Sand Mountain Volcanic Field
The Sand Mountain Volcanic Field is a volcanic field in the upper McKenzie River watershed, located in the United States in Oregon. Part of the Cascade Volcanic Arc, it lies southwest of Mount Jefferson and northwest of Belknap Crater and Mount Washington. Its highest elevation is.
Active during the Holocene epoch, the Sand Mountain Field began erupting about 4,000 years ago. The field includes 23 basaltic and basaltic andesitic cinder cones and lava flows, for a total of 42 volcanic vents within two roughly north–south trending alignments. Its total dense-rock equivalent eruptive output is, with subfeatures including a tephra field and a lava tube system. Future activity is possible, but according to the Volcano Hazards Program of the United States Geological Survey, the threat from the field itself is low.
The field lies within the Willamette National Forest near Santiam Pass. It supports some forested areas, which grow well on lava flows with tephra that serves as suitable soil for stands of Douglas fir and western hemlock and less commonly western red cedar. The surroundings represent an area of geological interest, and scoria has been quarried from one of the field's volcanic cones for highway supplies. Nearby tourist destinations include a ski resort, Tamolitch Falls, and Clear Lake.
Geography
The Sand Mountain Field is part of the Cascade volcanic arc and is located at the western edge of the High Cascades segment of the central Oregon Cascades, near the upper McKenzie River watershed in Oregon, in the United States. It encompasses an area of and has a maximum elevation of. Located within the Willamette National Forest in the McKenzie Ranger District near Santiam Pass, the Sand Mountain Volcanic Field is southwest of Mount Jefferson and northwest of Belknap Crater and Mount Washington.The field borders Clear Lake, the source for the McKenzie River, and lava flows from the field formed a dam that created the lake. Clear Lake encompasses near Oregon Route 126, with a shallow northern region and a deeper southern zone, which reaches a maximum depth of. Other nearby lakes include Lost Lake, Lava Lake, and Fish Lake. Fish Lake is an ephemeral lake that dries up during the summer season, formed by the damming of local water drainages by a lava flow from Sand Mountain Volcanic Field about 3,850 years ago. Local topography has influenced eruptive activity in the past, directing lava flows downhill to the west and, with wind, directing tephra to the northeast.
Annual precipitation in the Oregon Cascades averages, with about 80 percent falling in the winter season. In the High Cascades, discharge is steady throughout the year, and precipitation falls mostly as snow, which then melts, seeps into the ground, and reaches springs several years later.
Ecology
The southern part of the field supports mature forests, but other zones with younger lava deposits are barren. Mature forests correspond to lava flow areas with tephra; areas that do not have tephra cover lack soil and support only limited vegetation. Below elevations of, these forests are typical of the lowland temperate climate of the Pacific Northwest, with Douglas fir and western hemlock predominating. Notably, forests in the Sand Mountain volcanic field have fewer western red cedar trees than is common among other forests in the Pacific Northwest region. Fen wetlands surround Lost Lake, which is about north–northeast of the field.Within Clear Lake is a submerged forest of Douglas fir trees. Well-preserved under of water, they can be seen standing on the bottom of the lake. The cold water prevents most life from growing, giving the lake its name, Analysis of samples from the underwater trees in 1965 suggested that the trees were "drowned" about 3,000 years ago, when a lava flow from Sand Mountain created Clear Lake. The eruption altered the ecosystem and created new habitats in the lake, on its shores, and on the new lava flow deposits. Today, life in the lake includes naturally reproducing fish species in Clear Lake such as brook and cutthroat trout, and the water is restocked annually with rainbow trout.
Geology
The Cascade Range and Cascade Volcanic Arc result from the subduction of the Juan de Fuca tectonic plate under the North American tectonic plate. In the Central Oregon Cascades, there are two subsegments: the Western Cascades and the High Cascades. As the North American Plate has rotated in a clockwise fashion above the subduction zone, volcanism has moved east in the central Oregon Cascades and moved west in the northern Cascades.The graben of the High Cascades is bounded to the east by the Green Ridge fault zone and to the west by the Horse Creek fault zone. It has a width of and a depth of. Predominantly, it has generated mafic lava, producing about of magma within the past 15,000 years. Moreover, many of the identified eruptive vents from the Quaternary include scoria cones or other mafic vents. The High Cascades graben displays a unique geochemical signature with low-K tholeiite magma and a relatively enriched mantle source, produced by extension and heat flux.
The Sand Mountain Volcanic Field was formed after magma entered rock that was fractured by faulting related to subsidence of the High Cascades graben. Beginning as dikes, these bodies of magma moved through conduits to separate volcanic vents at the surface. The initial magma was basaltic, though this was replaced several hundred years later by more evolved, basaltic andesite magma. Eruptions at Sand Mountain Field were fed by two or three magma chambers, including a number of mafic magma sources over a brief span of distance and time.
The Sand Mountain Volcanic Field includes 23 basalt and basaltic andesite cinder cones and associated lava flows, which were produced by two roughly north–south trending alignments of 42 volcanic vents. The two groups cross near the Sand Mountain cinder cone; their alignments imply the existence of complex volcanic dikes under the field. Sand Mountain marks the largest cinder cone in the field, at a height of. According to the Global Volcanism Program, the field includes six major clusters of volcanic vents, all pyroclastic cones. These are the Central Group, Little Nash Crater, Lost Lake Group, Nash Crater, Sand Mountain Cones, and South Group. Slight variations in vent alignment within groups likely reflect the small differences in local tectonic stress fields. There is also evidence of vent reoccupation, suggested by lava deposits in close proximity at the Old Wagon Road vent and Great Spring vent eruptive units; one of the Great Spring cones buries part of an Old Wagon Road vent.
There are three major geochemical groups in the field: Sand, Lost Lake, and Nash. Together they amount to, three of which have not been associated with exposed volcanic vents and one cluster of volcanic cones that do not have associated lava flows. The Sand group includes, while the Nash and Lost Lake group both consist of four units. The has no associated lava deposits, and the has no associated volcanic vent. These two deposits exhibit unique compositions; SnoPark lava has relatively higher levels of barium and potassium oxide than the rest of the field, with the exception of the Jack Pine lava, which has even higher concentrations of barium, at 1081 ppm and 1343 ppm, respectively.
Single lava units at the field have uniform compositions, which is distinct from other young lava deposits within the surrounding region; major distinguishing elements include silicon dioxide, titanium dioxide, magnesium oxide, and calcium oxide. The Sand group has high compositional variation, ranging from basalt to basaltic andesite, while the Lost Lake group is mostly basalt, and the Nash group is completely composed of basaltic andesite. The Nash group shows distinctively high silica content and a unique ratio of iron oxide to magnesium oxide, and the Lost Lake Group is distinguished by its lower iron oxide to magnesium oxide ratio and high magnesium oxide content.
According to Wood and Kienle, the field has a lava composition of subalkaline basalt and basaltic andesite. Deligne et al argue that the field also has calc-alkaline basalt. Morphologically, Sand Mountain Field lavas have blocky appearances, reaching thicknesses up to though certain parts of the Lost Lake group have a ropy, pāhoehoe surface.
Subfeatures
The Jack Pine cone in the Sand Mountain Field is composed of absarokites, which are unique within the Cascade Range. Its geochemical signature suggests that it was fed by a distinct magma chamber. Absarokites occur within the forearc of the Cascades and the Central Cascades, so this unique magma may be the result of old, metasomatized material from the Earth's mantle.The basaltic andesite at Little Nash Crater includes many small plagioclase phenocrysts with less abundant olivine phenocrysts, with silica levels of about 56.8 percent. These flows are younger than 2,590 ± 150 years by radiocarbon dating. Basaltic lava from the Lost Lake cone group contained 2–3 percent olivine phenocrysts, which are slightly porphyritic; these deposits have been radiocarbon dated to 1,950 ± 150 years BP. At Nash Crater, there are basaltic andesite lava flows with sparse olivine phenocrysts and silica levels of about 53.5 percent. The lava from the main Sand Mountain chain varies from 51.6–53.2 percent silica, thus ranging from basalt to basaltic andesite in composition. Basalt lava at Sand Mountain contains sparse plagioclase and olivine phenocrysts, while basaltic andesite lava only exhibits olivine phenocrysts. These flows vary in age from are 3,850 ± 215 years BP to 2,750 years BP.
The field also includes an extensive tephra deposit, which encompasses an area of and has a volume of. This tephra field is notable because it has a much greater volume and extent than tephra produced by other mafic volcanoes in the central Oregon Cascades. Additionally, the tephra exhibits uniformity with a fine mode grain size of to and a lack of lapilli. These tephra deposits vary in age from 3,440 ± 250 to 1,600 years BP, according to radiocarbon dating. They reach thicknesses of greater than as far as from the Sand Mountain cinder cone and greater than up to from the cinder cone. Microfractures within Sand Mountain clastic rock along with the blocky, equant shapes and density of sideromelane and tachylite clastic rock in the field suggest that Sand Mountain Field lava interacted with water. According to McKay, this likely came from groundwater.
There is a lava tube called Lucy's Cavern that funneled the Sand Mountain Volcanic Field flows. It has a length of at least and reaches depths of. Its roof has a vaulted appearance with drip-like features. There are also two circular "vertical pits" that reach depths of and, respectively. These open vertical conduit lava caves, known as Century and Moss Pits, lie to the southwest of Sand Mountain on a ridge of spatter material that trends to the east. There are also remnants of a third open conduit in a former vent, which left a crater about in diameter. Both Moss Pit and Century Pit have vertical, circular entrances with smooth, remelted lining. Moss Pit has a diameter of at the entrance, with a depth of. It also features a small chamber near its base, which slopes down for another. Century Pit lies to the east of Moss Pit, with an entrance in diameter. Surrounded by a wall of spatter material ranging from in height, it has a vertical drop of, its lower half widening to a chamber with dimensions of at its base. The western margin of the base has an opening about deep, and at that depth it is blocked by debris. The lower chambers for both pits may be influenced by a fissure or may form part of an open fissure vent.