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Geology

Geologic Processes

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Mount Shasta and its immediate surroundings are the products of several geological processes operating in concert. Volcanism has played a major role in shaping this landscape, and the variety of volcanic features found in the southern Cascades reflects the diversity of lavas and eruptive styles common to this region. Episodes of volcanism have alternated with intervals of erosion during which GLACIERS, streams, and mass movements such as rockfalls, DEBRIS FLOWS, and DEBRIS AVALANCHES have modified the original volcanic landforms. This section briefly introduces the major agents that have shaped the Mount Shasta region, beginning with volcanic activity and the origins of the lavas that sustain it.

Volcanism and Volcanic Landforms

Silicon and oxygen are the two most abundant elements in Earth's crust and mantle. The characteristics of lavas depend so strongly on the abundances of these elements that volcanic rocks are classified according to the amounts of silica (SiO2) that they contain. Lavas erupted on and around Mount Shasta span a wide range of silica contents, from basalts with about 49 weight percent to dacites with 71 (Baker, 1988). In the field, these rocks can usually be distinguished by their colors (silica-poor rocks are typically darker than silica-rich ones) and by the types of early-crystallized MINERALS (phenocrysts) they contain (Figure 5).

A recent experimental study (Baker and others, 1994) indicates that some of the compositional differences among the lavas in the Mount Shasta area result from different degrees of partial melting in the mantle above the subduction zone. Most of the magmas that rise from this zone are silica-poor basalts and basaltic andesites and yet, surprisingly, about 90 percent of the mountain is built from more silica-rich andesites and dacites. Clearly, major changes are occurring as magmas rise from the subduction zone, cool, and interact with crustal rocks on their way to the surface. Detailed studies of Mount Shasta's lavas show that three processes -- fractionational crystallization, assimilation, and magma mixing -- play important roles in bringing about these changes (Figure 6).

Basalts and Basaltic Andesites: Because of their low silica contents and high eruptive temperatures, basalt and basaltic andesite lavas are "runny" compared to more silica-rich ones. The basaltic lavas that reach the surface around the flanks of Mount Shasta form long tube-fed flows, broad SHIELD VOLCANOES, and steep loose TEPHRA CONES (Figure 7). None of these lavas is directly related to any of Mount Shasta's eruptive episodes, however, and studies by Baker and others (1994) suggest that they are formed by small degrees (6 to 10 percent) of partial melting of nearly-dry peridotite.

The most mafic magmas directly related to Mount Shasta are basaltic andesites that form small flank vents such as Green Butte. Some of the basaltic andesites in the Mount Shasta area are unusually rich in magnesium (up to 11 weight percent MgO at >52 weight percent SiO2) and cannot be formed from less magnesian basaltic andesites by fractional crystallization of the minerals they contain as phenocrysts. Rather, experimental evidence suggests they have been formed by large degrees (20 to 30 percent) of partial melting of relatively water-rich parts of the mantle. The origins of the less magnesian basaltic andesites in the Mount Shasta area have not been studied in detail except to note that they can be related to one another by crystal fractionation. At the nearby Medicine Lake volcano, however, the development of similar lavas is thought to have resulted from mixing of primitive and fractionated basalts that have undergone subsequent crustal contamination (Baker, 1988).

Andesites and Dacites: Because of their higher silica contents and lower eruptive temperatures, andesite and dacite lavas are "pastier" than basaltic ones. They tend to form stout flows or to pile up on top of their vents as steep-sided DOMES (Figure 8). The pastiness of andesite and dacite magmas also prevents them from releasing dissolved volatiles readily as they rise towards the surface. When they erupt, the rapid expansion of these volatiles commonly causes explosions that shatter the lavas into PYROCLASTIC MATERIALS. The eruptive behavior of andesite and dacite lavas depends strongly on their volatile contents, and commonly changes during the course of an eruption. As a result, the same vent may alternately produce both the lava flows and deposits of pyroclastic material that will build a layered stratovolcano.

The origins of the andesites and dacites that have been erupted at Mount Shasta are apparently quite complex and, according to Baker (1988), require six unique crustal and mantle sources to account for all of their compositional differences. Some andesites have apparently been formed by complex mixing of basaltic andesites and more fractionated magmas, followed by additional fractional crystallization and crustal contamination. In some instances the mixing of these different magmas was incomplete and yielded "mingled" rocks such as the banded pumices of the Red Banks (Figure 9). In most cases, however, the evidence for mixing of magmas beneath Mount Shasta comes from subtle differences in the mineral compositions and textures of the lavas. Even though it can be difficult to detect, the importance of mixing is underscored by compositional modeling which suggests that fractionation of the observed phenocrysts cannot relate Mount Shasta’s andesites to basaltic andesite parents, nor its dacites to andesite parents (Baker, 1988).

Glaciers and glacial erosion

A glacier is mass of land ice so large that it flows downhill under its own weight. Glaciers develop in cool, wet areas where at least some of the snow that falls during one winter does not melt or evaporate by the next. Masses of snow and ice typically accumulate at the upper ends of valleys on peaks like Mount Shasta and, when the become thick enough, flow down these valleys as glaciers (Figure 10). Glaciers are powerful erosive agents because flowing ice can pluck blocks from the underlying bedrock, and use rock fragments embedded in itself as abrasives to grind away the valley floor and walls. Glacial erosion produces several distinctive features -- including bowl-shaped CIRQUES, ragged ARETES, and broad U-shaped valleys -- all of which can be found on Mount Shasta (Figure 11). At their downslope ends, glaciers come to a halt where warmer temperatures melt and evaporate the ice faster than it can flow downhill. Where glaciers stop, all of the rock material that they have been carrying is either deposited as piles of poorly-sorted rock fragments called MORAINES or carried away as fine suspended sediment in meltwater streams (Figure 12).

Seven major glaciers are recognized on Mount Shasta today (Figure 13), and they have a total volume of about 140 million cubic meters (Driedger and Kennard, 1986). As impressive as this sounds, it is small by comparison to the amount of ice that mantled the mountain at least twice during Pleistocene time. Today's glaciers are not actually the remnants of these larger ice age glaciers, but developed independently about 700 years ago during a period of modest global cooling (Guyton, 1998). Mount Shasta's glaciers are of concern because they hold an enormous volume of water which, if released suddenly, could pick up large amounts of poorly-consolidated glacial and pyroclastic materials from the mountain's slopes and produce devastating debris flows.

Streams and Debris Flows

Although Mount Shasta receives an average of about 168 centimeters of precipitation each year, there are only a few permanent streams on the mountain. Because much of Mount Shasta is composed of permeable pyroclastic materials, water percolates into its slopes and finds its way into fractures and other openings in the underlying bedrock. Much of this water later emerges at springs around the base of the peak. Only during heavy rains or periods of rapid melting of snow and ice -- when water is supplied faster than it can soak in -- do stream discharges rise. Under normal circumstances these streams carry small parts of their sediment loads as dissolved or suspended materials, and push or roll heavier rocks along their beds. As stream flow increases, however, the water rapidly erodes poorly-consolidated pyroclastic and glacial deposits and entrains large amounts of suspended sediment. When this happens the water becomes a slurry that may be twice as dense as pure water (de la Fuente and Bachmann, 1999) and is capable of buoying up and carrying much larger rocks than normal. Under these conditions the stream is transformed into a debris flow, as Whitney Creek was in August, 1997 (Figure 14) [video clip]. Such flows are one of the greatest hazards Mount Shasta poses to people and property on its lower slopes.

Mass movements

Mass movements are downslope falls or flows of weathered rock material driven by gravity. Although the speeds and coherencies of the moving materials differ, all mass movements are promoted by three factors: steeper slopes, weaker rock materials, and an abundance of water. As you would expect from these considerations, many of the mass movements on Mount Shasta have originated from steep parts of the mountain that have been altered by the discharge of volcanic gases, and have occurred after periods of heavy rain or the rapid melting of snow and ice.

Rockfalls are perhaps the most common mass movements in the Mount Shasta region, and take place when coherent blocks of rock break loose from steep outcrops and cascade down slopes to accumulate as talus below. Debris flows are also rapid mass movements, but the moving masses are incoherent sediment-water slurries rather than solid blocks. These flows, in turn, are overshadowed by faster and potentially more devastating debris avalanches like the one that brought down the northern flank of ancestral Mount Shasta sometime between about 360,000 and 160,000 years ago. Collapse of a large mass high on the mountain produced an avalanche that swept 55 kilometers northward and buried the western Shasta Valley beneath a deposit of large blocks of volcanic rock surrounded by finer matrix of shattered volcanic and sedimentary materials (Figure 15). The Shasta Valley debris avalanche is one of the world's largest and best-studied debris avalanches, but it is still not clear what triggered the avalanche and how it was able to travel so far. There is no evidence that a volcanic eruption initiated the collapse of ancestral Mount Shasta's northern flank, and other possible causes for the avalanche include an earthquake or the saturation of the slope by rain or snowmelt. Our current understanding of debris avalanches suggests that their long runouts may be the result of the formation of a layer of shattered rock and air at the base of moving mass that behaves as a fluid and enables the avalanche to travel with relatively little friction over a great distance (Bishop, 2001). Studies indicate that many stratovolcanoes, including Mount Shasta, have been intensely altered by the circulation of hot, acidic groundwater as they age (Crowley and other, 1999). It is this sort of alteration, which turns hard lavas into softer clay-rich rocks, that may be setting the stage for future debris avalanches in the High Cascades.

 

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