Volume XXXII-XXXIII - 2001/2002

Visitors to this national park marvel at the spectacle of a large volcano that collapsed nearly eight thousand years ago to produce a basin now filled with indescribably blue water. Indeed, the geological story of what happened to produce the Crater Lake basin that we see today may be the best description of a "young" caldera in the world. There are, however, numerous other interesting volcanic features in the park. At least 20 cinder cones have been identified within the boundaries of Crater Lake National Park. And, of course, Wizard Island, the most famous of these, rises more than 700 feet above the lake's surface near the western shoreline and is well known to all visitors.

Cinder cones as landforms

Figure 1: Location of the major cinder cones in Crater Lake National Park, Oregon. The dashed line inside the caldera represents the location of Merriam Cone and the ring fracture zone where post caldera eruptions reached the surface. The Central Platform is outlined by the dotted line. Sketch by the author.

One simple method of classifying volcanoes groups them into three categories: shield volcanoes, strato-volcanoes, and cinder cones. Shield volcanoes are massive, low, dome-shaped features produced primarily by fluid lava flows, and usually composed of basalt. The islands of Hawaii are examples of these huge volcanic features but only a small portion is visible above sea level. A strato-volcano is constructed layer by layer of more or less alternating lava flows and pyroclastics (loose fragments erupted during explosive activity) that tend to be large features on land with relatively steep slopes. They are the subjects of picture postcards and calendars. In the Cascade Range, Mount Rainier and Mount Shasta are good examples of strato-volcanoes.

Cinder cones are the "baby" members of the volcano family listed above. They seldom exceed a mile across at the base and a thousand feet high. Unlike the shield or strato-volcanoes, this smallest member of the volcano family have an extremely short life span—being active for just days to a few months in most cases. Only in rare occasions does their activity extend for a year or more. From a geological perspective, this is like an instant or a snapshot in the volcanic record of an area. Due to their origins, cinder cones are called "monogenetic," with the entire feature built during one eruption episode from one source of magma.

A typical cinder cone eruption begins by venting magma rich in gas that expands, producing solid fragmental volcanic products like blocks, bombs, scoria, ash and dust. This ejected material, the pyroclastics noted above, accumulates at a single location to build an inverted cone around the vent. The larger and heavier particles collect closest to the vent, while smaller and lighter ones drift farther—or are carried away by the wind. As the cone grows, a central region may develop composed of the larger pieces (blocks, bombs and scoria) mixed with more fluid magma that hardens into a rigid core. On a young cone this core is seldom seen as it is usually buried by additional pyroclastic material blown out of the vent later.

Figure 2: Schematic cross section of magma sources for cinder cones and the climatic eruptions at Crater Lake National Park. The outer margins of the Climatic Eruptive Magma Source defines the shadow zone, that portion of Mount Mazama that collapsed to form the caldera. Sketch by the author.

In time, the gas dissolved in the magma that powered the explosive initial eruption is exhausted and magma reaches the surface as lava. When this happens, lava flows may break out from the base of the cinder cone and flow away from the volcano. Magma forming cinder cones is typically basaltic or basaltic-andesite in composition. Lavas with this composition tend to be relatively hot and flow readily, sometimes for great distances. Although the most prominent part of these small volcanic features is the cone that develops, associated lava flows may actually contain up to ten times more volcanic material. Since cinder cones are relatively small features, they are often overlooked. They are, however, the most abundant kind of volcanic cone—being common worldwide and numbering in the thousands. One report suggests that at least 400 cinder cones have formed in the Oregon portion of the Cascade Range alone. Most of those in Crater Lake National Park are associated with the construction of Mount Mazama. Wizard Island, of course, was formed in the caldera following the collapse of Mazama's summit.

Cinder cones generally develop in volcanic areas that do not have larger volcanoes, or are associated with more massive volcanic cones. Those related to larger volcanic features, like shield and strato-volcanoes, usually develop along weak areas in existing rock. Such zones are often referred to as basement fractures and tend to be radial to the larger volcano—something like the spokes of a wheel. Although this is not obvious for the cinder cones at Crater Lake, connecting certain pairs does produce a crude radial pattern. Examples are Desert Cone and Red Cone, or Scoria Cone and the adjacent Hill 6545.

Cinder cones in the park

The southern flank of Williams Crater along Rim Drive on the west rim of the caldera. Basaltic magma rising to the surface vent here appears to have "tapped" some magma from the Climatic Eruptive Magma Source to produce "mixed magma" rocks. Photo by the author.

The cinder cones in Crater Lake National Park fit into two categories: those associated with the small basaltic shield cones of Union Peak and Timber Crater, and all the others that are related to Mount Mazama. Hill 6902 and the cinder cone on its summit appear to be part of the Timber Crater shield volcano in the northern portion of the park. In a like manner, the southwestern quadrant holds Castle Point and its summit cones, along with several smaller cinder cones belonging to the Union Peak shield volcano.

Red Cone and Desert Cone, clustered in the Pumice Desert in the northwestern section of the park, are good examples of cones related to Mount Mazama. Both appear symmetrical when viewed from the North Entrance road, a distance of about a mile. As with most other cinder cones throughout the park, both have a symmetrical appearance when viewed from above. Other prominent Mount Mazama cinder cones include Bald Crater, Crater Peak, Scoria Cone and Maklaks Crater—the latter is called Diller Cone on older maps.

Howell Williams at Crater Lake in 1965. NPS photo by Ed Paine.

The cinder cones associated with Mount Mazama exhibit a large range in volume. Of the eight more prominent cones for which a volume has been determined, Crater Peak is the largest with a volume of just under a tenth of a cubic kilometer. Hill 6545, near Scoria Cone, is the smallest measuring somewhat less than a tenth the size of Crater Peak. Wizard Island's volume falls about midway between these extremes.

Unlike the typical cinder cones described above, cinder cones located away from the caldera exhibit few lava flows. Since all of these cones predate Mazama's climactic eruption, it could be that the flows are buried by eruptive material. The tendency to erode easily is another factor since the edifice of cinder cones is composed of loose debris. Older cinder cones are thus more rounded with lower slopes that result from erosional effects. In a like manner, summit craters are typical in younger cinder cones, like the crater nearly 100 feet deep on Wizard Island. For the other Crater Lake cones, however, only shallow depressions remain of any craters that may once have been present.

Red Cone illustrating pyroclastic debris and forested northern slope. This cinder cone is typical of the small monogenetic volcanoes related to Mount Mazama outside the caldera. Photo by the author.

All of the Crater Lake cinder cones are similar in composition. One method of describing and comparing the composition of volcanic rocks is expressed by how much silicon (Si) and oxygen their rocks contain. Reports of chemical analysis combine these two elements and express them as the percentage of SiO₂. The great majority of cinder cones found in the park have a SiO₂ range of between 52% and 58%, thus producing igneous rocks called basalt or basaltic andesite.

Based on the amount of weathering, soil cover, erosion and a few radiometric dates (the measurement of geological time by means of the rate of disintegration of certain radioactive elements), most cinder cones in Crater Lake National Park appear to be less than 50,000 years old. Radio-metric dates (using the potassium-argon method of dating, or K-Ar) have been made on rocks from four cones: Timber Crater, Red Cone, Scoria Cone and Desert Cone. Only the results for Desert Cone indicate an earlier formation with a date of about 200,000 years. It should be noted, however, that the ages resulting from such techniques have a very large range. For example, the age for Red Cone rocks is recorded as 36,000 years before present, plus or minus 12,000 years.

Williams Crater & Wizard Island

Figure 3: This east-west profile of Red Cone illustrates the typical shape of cinder cones at Crater Lake. Slopes for cones outside the caldera suggest some decrease from the original shape due to erosion. Angles for young, fresh cinder cones tend to be about 30°, similar to that of Wizard Island which has a slope of 29°. Sketch by the author.

Just outside the west caldera rim a small cinder cone can be found called Williams Crater, previously labeled as "Forgotten Crater" on older maps. This feature may provide an interesting insight into the nature of the magma sources that produced Mount Mazama and the climatic eruptions. Some erupted materials appear to be formed from "mixed magmas" and contain both low and high SiO₂ compositions. These have been interpreted as a mixing of the two different magma sources associated with the Mount Mazama region - the deep magmas and the climatic eruption magma source. Based on glacial erosion and other evidence, Williams Crater appears to have been active between 22,000 and 30,000 years ago.

All the volcanic features on the floor of the Crater Lake caldera developed after the collapse of Mount Mazama. Soon after the climatic eruption and collapse that created the caldera, renewed volcanic activity formed the central platform, Merriam Cone, Wizard Island, and other features. An estimated 3 km³ of post caldera volcanic material was erupted through a ring fracture system - most of it part of the Wizard Island edifice. Merriam Cone, rising some four hundred meters above the caldera floor, has the general appearance of a small cinder cone. Recent evidence, however, suggests it was formed below water. Wizard Island's cinder cone rests on top of a pile of lava flows that extend eastward over the central platform.

Several research devices and techniques were used to map the topography of the caldera floor and investigate the nature of the rocks and sediments that occur in the Crater Lake basin. Rock samples collected from some 250 feet below lake level, on the flanks of Wizard Island, appear to have been in placed under water. The composition of these samples is identical to the youngest sub-aerial flows of the island's cinder cone located above them. All this suggests that Wizard Island was built on top of lavas erupted to create the central platform in the rising waters of Crater Lake.

Timber Crater across Pumice Desert as viewed from the North Entrance road. This cone, resting on the summit of a small shield cone, is the largest cinder cone in Crater Lake National Park. Photo by the author.

The maximum age of Wizard Island and Merriam Cone is constrained by the date of Mount Mazama's collapse a little less than eight thousand years ago. There are various estimates of how long it took for the lake to fill to its current level. Other evidence suggests that Wizard Island formed very early in the history of the basin. Using radiocarbon (C-14) dates for the collapse, and allowing a few hundred years for the lake to fill, places Wizard Island's age at about 7,000 years before the present. Its conical shape gives Wizard Island the look of a young feature, having suffered little erosion. Since there are no "hot" areas outside the caldera to suggest remnant volcanism, the lava flows at the western base of Wizard Island may represent the most recent volcanic activity in the park.

Over the past 20 years, a large, number of publications have resulted from intensive geological fieldwork. These efforts have concentrated on the construction, climatic eruptions, and ultimate collapse of Mount Mazama, resulting in the Crater Lake caldera. This is, of course, the geological story to be presented at Crater Lake National Park—the very reason the park was established. Other geologic features, such as the cinder cones, however, are also worthy of further investigation.