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Broken Top is a glacially eroded
complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...
stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...
. It lies in the
Cascade Volcanic Arc The Cascade Volcanoes (also known as the Cascade Volcanic Arc or the Cascade Arc) are a number of volcanoes in a continental volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to No ...
, part of the extensive
Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...
in the
U.S. state In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its so ...
of
Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
. Located southeast of the Three Sisters peaks, the volcano, residing within the Three Sisters Wilderness, is west of
Bend, Oregon Bend is a city in central Oregon and the county seat of Deschutes County, Oregon, Deschutes County, Oregon, United States. It is located to the east of the Cascade Range, on the Deschutes River. The site became known by pioneers as a ford (cros ...
, in Deschutes County. Eruptive activity stopped roughly 100,000 years ago, and erosion by glaciers has since reduced the volcano's cone to where its contents are exposed. There are two named glaciers on the peak, Bend and Crook Glacier. Diverse species of flora and fauna inhabit the area, which is subject to frequent snowfall, occasional rain, and extreme temperature variation between seasons. Broken Top and its surrounding area constitute popular destinations for hiking, climbing, and scrambling.


Geography

Broken Top lies in the
Cascade Volcanic Arc The Cascade Volcanoes (also known as the Cascade Volcanic Arc or the Cascade Arc) are a number of volcanoes in a continental volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to No ...
, roughly located at . It is part of the
Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...
in
Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...
, in the Three Sisters Wilderness, which lies in Deschutes County. The mountain has an elevation of according to Hildreth (2007).


Wilderness

The Three Sisters Wilderness covers an area of , making it the second-largest
wilderness area Wilderness or wildlands (usually in the plural) are Earth's natural environments that have not been significantly modified by human activity, or any nonurbanized land not under extensive agricultural cultivation. The term has traditionally ...
in Oregon, after the Eagle Cap Wilderness area. Designated by the
United States Congress The United States Congress is the legislature, legislative branch of the federal government of the United States. It is a Bicameralism, bicameral legislature, including a Lower house, lower body, the United States House of Representatives, ...
in 1964, it borders the Mount Washington Wilderness to the north and shares its southern edge with the Waldo Lake Wilderness. The area includes of trails and many forests, lakes, waterfalls, and streams, including the source of Whychus Creek. The Three Sisters and Broken Top account for about a third of the Three Sisters Wilderness, and this area is known as the Alpine Crest Region. Rising from about to in elevation, the Alpine Crest Region features the wilderness area's most-frequented glaciers, lakes, and meadows.


Physical geography

Weather varies greatly in the area due to the
rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from body of water, bodies of water (such as oceans and larg ...
caused by the Cascade Range. Air from the Pacific Ocean rises over the western slopes, which causes it to cool and dump its moisture as rain (or snow in the winter). Precipitation increases with elevation. Once the moisture is wrung from the air, it descends on the eastern side of the crest, which causes the air to be warmer and drier. On the western slopes, precipitation ranges from annually, while precipitation over the eastern slopes varies from in the east. Temperature extremes reach in summers and during the winters. Before settlement of the area at the end of the 19th century,
wildfire A wildfire, forest fire, or a bushfire is an unplanned and uncontrolled fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identified as a ...
s frequently burned through the local forests, especially the
ponderosa pine ''Pinus ponderosa'', commonly known as the ponderosa pine, bull pine, blackjack pine, western yellow-pine, or filipinus pine, is a very large pine tree species of variable habitat native to mountainous regions of western North America. It is t ...
forests on the eastern slopes. Due to fire suppression over the past century the forests have become overgrown, and at higher elevations they are further susceptible to summertime fires, which threaten surrounding life and property. In the 21st century, wildfires have been larger and more common in the Deschutes National Forest. In September 2012, a lightning strike caused a fire that burned in the Pole Creek area within the Three Sisters Wilderness, leaving the area closed until May 2013. In August 2017, officials closed in the western half of the Three Sisters Wilderness, including of the
Pacific Crest Trail The Pacific Crest Trail (PCT), officially designated as the Pacific Crest National Scenic Trail, is a long-distance hiking and equestrian trail closely aligned with the highest portion of the Cascade and Sierra Nevada mountain ranges, which lie ...
, to the public because of 11 lightning-caused fires, including the Milli Fire. As a result of the increasing incidence of fires, public officials have factored the role of wildfire into planning, including organizing prescribed fires with scientists to protect habitats at risk while minimizing adverse effects on air quality and environmental health.


Geology

Broken Top is a
complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...
stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...
. It lies northwest of Ball Butte and southeast of South Sister (part of the Three Sisters complex volcano), which is located at longitude 121.7° W and latitude 44.08° N. It has an elevation of , with a volume of . Like other Cascade volcanoes, Broken Top was fed by magma chambers produced by the
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
of the Juan de Fuca tectonic plate under the western edge of the North American tectonic plate. The mountain was also shaped by the changing climate of the
Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...
Epoch, during which multiple
glacial period A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
s occurred and glacial advance
eroded Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. Erosion is disti ...
local mountains. Broken Top joins several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends from north–south. This includes the Three Sisters complex, Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack, and Mount Bachelor. Constructed towards the end of the
Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...
epoch, these mountains are underlain by more ancient volcanoes that sank within parallel north–south trending faults in the surrounding region. The Three Sisters form the centerpiece of a region of closely grouped volcanic peaks, an exception to the typical spacing between volcanoes elsewhere in the Cascades. This vicinity is among the most active volcanic areas in the Cascades and one of the most densely populated volcanic centers in the world, as well as the second largest
volcanic field A volcanic field is an area of Earth's Earth's crust, crust that is prone to localized volcano, volcanic activity. The type and number of volcanoes required to be called a "field" is not well-defined. Volcanic fields usually consist of clusters ...
of silicic rock within the
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
Cascades. The area from the Three Sisters to Broken Top and Mount Bachelor features at least 50 eruptive vents for
rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...
and rhyodacitic lava. East of Broken Top, the c, a area of andesitic and mafic scoria cones, features similarly rhyolitic and rhyodacitic lava deposits. The Tumalo volcano spread ignimbrites and
plinian Plinian eruptions or Vesuvian eruptions are volcanic eruptions characterized by their similarity to the eruption of Mount Vesuvius in 79 AD, which destroyed the ancient Roman cities of Herculaneum and Pompeii. The eruption was described in a le ...
deposits in ground eruptions across the area (Plinian eruptions are similar to the eruption of Vesuvius that destroyed Pompeii). These deposits spread from Tumalo to the town of Bend.


Eruptive history

Broken Top's initial eruptions, beginning about 300,000 years ago during the middle of the
Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...
epoch and overlapping with eruptions at the North Sister volcano, consisted of basaltic andesite lava that contributed to a base shield. The center of this edifice consists of
oxidized Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
agglomerate that was invaded by intrusive dikes and geologic sills. The scoria, dike rocks, and lava that comprised this cone had a uniform composition made up of
phenocryst image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid sh ...
s with
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
,
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
, two types of
pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...
, and
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
. Erratic eruptions continued afterward, erupting
silicic Silicic is an adjective to describe magma or igneous rock rich in silica. The amount of silica that constitutes a silicic rock is usually defined as at least 63 percent. Granite and rhyolite are the most common silicic rocks. Silicic is the g ...
lava off and on until 150,000 years ago, including pyroxene andesites that lacked olivine, as well as
obsidian Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...
, which was rare among local volcanoes during that time period. Much of the current cone is occupied by flows from eruptions of
mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
lava (rich in magnesium and iron) including
andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...
,
dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
, rhyodacite, and
pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s. These deposits extend from the subordinate volcano to the summit, and reach Todd Lake Volcano as well as rhyodacitic lava at Tam MacArthur rim and Whychus Creek Falls. Another set of rhyodacitic lava domes runs for from south of Broken Top to Demaris Lake to its north, but they were partially covered by andesitic deposits from eruptions at the South Sister volcano. In total, throughout its 150,000 years of eruptive activity, Broken Top erupted between of lava. Though most of the material that forms Broken Top's volcanic cone originated from its main vent, the edifice was also built up by auxiliary vents on its flanks and parasitic cones that surrounded the main edifice. The side vents erupted to form
fissure vent A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilo ...
s, producing basalt and andesite, while the parasitic vents erupted scoria. Broken Top's
volcanic crater A volcanic crater is an approximately circular depression in the ground caused by volcanic activity. It is typically a bowl-shaped feature containing one or more vents. During volcanic eruptions, molten magma and volcanic gases rise from an ...
, now in diameter, was most likely created through
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...
, which created a depression that was filled with thick basaltic andesite lava flows topped by thin lavas. Once the volcano's central conduit congealed, it underwent hydrothermal alteration over time, which has been revealed by several
cirque A (; from the Latin word ) is an amphitheatre-like valley formed by Glacier#Erosion, glacial erosion. Alternative names for this landform are corrie (from , meaning a pot or cauldron) and ; ). A cirque may also be a similarly shaped landform a ...
s. The summit cone has also undergone extensive
glacial erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
, with
embayment A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a ''gulf'', ''sea'', ''sound'', or ''bight''. A ''cove'' is a small, ci ...
s and a major cirque on its southern side. It forms an
amphitheatre An amphitheatre (American English, U.S. English: amphitheater) is an open-air venue used for entertainment, performances, and sports. The term derives from the ancient Greek ('), from ('), meaning "on both sides" or "around" and ('), meani ...
shape that opens to the southeast. Due to this extensive erosion, the contents of Broken Top's cone are exposed, which allows volcanologists to classify Broken Top as a complex stratovolcano.Wood and Kienle 1992, p. 183. Erosion has destroyed the original summit of the mountain, along with its southeastern slope and much of the volcano's interior, revealing purple, red, and black scoria layers that alternate with yellow, brown, and orange pumice and
tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...
s, along with white pumice interbedded in a matrix with black lava fragments.


Glaciation and flash floods

During the late Pleistocene, Broken Top and its surroundings were almost entirely covered by glaciers. The only features in the area not buried by glaciers were the summit of Tumalo Mountain and the summit of Broken Top, including the northwest ridge crest. No geologic evidence suggests that any eruptive activity took place subglacially. Currently, the Bend and Crook glaciers, located at elevations of and , respectively, continue to erode the mountain's interior contents. Both small
ice stream An ice stream is a region of fast-moving ice within an ice sheet. It is a type of glacier, a body of ice that moves under its own weight. They can move upwards of a year, and can be up to in width, and hundreds of kilometers in length. They t ...
s, they have decreased significantly in size over the past few decades, and retreat continues at a rapid rate. When
Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Mat ...
glaciers retreated during the 20th century, water filled in the spaces left behind, forming
moraine-dammed lake A moraine-dammed lake, occurs when the terminal moraine has prevented some meltwater from leaving the valley. When a glacier retreats, there is a space left over between the retreating glacier and the piece that stayed intact which holds leftov ...
s, which are more common in the Three Sisters Wilderness than anywhere else in the
contiguous United States The contiguous United States, also known as the U.S. mainland, officially referred to as the conterminous United States, consists of the 48 adjoining U.S. states and the District of Columbia of the United States in central North America. The te ...
. The local area has a history of
flash flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice and snow. Flash f ...
s, including at Broken Top itself, where a flash flood took place on October 7, 1966. Analysis of the event by forest ranger David Rasmussen determined that it had originated from a lake on Broken Top, located at an elevation of with an area of . This lake sits at the foot of Crook Glacier and is fed by Bend Glacier. The event was likely initiated by glacial ice that fell into the lake, spawning a large wave that breached the moraine and drained of water into a channel underneath the lake. The resulting flood reached depths of , cascading down the eastern and southern flanks of the mountain and reaching the Cascade Lakes Scenic Byway. The flood moved logs, rocks, and mud to the Sparks Lake meadow basin; it covered between 35 and 45 percent of the lake's basin with
silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
and deposited at least of
sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...
in the lake and surrounding meadow.


Additional cones

Todd Lake volcano, which erupted
porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...
andesite (andesite with distinct differences among its crystal size) with a pale gray color, sits at Broken Top's southern foot. Its lava flows have been exposed and are covered with red scoria and agglutinate from a separate volcanic vent. It began to erupt as Broken Top stopped eruptive activity, since the two have overlapping layers of lava. As Pleistocene glaciers retreated, basaltic cinder cone volcanoes formed south of Todd Lake right as Cayuse Crater was forming. Cayuse Crater, an unrelated
postglacial The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
volcano which formed 11,000 years ago, also sits on Broken Top's southern flank. It is made up of basaltic lava and
scoria Scoria or cinder is a pyroclastic, highly vesicular, dark-colored volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air to form discrete grains called clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackso ...
that erupted after Broken Top ceased activity, suggesting that it formed due to an underlying magma chamber closer to South Sister. Cayuse Crater also consists partly of yellow
lapilli tuff Lapilli (: lapillus) is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' is Latin for "little stones". By definition lapilli range from in diam ...
and tuff breccia. It has a double rim, its southern wall having been breached by lava flows, with two additional smaller cones, about in elevation, to the northwest. More recent Holocene activity on the volcano's flanks constructed basaltic lava flows, cones, and ash that have become interbedded with
moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a gla ...
and material from
outwash plain An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the glacier terminus, terminus of a glacier. As it flows, the glacier grinds the underlying r ...
s from the
Neoglacial The neoglaciation ("renewed glaciation") describes the documented cooling trend in the Climatology, Earth's climate during the Holocene, following the retreat of the Wisconsin glaciation, the Last glacial period, most recent glacial period. Neoglac ...
period. Ball Butte refers to an additional cone, in elevation, which is considered one of Broken Top's subfeatures. There are also a number of parasitic cones in the area, which helped build Ball Butte over time. They also produced much red scoria and formed
hot spring A hot spring, hydrothermal spring, or geothermal spring is a Spring (hydrology), spring produced by the emergence of Geothermal activity, geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow ...
s and
fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or another rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...
s (openings in the earth that emit steam and gases). The remnants of a large
scoria cone A cinder cone or scoria cone is a steep, conical landform of loose pyroclastic fragments, such as volcanic ash, clinkers, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions or l ...
near Bend Glacier, for the most part destroyed by ice, include
volcanic bomb A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...
s that extend up to in length. During the late Pleistocene, six cinder cone and auxiliary vents between Broken Top and Tumalo Mountain erupted, yielding glomeroporphyritic basaltic andesite. They all likely derived from the same magma source.


Recent history and potential hazards

When the first geological reconnaissance of the surrounding region was published in 1925, its author, Edwin T. Hodge, suggested that the Broken Top, the Three Sisters, and several other mountains in the area constituted the remains of an enormous collapsed volcano that had been active during the Miocene or early Pliocene epochs. Naming this ancient volcano , Hodge theorized that it had collapsed to form a caldera just as
Mount Mazama Mount Mazama ( Klamath: ''Tum-sum-ne'') is a complex volcano in the western U.S. state of Oregon, in a segment of the Cascade Volcanic Arc and Cascade Range. The volcano is in Klamath County, in the southern Cascades, north of the Oregon– ...
collapsed to form
Crater Lake Crater Lake ( Klamath: ) is a volcanic crater lake in south-central Oregon in the Western United States. It is the main feature of Crater Lake National Park and is a tourist attraction for its deep blue color and water clarity. T ...
. In the 1940s, Howel Williams completed an analysis of the vicinity and concluded that Multnomah had never existed, instead demonstrating that each volcano in the area possessed its own individual eruptive history. Williams' 1944 paper, titled "Volcanoes of the Three Sisters Region, Oregon Cascades", defined the basic outline of the region, though he lacked access to chemical techniques and radiometric dating. geologist and volcanologist Edward Taylor's analysis in 1978 determined that the current Broken Top cone was constructed on basaltic andesite lavas that built a platform. As dikes and sills invaded the edifice over time, it formed a volcanic cone made of different lavas including basaltic andesite, dacite, and rhyodacite, as well as tephra and pyroclastic materials from more explosive eruptions. Broken Top poses significant hazards to the surrounding area, especially within the proximal hazard zone with a diameter of that surrounds Broken Top and the Three Sisters. A
landslide Landslides, also known as landslips, rockslips or rockslides, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, mudflows, shallow or deep-seated slope failures and debris flows. Landslides ...
at Broken Top, or an eruption from the nearby South Sister, could initiate a
lahar A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley. Lahars are o ...
(rapid flows of water, rock, and mud) that could cascade down river valleys in the surrounding area. While smaller landslides of less than are more probable than huge avalanches, they can still cause significant damage and cause lahars. Larger landslide events would most likely be the result of magma intrusion, and thus they would be predicted by
seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...
s and volcanic surveying devices. An eruption from South Sister would pose a threat to nearby life, as the proximal danger zone extends from the volcano's summits. During an eruption, tephra could accumulate to in the city of Bend, and
mudflows A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/se ...
and
pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
s could run down the sides of the mountain, threatening any life in their paths. Over the course of the past century, five or more lahars have occurred at the Three Sisters and Broken Top, either as a result of glacier-outburst flooding or the failure of
moraine-dammed lake A moraine-dammed lake, occurs when the terminal moraine has prevented some meltwater from leaving the valley. When a glacier retreats, there is a space left over between the retreating glacier and the piece that stayed intact which holds leftov ...
s. However, these only affected undeveloped areas, as they extended less than roughly from their original source. Future dam failures leading to similar events would likely still only affect the proximal hazard zone with a diameter of , but they might reach more distal areas if they originate from certain lakes, such as Carver Lake or the unnamed lake on the eastern side of Broken Top draining to Sparks Lake. Lakes with less threat of reaching the distal hazard zone include small lakes in the
headwaters The headwater of a river or stream is the geographical point of its beginning, specifically where surface runoff water begins to accumulate into a flowing channel of water. A river or stream into which one or many tributary rivers or streams flo ...
of Whychus Creek and the basin below .


Recreation

Broken Top and the nearby Three Sisters represent a popular climbing destination for hikers and
mountaineers Mountaineering, mountain climbing, or alpinism is a set of outdoor activities that involves ascending mountains. Mountaineering-related activities include traditional outdoor climbing, skiing, and traversing via ferratas that have become sports ...
. The United States Forest Service requires permits (of which a limited number are available) for use during the busier parts of the year, and dogs must be kept on leashes on the Green Lakes, Moraine Lakes, South Sister, Soda Creek, Todd Lake, and Crater Ditch trails from July 15 through September 15. Horses are prohibited, and
ice axe An ice axe is a multi-purpose hiking and climbing tool used by mountaineers in both the ascent and descent of routes that involve snow or ice covered (e.g. ice climbing or mixed climbing) conditions. Its use depends on the terrain: in its si ...
s are sensible during the winter climbing season for safety reasons. Motor vehicles and other mechanical means of transport such as bicycles, wagons, motorboats, and helicopters are generally prohibited within the Three Sisters Wilderness under the rules of the National Wilderness Preservation System. The hiking trail on Broken Top begins at the Green Lakes Basin at the trailhead east of Bend, running and gaining in elevation. It can be scrambled, though the route demands
Yosemite Decimal System The Yosemite Decimal System (YDS) is a five-part Grade (climbing), grading system used for rating the difficulty of rock climbing climbing routes, routes in the United States and Canada. It was first devised by members of the Sierra Club in Southe ...
class 4 or lower class 5 climbing and has significant exposure. The Broken Top trail begins at a junction with the Green Lakes Trail. It runs for to Crater Creek, including of trail that passes right by the volcano and Crook Glacier. There is also an unofficial trail that runs up the moraine of Crook Glacier. The northern slopes of Broken Top can be accessed by trailheads north of Tam McArthur Rim, while its southern slopes can be reached by trailheads near Crater Creek.


See also

* Geology of the Pacific Northwest


Footnotes

* Sources disagree on the number of cirques that carved into Broken Top; Wood and Kienle (1992) say three cirques, but Williams (1944) claims that five cirques altered the volcano's core.


Notes


References

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External links

* * {{Volcanoes of Oregon Mountains of Oregon Subduction volcanoes Cascade Volcanoes Pleistocene stratovolcanoes Volcanoes of Deschutes County, Oregon Mountains of Deschutes County, Oregon Stratovolcanoes of Oregon