University of Colorado GEOLOGY 1010 Class Note 4

Volcanoes

This is an aa flow.

This is a pahoehoe flow. Here scientists use a sensitive thermal probe (marshmallow) to test the thermal emissivity of the lava.


Magma is the term for any molten silicate material, whether below the surface or on top.

Lava is magma that is flowing across the surface, an extrusive magma.

Although the crust and mantle are solid there may occasionally may be small amounts of magma generated, usually near the surface. At mid-ocean ridges (divergent plate boundaries) primary magma is generated in the upper mantle. These primary magmas are basaltic in composition and have a characteristic chemistry and are called mid-ocean-ridge basalts (MORB).

Magmas may contain 45 to 75% silica (SiO2). The more silica, the more viscous the lava and the more explosive the eruption.

Primary oceanic lavas are low in silica and are said to be mafic in composition. Basalt is a mafic volcanic rock typical of ocean basins. It is dark in color and dense (density = 3.1-3.2).

Basaltic eruptions are commonly non explosive and magma may built up a cinder cone when the magma first reaches the surface. The cone may later be breached and a lava flow may emerge from the vent. The first part of the flow is relatively cool and viscous. This part of the flow forms a rubbly surface called an aa flow. As eruption continues and more and more magma flow out the temperature of the magma delivered to the flow front may increase which decreases the viscosity. The high temperature lava forms a ropey surface called a pahoehoe flow. Temperatures of basaltic lavas are typically 1100 to 1200 degrees C. Kilauea volcano on the Big Island of Hawaii is an example of a basaltic volcano.

Continental magmas are richer in SiO2 and are said to be felsic or silicic in composition. Rhyolite is a felsic volcanic rock typical of continental regions. It is usually light in color and has a relatively low density (density = 2.5-2.7 g/cm3). It is compositionally equivalent to granite (An intrusive rock).

Rhyolite eruptions tend to be large, explosive and catastrophic. The magma is commonly ponded in a near-surface magma chamber which may be up to tens of kilometers in diameter. The magma may contain 1 to 2% water by weight which causes the magma to foam when pressure is released. The rhyolite glass foam produced early in the eruption is called pumice. As eruption proceeds, the bubbles burst leaving tiny glass shards in an explosive cloud of gas. As the hot gas cloud collapses the hot, dense cloud descends the mountain flanks in a glowing cloud or nuee ardente. The deposit produced is called an ash flow, and the rock made up of consolidated ash is called tuff. If a flow is emplaced at a temperature above the softening point of the glass (500-600 º C) the tuff will weld to form a hard, dense rock. If the rhyolite is low in water and other volatiles a lava flow may be produced. Commonly such flows cool too rapidly to crystallize and an obsidian (dense glass) is formed.

Here we see an ash flow deposit showing the various layers. The light colored basal unit is non-welded ash. The dark unit is welded glass called a vitrophyre and the tan-colored unit above is welded, devitrified (crystallized) tuff.

Mt Pinatubo was a rhyolite eruption. Geologically recent major rhyolite eruptions in the western US include Yellowstone (750,000y), Long Valley, CA (850,000y), and Bandelier, NM (1.1 and 1.0 My).

Andesite is a volcanic rock intermediate in composition between basalta and rhyolite. It is named for the Andes and is typical of continental margins above a subduction zone (continental convergent plate boundary). Mt. St. Helens is andesitic in composition. Here are some pics of volcanoes.

Here is a slide show of the Mt. St. Helens Eruption.


GEOL 1010 Syllabus

GEOL 1010 Class Note 5

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