133 Formation of the Soils – Parent Material

Formation of the Soils
 Parent Material

The geologic history of the park is one of fire and ice. Volcanic eruption material and glaciation combined over time to form the soils in the park. Because of the complexity of the geologic history, only the part that relates to soil development is discussed in this section. The weathered remnants of Union Peak and Timber Crater represent volcanoes that were active approximately 0.5 to 1.5 million years ago. These peaks have undergone extensive erosion by water and ice, leaving a central spire that formed from the resistant rock of the core plug and the surrounding andesite lava flows. The less resistant ashflow deposits of the upper portions of the volcanoes have been eroded away.

The next volcanic generation in the park is that of Mount Mazama, which is about 400,000 years old. The mountain is a composite cone that formed from five closely spaced volcanic vents. Regular eruptions of pumice and ash and lava flows of andesite and dacite built a peak reaching an elevation of 10,000 to 12,000 feet (Bacon and others, 1997). About 7,700 years ago, a major eruption began that covered much of Oregon and the rest of the Northwest with a layer of pumice and ash. Near the mountain, pyroclastic ash and cinder avalanches covered much of the flanks and nearby lowlands. The massive eruption emptied the magma chamber under Mount Mazama; thus, the mountain collapsed and formed a caldera about 4,000 feet deep (Williams, 1942). The caldera has partially filled with water, creating the spectacular Crater Lake. Approximately 7,400 years ago, eruptions within the caldera formed several cones, one of which extends above the surface of the lake and is known as Wizard Island.

The soils that formed from the eruption deposits can be divided into groups according to the characteristics of the parent material. Parent material from the cataclysmic eruptions consists of ash, cinders, and pumice. The weathering of this material produced soils with characteristics that directly correspond to the percentage of pumice, ash, hard rock fragments, and cinders in each of the stages of the eruptions. The initial eruption produced a plume of fine sand-sized ash that covered a large portion of Oregon and the rest of the Northwest. This ash produced the parent material for the Steiger soils, which are mainly on the drier xeric part of the eastern flank of Timber Crater. Castlecrest soils formed in similar material in the wetter udic zone.

The initial pumice and ash pyroclastic flows produced thick accumulations of cobble-sized pumice within and extending far beyond the boundary of the park and settling mainly in the valleys and the low-lying lava plains. The soils that formed in these accumulations are those of the Umak and Maklak series. Portions of this early airfall material produced thick accumulations of gravel-sized pumice to the north and east of the mountain and extending far beyond the park. Timbercrater soils are mainly in the wetter areas on mountainsides and buttes, where the gravel-sized pumice deposits were at a high enough elevation to avoid being buried by subsequent ashflows. In similar positions along the eastern boundary of the park are the drier Lapine soils.

In later eruptions, the ashflows were of smaller extent and were dominantly crystalline-rich ash and cinders with a smaller percentage of pumice. Soils on these ashflows are those of the Castlecrest, Cleetwood, and Collier series. There are localized areas within these flows that are dominantly andesite rock and cinder fragments ranging in size from gravel to boulders. The Sunnotch soils formed in these areas. Some ashflows on the uppermost slopes of the caldera were deposited while sufficiently hot to weld the particles of ash together. Soils of the Unionpeak series, which generally have a weakly cemented, root-restricting layer in the subsoil, formed in this material.

The soils of the Llaorock series formed in areas on the older volcanoes of Union Peak and Timber Crater, where the slightly weathered andesitic or dacitic bedrock is near the surface and in uncovered areas of the younger andesite lava flows of Mount Mazama. The Llaorock soils are characterized by large angular rock fragments mixed with airfall ash.

Somewhat overshadowed by the effects of recent volcanic eruptions is the long history of glaciation in the park. Valley glaciers have been present on Mount Mazama throughout its history. During the height of the Ice Age, large icecaps covered most of the Cascade Range. Mount Mazama commonly had many valley glaciers, which over several ice advances slowly carved large valleys. Some of the valleys were totally or partially filled in during eruptions, especially those on the northern side of Mount Mazama. The cataclysmic eruption occurred during a period that was warmer than the present climate, and the valley glaciers had retreated beyond the present caldera rim. The collapse of the mountain truncated the glacial valleys. Most of the valley glacial deposits were incorporated into or covered by eruption debris; however, there are remnants of deposits from the icecaps, mainly upwind of the airfall deposits and at elevations high enough to escape burial by the ashflows. These remnants lie to the west and south, near the border of the park. The Grousehill and Oatman soils formed in these deposits, which represent some of the oldest parent material in the park. The ice receded about 15,000 to 25,000 years ago; therefore, these soils are the most developed of any in the park.


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