Rocky Butte
Rocky Butte is an extinct cinder cone butte in Portland, Oregon, United States. It is also part of the Boring Lava Field, a group of volcanic vents and lava flows throughout Oregon and Washington state. The volcano erupted between 285,000 and 500,000 years ago.
As part of the Boring Lava Field, Rocky Butte is considered an outlier of the Cascade Range. It was produced by the subduction of the oceanic Juan de Fuca tectonic plate under the North American tectonic plate; it is the core remnant of intrusive rock from kilate Pleistocene volcano. The butte has a calc-alkaline composition and consists of basaltic andesite with olivine phenocrysts.
Historically, the butte was the home of the Rocky Butte Jail, Judson Baptist College, and Hill Military Academy, as well as an extensive Works Progress Administration construction project, Portland Bible College and a campus for the City Bible Church; at the summit of Rocky Butte there is a still functioning but decommissioned rotating airway beacon. The slopes of the butte currently support Joseph Wood Hill Park and the Rocky Butte Natural Area, which includes a wide variety of flora and fauna and supports rich forest stands. Located adjacent to Interstate 205, the butte is a popular destination for hiking, climbing, and viewing mountains from its summit viewpoint.
Geography
Rocky Butte lies in the northeast part of Portland within the city limits, in Multnomah County, part of the U. S. state of Oregon. It is one of the few smaller volcanic cones within or near Portland, along with Mount Tabor, Kelly Butte, and Powell Butte. According to the U.S. National Geodetic Survey, Rocky Butte has an elevation of. It is surrounded on almost all sides by the Portland Delta.With a variable topography, the Portland area ranges from river valley floors to terraces reaching elevations of. The Willamette Valley is separated within by hills reaching heights more than, and it is also physically separated from the lower Columbia River valley. The Columbia River flows west from the eastern Portland region, merging with the Willamette near Portland before moving north. Tributaries for the Willamette include the Pudding, Molalla, Tualatin, Abernethy, and Clackamas Rivers, while the Washougal and Sandy Rivers mark notable tributaries for the Columbia River. The Columbia River has significantly shaped the geology of the area.
Portland's climate is moderate, with long growing seasons, moderate rainfail, mild winters, and warm, dry summer seasons. The area typically does not experience frost, with more than 200 frost-free days annually. Temperature can vary widely, reaching a historic maximum of, though the usual July maximum is below, and the average minimum for January is above. Yearly, precipitation averages between in most river valleys, with a mean of from 1871 through 1952. It shows variability, however, with a historic low of at Portland in 1929 and a maximum of in 1882. More than 75 percent of this precipitation occurs between October and March; July and August mark the driest months with means below, while November, December, and January represent the wettest with averages greater than. Prevailing winds originate from the south during winter and from the northwest during the summer season, with the exception of prevailing winds at the mouth of the Columbia River Gorge, where winds predominantly move to the east. The southern winds have the highest velocities of the three, only rarely occurring with potentially destructive force.
Geology
There are a number of volcanic centers within a radius of Troutdale and more than 32 vents within a radius of Kelly Butte. Mostly small cinder cone vents, these volcanoes also include some larger lava domes from shield volcanoes at Mount Sylvania, Highland Butte, and Larch Mountain. Known as the Boring Lava Field, this cluster includes more than 80 known small vents and associated lava flows, with more volcanic deposits likely present. The Global Volcanism Program reports that the field includes somewhere between 32 and 50 shield volcanoes and cinder cones, with many vents concentrated northwest of the town of Boring.Considered an outlier of the Cascade Range, the Boring Lava Field lies about to the west of the major Cascade crest. It marks one of five volcanic fields along the Quaternary Cascade arc, along with Indian Heaven, Tumalo in Oregon, the Mount Bachelor chain, and Caribou in California. Like the Cascade Range, the Boring field was also generated by the subduction of the oceanic Juan de Fuca tectonic plate under the North American tectonic plate, but it has a different tectonic position, with its eruptive activity more likely related to tectonic rifting throughout the region. The Boring Lava Field has erupted material derived from hot mantle magma, and the subducting Juan de Fuca plate may be as shallow as in depth at their location.
Many volcanic features in the Portland Basin area were destroyed by the Missoula Floods, which took place between 21,000 and 15,000 years ago and probably destroyed small cinder cones and maar craters, burying them under up to of silt from slack water. Volcanic vents with either extensive eruptions of lava flows or volcanic plugs that filled their vents survived, including Rocky Butte, Prune Hill, and Beacon Rock. However, the upstream side of Rocky Butte was heavily eroded by the flooding.
Rocky Butte is a prominent, but isolated hill, with two volcanic vents. It is the core remnant of intrusive rock from a heavily eroded, late Pleistocene volcano that erupted basaltic andesite. Like many other vents in the Boring Lava Field, Rocky Butte consists of basaltic andesite with olivine phenocrysts, and it has a calc-alkalic composition. This basaltic andesite can be observed in exposures that extend from its summit park area to the roads below, as well as in a sill between layers of fluvial gravel from the Troutdale Formation on the Butte's eastern slope, within a flow scarp. Vug cavities were identified from rock in the historic quarry on the eastern side of Rocky Butte by scientists from the United States Geological Survey, including optically positive hornblende with a pale brown color. They exhibit prismatic cleavage and extend for a few millimeters in length. Other minerals identified were dark green, pyroxene crystals with 70 percent hedenbergite and 30 percent diopside; white aggregates of tridymite; brown, granular olivine crystals with 80 percent forsterite and 20 percent fayalite; and tiny black, hexagonal plates of hematite with beveled edges. These minerals constitute a rare combination, and Trimble was unable to explain why free silica tridymite would be found together with low silica olivine.
The hill descends to meet the floodplain of the Columbia River, and at lower elevations, the mountain has cliffs on its northern and eastern sides. At the foot of Rocky Butte, there is a fosse, which extends for before it opens toward the Columbia River to the north and divides to the south into two arms that run west and southwest. This depression has a width of less than and an average depth between. There is another terrace, with a height of, which sits next to Rocky Butte. It runs for to the west, with a descent of over that distance. To the south, this terrace also has a long depression near its base, which extends for from Rocky Butte. These features are likely the product of more than just erosional forces; Bretz argued that the fosse resulted from fluid eddies that pushed downward where currents interacted with the eastern side of Rocky Butte, and the terrace formed from accumulation of deposits in water under the blockage.
Eruptive history
Evarts et al. assign Rocky Butte an age younger than 500,000 years. One Argon–argon dating experiment determined an age for Rocky Butte lava of 285,000 years ± 16,000 years ago, which matches the normal magnetic polarity for the lava deposits at the Butte. There is a magnetic anomaly about northwest from Rocky Butte in Portland, but it is unlikely to be associated with the volcano or the Boring Lava Field in general. Local newspaper sources report that Rocky Butte is extinct.Trimble noted that an eruption at Rocky Butte covered a hill composed of Troutdale Formation rocks, which are exposed as conglomerate rock at a height of on Rocky Butte's western slope. The exposure of rock from the Troutdale Formation suggests that Rocky Butte erupted onto a topographically irregular surface and that this surface later underwent erosion.
Ecology
The moderate climate and ample precipitation of the region lead to rapid regrowth of vegetation on untended sites, which can hamper fieldwork in the area. Many forests that covered the area were partly cleared for agriculture, timber, or burials in the early 20th century. These cleared and burned land plots sustain rich stands of secondary forest, featuring gorse, huckleberry, nettles, poison oak, salal, and blackberry. Myriad species of fern, as well as rapid-growth deciduous trees like alder and vine maple are also frequent. Forests support stands of Douglas fir, western hemlock, western redcedar, Pacific dogwood, bigleaf maple, Oregon ash, red alder, cascara buckthorn, Pacific madrone, and Oregon white oak; within swamps and moist areas in creeks, the shrub Devil's club can be observed. Other trees that sometimes dominate forest areas include black cottonwood. Forest communities have many additional shrubs including Indian plum, western hazel, and snowberry. Ground layer plants include the herbaceous sword fern and stinging nettle.In contemporary times, clearing of forests for housing development has left about half of the Boring Lava region still forested. As a result, water quality has decreased due to higher sedimentation and turbidity, and flooding has gotten worse over time. Streams within the area are of either first or second order, with moderate to low flows and average gradients between 10 and 12 percent. Cool and clear, many sustain macroinvertebrates, and a smaller number support amphibians and fish. The Riparian zones in the Lava Field area host diverse species, and they are influenced by uplands that serve as migration connections for birds, mammals, reptiles, and some amphibians. Gravel bars extend to the west from Rocky Butte, creating barriers for a lowland marsh.
The United States Fish and Wildlife Service provided a list of potentially threatened or endangered species in the Boring Lava area, calling them "sensitive" species. Among plant species, they determined the following species to be sensitive: white top aster, golden Indian paintbrush, tall bugbane, pale larkspur, peacock larkspur, Willamette daisy, water howellia, Bradshaw's lomatium, Kincaid's lupine, Howell's montia, Nelson's checkermallow, and Oregon sullivantia. For animal and marine life, northwestern pond turtles, Willow flycatchers, long-eared myotises, fringed myotises, long-legged myotises, Yuma myotises, Pacific western big-eared bats, and northern red-legged frogs have been identified as species of concern; pileated woodpeckers, bald eagles, cutthroat trout, and coho salmon are also considered sensitive.