| Geology of Lassen Volcanic National Park, California:
An Overview and Field Trip Log |
| Introduction |
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| Lassen Peak is the southernmost of the fifteen or so major volcanoes that dominate the High Cascade Range - a chain of volcanic cones that stretches from northern California to southern British Columbia (Fig. 1; Christiansen, 1982). Prior to the 1980 eruption of Mount St. Helens, Lassen Peak was also the most recently active volcano in the High Cascades. Its eruptions in 1914 through 1917 focused public attention on northeastern California and led to the designation of the area surrounding the peak as a national park. The Lassen region remains thermally and seismically active today, a reminder that the 600,000-year old volcanic system centered there is still "alive." Because future eruptions are likely to produce fast-moving pyroclastic flows and volcanic mudflows that could devastate low-lying areas tens of kilometers from the volcano (Hoblitt and others, 1987), the Lassen area continues to be closely monitored by geologists. |
| This paper presents a brief summary of the geology of the Lassen Peak region that will serve as an introduction to the features we will be visiting during today's field trip. Much of the information presented in this handout was taken from works by Clynne and Muffler (1989) and Kane (1980), and the complete list of the references cited is given at end of paper. Definitions of words that are italicized in the text will be found in a glossary that follows the references. |
| Regional Geologic Setting |
| The eruptive activity at Lassen Peak and the other volcanoes in the High Cascades has been fed by batches of magma that have risen from Earth's mantle in response to subduction along the Pacific Northwest coast. The lithospheric plate that carries North America is overriding three small oceanic plates that lie to the west (Fig. 2; Guffanti and Weaver, 1988). As the southernmost of these - the Gorda plate - slides beneath northern California, it carries water bound into its surface layer deep into the mantle. As the plate descends it is heated by the surrounding mantle and, at a depth of about 80km, releases its bound water. This water attacks the bonds between Si and O atoms in the mantle rocks above the plate and causes these rocks to melt, producing magmas. The magmas are less dense than the surrounding rocks and rise gradually until they either reach the surface, to erupt as lavas, or cool and solidify underground. If you would like to learn more about the details of the subduction process, Stern (1998) has recently written an excellent review. |
| Geologic History of the Lassen Volcanic Center |
| The rise of magmas from the Cascadia subduction zone began to build the High Cascade volcanoes several million years ago. In the Lassen region at least five volcanic centers, consisting of a central stratovolcano flanked by smaller domes,have developed during the past 3 Ma (Clynne, 1990a). The growth and "death" of each of these centers followed a similar pattern. First, silica-poor lavas called andesites and basaltic-andesites erupted from a central vent and built up a cone of alternating lava flows and layers of pyroclastic materials. Next, thick lava flows of more silica-rich andesite spilled down the sides of this early cone and completed its construction. Finally, silica-rich lavas called dacitesandrhyolites erupted from vents around the flanks of the cone and formed domes and short, thick lava flows. |
| Beneath each volcanic center, however, the .magma which fed the final phase of its activity continued to release a tremendous amount of heat as it cooled and crystallized. This heat warmed the groundwater below the central cone and formed a hydrothermal circulation system beneath the old vent. The rising water carried sulfur and Chlorine compounds which had been expelled from the magma, and oxidation of hydrogen sulfide to sulfate rendered the water acidic as it reached the surface. The reaction of this hot, acidic water with the fresh lavas of the cone produced soft clays and opal. Once the center of the cone had been "softened-up" by hydrothermal alteration it was preferentially removed by later stream and glacial erosion. This process left only segments of the unaltered flank lavas to mark the original extent of the cone. |
| The Lassen volcanic center is the youngest of the five centers in the vicinity of the park, and its history has followed the model outlined above fairly closely. It is also the only regional volcanic center in which the hydrothermal system is still active. The growth of the stratovolcano that marked the center's first two phases of activity (1 and II on Fig. 3; Clynne and Muffler, 1989) began perhaps 0.7 to 0.8 Ma, and ended about 0.61 Ma. The cone, called the Brokeoff Volcano (or Mount Tehama), is estimated to have been about 12 km in diameter and to have had a summit elevation of about 3,350 m (11,000 ft) by Clynne (1990b), Its vent appears to have been centered above what is today the Sulfur Works in the southwestern part of the park. Hydrothermal activity , which continues today at sites such as the Sulfur Works and Bumpass Hell, extensively altered the core of the old cone. Streams and Pleistocene glaciers have removed most of this altered rock, leaving only the relatively unaltered masses of flank lavas such as Brokeoff Mountain, Mount Diller, and Mount Canard to outline its original extent. |
| The third phase of activity at the Lassen volcanic center has continued sporadically over the past 600,000 years, with eruptions having occurred mostly in three pulses. The first pulse, which be an about 614,000 years ago (Lanphere and others, 1999), formed a small caldera on the northern flank of the cone and produced rhyolite pyroclastic deposits (the Rockland Tephra) as well as several domes and flows (IIIR on Fig. 3). Little trace of this caldera remains because it has apparently been filled by younger third-phase lavas. The second pulse of activity occurred between 250,000 and 200,000 years ago, and built a series of dacite domes and lava flows on the northern flank of the old cone. Bumpass Mtn., Ski Heil Peak, and Reading Peak are some of the domes built during this episode (IIIB on Fig. 3). The third pulse of activity has occurred during the past 100,000 years and has produced a distinctive suite of dacite lavas that contain quenched inclusions of more mafic (andesite and basalt) magmas. Eagle Peak (57,000 years ago), Lassen Peak (27,000 years ago; Fig. 4), and the Chaos Crags (about 1, 100 years ago) are three of the more prominent domes or dome complexes that have been built during this pulse of activity (IIIB on Fig. 3). |
| A decrease in the strength of Earth's gravitational field within a 25 km-wide oval area encompassing Lassen Peak and the Central Plateau to the east may reflect the presence of a body of low-density magma at depth. Clynne (1989) has argued that such a body would probably consist of partially-molten dacitic magma, be 5 to 8 km across, and lie at a depth of 10 to 20 km. The presence of quenched basalt and andesite inclusions in recent eruptive products indicates that magmas from the mantle are still adding heat and mass to this dacitic magma chamber. Thus far, efforts to image the body using seismic waves have been unsuccessful. However, because rock is a poor conductor of heat, it is reasonable to expect that the cooling and crystallization of such a large mass of magma will sustain activity at the Lassen volcanic center for several hundred thousand years to come! |
| Origin of the Lavas in the Lassen Region |
| Magmas entering the crust from the underlying subduction zone are basalts and basaltic andesites, and have continued to erupt around the margins of the center throughout its history (Clynne and Muffler, 1989). The Hat Creek Basalt, in which the Subway Cave lava tube has formed just north of the park, is typical of this suite of magmas. The initial compositional variability of these magmas is probably due to differences in the degrees of partial melting or water contents of their mantle source regions, as at nearby Mount Shasta (Baker and others, 1994). Early in history of the Lassen volcanic center these magmas ascended through relatively "cool" crust and underwent relatively little interaction with it. As the cone-building phase progressed, however, rising magmas warmed the crust and towered its density and viscosity . These changes slowed the ascent of tater batches of magma and led to the development of small magma chambers in the crust. Within these chambers the compositions of the rising magmas were modified by assimilation of the surrounding crustal rocks and by the fractionation of early-crystallized minerals. These processes led to the development of the progressively more silica-rich andesites and dacites that have dominated the second and third eruptive phases. The present chamber is envisioned as a body that is vertically-stratified from a dacitic cap, through an andesitic dominant volume, to a basaltic base (Fig. 5; Clynne and Muffler, 1989). Basaltic magmas cannot rise directly through the chamber because of the lower densities of the magmas that overlie them. They do occasionally intrude into the dacite, however, and mix or mingle with it to form the distinctive mafic inclusions characteristic of the lavas that have been erupted from the center during the past 100,000 years. |
| Volcanic Hazards Posed by the Lassen Volcanic Center |
| During its recent history the Lassen volcanic center has typically produced dacitic and rhyolitic lavas. Because of their high silica contents these lavas are very viscous and may retain volatiles such as water, carbon dioxide, and hydrogen sulfide until high vapor pressures are reached. When the magmas approach Earth's surface these volatiles form rapidly expanding bubbles that tear the lava apart in explosive eruptions. Such eruptions are expected to produce towering ash clouds that will spread air fall tephra tens of kilometers downwind from the volcano (Fig. 6; National Park Service, 1936). These clouds may also "collapse" to form ground-hugging pyroclastic flows that will devastate lowland areas over similar distances. Smaller pyroclastic flows are also likely to be formed when the steep sides of silicic domes or lava flows collapse (Hoblitt and others, 1987). |
| If an eruption were to occur in the winter or spring, hot tephra or pyroclastic material could melt the thick snow pack that covers the area and produce floods and volcanic mudflows (lahars) that would devastate river valleys well downstream from the volcano. Finally, the presence of a relatively shallow dacitic magma chamber and the formation of at least one caldera during the past several hundred thousand years suggests that another caldera-forming eruption is a distinct possibility (Christiansen, 1982). |
| Field Trip Log |
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| We will plan to visit five stops today and allow some time for a visit to the Park Service's interpretive displays at the Loomis Museum towards the beginning of the trip. Please wear appropriate clothing and bring a lunch, water, a hat, and sunscreen. |
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