountainshills

cliffs ridges

CRATER LAKE, OREGON.


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.


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.


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.


Igneous Rocks As Geological ComponentsIgneous rocks contribute to local geology in 3 ways:1. Plutons, 2. Lava Flows3. 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.


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.


LACCOLITHS are smaller dome-shaped masses intruded between pre-existing rock layers. Often the country rock is deformed into a dome by the intrusion.


Exposed laccoliths may also form resistant uplands..


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.


A SILL is the horizontal equivalent of a dyke, intruded horizontally between layers of pre-existing rock.

Salisbury Crags, Edinburgh.


Palisades, New York.


Multiple sills, Big Bend.


Lava 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.


Kamoamoa 2011


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.


Pyroclastic 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...

TUFF


to resistant volcanic debris flow conglomerates.


often, these pyroclastics are interbedded with lava flows..

Flow lines – a common feature of lava flows.


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.


Mount St. Helens (1980) lahar deposits infilling a valley.


Lahar deposits.


Depth of lahar indicated on tree.


Lahar damage.

harry williams, geomorphology1 volcanic structures introduction two aspects of volcanism are...

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