The region between Becharof Lake and Lower Ugashik Lake on the Alaska Peninsula is recently volcanic. Among other things, it includes a small stratovolcano – Peulik built on the remains of a larger flank collapse and caldera stratovolcano – Ugashik. Perhaps 10 km to the west are the most recent Maars in Alaska, the Ukinrek Maars. Finally the Gas Rocks, a group of three dacite domes and a pair of mafic cones sit between the Maars and the southern shore of the larger Becharof Lake. It is referred to as the Ugashik – Mount Peulik Volcanic Center.
There is an additional dome complex called Blue Mountain some 20 – 25 km SSW of Ukinrek Maars and some 30 km WSW of Ugashik – Peulik. Lower Ugashik Lake sits between Blue Mountain and the rest of the Ugashik – Mount Peulik Volcanic Complex. Blue Mountain contains 13 dacite domes in a cluster some 6 km long. Each dome has blocky lava flows. There are two large, central domes with smaller groups of satellite domes.
The entire region sits roughly midway between Aniakchak to the south and Katmai – Novarupta to the north.
The region is sparsely populated with around 100 people living within 100 km of the volcanoes, most of them to the west along Bristol Bay. The volcanoes and maars sit roughly equidistant between Egegik to the west, King Salmon and Naknek to the NNW, Pilot Point and Ugashik to the SW, and Karluk on Kodiak Island to the east. The complex lies some 550 km SW of Anchorage.
The land is relatively low, mostly covered with glacial till. The spine of the Alaska Peninsula sits to the east of the complex. There are several large, shallow lakes in the region, all scoured by the action of glaciers during recent ice ages.
The economics of the villages on the Bristol Bay side of the peninsula are primarily commercial fishing and salmon processing. The Becharof National Wildlife Refuge contains Becharof Lake and roughly bisects the volcanic complex along an E – W border. Land to the south belongs to the State of Alaska. That to the north is part of the refuge.
The complex sits beneath heavily traveled air routes between the US and Asia. Significant activity from any of the members can put ash 15 km into the air with significant ash fall 250 – 300 km downwind.
Best resource for information is a USGS report published in 2004 entitled Geology of the Ugashik – Mount Peulik Volcanic Center, Alaska by Thomas Miller.
The pre-caldera edifice of Ugashik is thought to have been a dome complex rather than a stratovolcano (think Unzen or Blue Mountain). Original magmas are andesites, basaltic andesites, switching over to dacites and rhyolites as the magma evolved. Breccias on the south and west rims resemble dome-margin crumble breccias and do not include basaltic, low silica andesitic magmas. They most resemble magmas at Augustine and Dutton. Dacites from the south rim of the caldera are 171,000 years old.
The caldera forming eruption was late Pleistocene, destroying the pre-existing dome complex and creating the 5 km wide caldera. The eruption ejected perhaps 5 – 10 km3 of material. The circular caldera suggests a collapse rather than a lateral blast and the shape of margins suggest the collapse was a single event. Intervening glacial action removed much of the pyroclastic material and reworked the rest. The eruption is thought to take place perhaps 40,000 years ago.
Post-caldera eruptions formed a nested cluster of dacite – rhyolite domes which cover most of the caldera floor. Much of the surface of the domes is smooth, suggesting glaciation after their formation.
Following the caldera-forming eruption at Ugashik 40,000 years ago, activity shifted a few kilometers to the north and started building ancestral Peulik. This volcano would build to a size similar to what we see today and suffer multiple episodes of sector collapse, dome destruction, and debris avalanches. This activity left the horseshoe shaped crater open to the NW and the somma we see today. The cone rebuilt and was destroyed in another flank collapse and debris avalanche covering 75 km2 NW of the volcano. Most recent activity shifted to rebuilding the cone and extruding short, stubby lava flows from the lower flanks of the volcano.
Mount Peulik volcano is a small truncated stratovolcano rising 1,474 m that partially overlaps the north flank of the Ugashik caldera. It is 10 km in diameter at its base and is topped with a summit crater 1.5 km in diameter. That crater is breached to the west and contains a small dome perhaps a half a kilometer in diameter. There are block and ash flows on 40 km2 of the western flank of the volcano. There is a smaller dome on the east flank that was the source of another block and ash flow. Lava flows from Peulik cover 8 km2 north of the volcano stretching all the way to the lake.
Peulik means “Smoking Mountain” in Aleut. While historic eruptions of Peulik are poorly documented, ocean observations in 1814 and 1852 of smoke are thought to be credible mostly due to its large distance from other prominent active volcanoes in the region.
There are multiple episodes of summit and flank collapse from Peulik. The largest deposit stretches 5 – 15 km from the current cone. While some of the dome debris is breadcrusted indicating an explosive magmatic component, there does not appear to be a lateral blast like St Helens or Bezymianny. Brown pumice lapilli came from later eruptions. Bulk volume of the avalanches is in the range 0.2 – 0.4 km3.
The most recent activity emplaced current summit domes(s). Their predecessors were destroyed in multiple episodes creating the 1 – 2 km wide horseshoe shaped crater and apron of pyroclastic flow deposits. The magma body is around 6.6 km below the volcano. The volcano inflated Oct 1966 – Sept 1997.
There was an M 4.1 earthquake on March 26, 2015 at the Ugashik – Peulik volcanic center. The quake and aftershocks took place near the location of the 1998 Becharof swarm. An inspection of hot springs three weeks later found frothy warm water flowing from the caldera into Ugashik Creek. Salmon were not entering the stream like in past years. Acidity was similar to samples taken in June 2004. Outside the caldera, creek water was tinted with sediment. There was no volcanic unrest noted.
Blue Mountain and Gas Rocks Dome Complex
Blue Mountain and The Gas Rocks are two clusters of dacite lava domes separated by 25 km in the Bristol Bay lowlands of the Alaska Peninsula. Both clusters are located on a treeless and roadless plain of glacial deposits, till, outwash, bogs and similar deposits. There are huge moraine-dammed lakes neighboring both sites that are very shallow and sit within 10 m of sea level. Both Blue Mountain and The Gas Rocks have been scoured by multiple instances of glaciation.
The majority of eruptive products in the rear arc of the Aleutian Trench are primarily basaltic and andesitic. The clusters of rear arc dacite domes are anomalous in this region.
Blue Mountain are two groups of dacite domes, with a cluster of three a kilometer to the north of the western end of the main cluster. These are the oldest volcanic structures in the region at 632,000 years old. Glacial action stripped the domes of pumice or glassy surfaces, removing perhaps 20% of the original volume. Both of the two main domes erupted a stubby 200 m thick lava flow to the south. The largest domes are 300 m above the surrounding apron of glacial deposits. The smallest domes are 100 above local ground. Total surviving volume of Blue Mountain dacite is just over 1 km3.
About 10 km SW of Blue Mountain is an isolated exposure of olivine basalt called Lone basalt. It is similar to but not as primitive as the basalts of Ukinrek maars. It differs from the mafic rocks of Mount Peulik and the other volcanic centers along the volcanic front, and is not related to the Blue Mountain dacite. It is dated around 593,000 years old. The Lone basalt may be related to an unknown subsurface extension of the Bruin Bay Fault.
The Ukinrek maars were created by the rise of primitive basalt and its phreatomagmatic interaction with overlying glacial till. They are a pair of craters created over a 10-day long phreatomagmatic eruption in March – April 1977. They are located 1.5 km south of Becharof Lake and 12 km west of Peulik volcano. The West Maar is elliptical measuring 105 x 170 m and 35 m deep. The East Maar is 600 m to the east, 300 m in diameter and 70 m deep. The West Maar was first to form. Both maars are now filled with a crater lake. The East Maar has a 49 m high lava dome emplaced at the end of the eruption.
CO2 gas has been continuously emitted from the vicinity of the maars, Gas Rocks, and beneath Lake Becharof since before the formation of the maars. Airborne sampling of CO2 emissions from Ukinrek Maars in 1998 found 187 metric tons / day. https://avo.alaska.edu/pdfs/cit4384.pdf
Gas Rocks are three dacite domes and one stratified mafic cone on the southern shore of Becharof Lake. The domes include extorted lava flows 10 – 15 m thick. The domes top out around 200 m above the lake surface. Domes are dated between 23,300 – 25,700 years old and the error envelopes for the ages overlap.
Cone 431 is the SE knob of The Gas Rocks. It is the remnant of a basaltic ejecta cone that rises 120 m above the lake. Materials building the cone suggest phreatomagmatic or hydrovolcanic eruption. Like the rest of the region, it had been heavily eroded by glacial action. Its age is similar to the three domes of The Gas Rocks.
There is a final basaltic cone, Cone 254, 4 km to the west. It is 25 – 30 m high. Layering suggests hydrovolcanic surge emplacement under the lake.
Gas discharge form The Gas Rocks takes place with up to three vigorous upwellings of cold lake water charged with CO2 a few meters offshore from the NE side of the dome complex. Hot springs were first documented in second hand reports in 1917. Discharge of hot water and CO2 picked up during the 1977 eruption of Ukinrek Maars. It has waned since that time. The gas flux is unrelated to the Gas Rocks dacites and seem to be related to the 1977 Ukinrek basalt and four other rear arc basalts. This suggests that mantle-derived fluids find a relatively easy path to the surface. The three main sites of gas emissions are not well aligned along the suggested discontinuity line of the Bruin Bay Fault.
Knob 500 is roughly midway between Ukinrek Marrs and Ugashik – Peulik. It is a circular mesa perhaps 1 km in diameter rising 100 above surrounding debris avalanche emplacement. It is compositionally similar to Ugashik – Peulik. It is either a flank eruption or as secondary vent for invasive lava the burrowed between the avalanche deposit/
Ukinrek Maars Eruption of 1977
1977 Eruption of Ukinrek Maars put tephras up to 10 m thick at the maar craters. It erupted undersaturated alkaline basalts resembling back arc magmas that is unrelated to Mount Peulik volcanism.
The phreatomagmatic explosions ejected both juvenile and non-juvenile materials from the crater creating a low rim of ejecta (less than 25 m high). There was minor base surge activity. Viscous basalt appeared in the East Maar almost immediately and created a gradually growing lava dome during the eruptions from the maar. Feeder dike pieces ejected during the eruption of the West Maar indicate that the explosion chamber migrated downward during the eruption sequence.
Following the eruptions, the maars filled with debris which was in turn covered by water from the surrounding water table. These are the best documented maar eruptions in historical time.
The maars continue to emit diffuse CO2 at high rates in places. Total estimated emissions are 21 – 44 tons/day. There are conspicuous zones of plant damage / kill covering 30,000 – 50,000 m2 near the maars. The CO2 emission flux is similar to the Horseshoe Lake – Mammoth Mountain discharge in Long Lake, an area closed to camping also with a tree kill due to CO2 emissions in 1990.
The most recent earthquake swarm in the region took place May – June 1998. It started May 8 – 9 with 5 earthquakes M 4.7 – 5.2. There were dozens of earthquakes in the M 2.5 – 3.5 range. 1 – 2 earthquakes were still being felt on May 12. By the last week in May there were 12 earthquakes from M 1.7 – 3.3. 1 – 5 June were 20 earthquakes between M 1.8 – 4.3. Activity decreased significantly 6 – 12 June with only 4 earthquakes, all over m 3.0. The area was not actively monitored at the time (it is now), and is also on a local air traffic route. There were no reports of surface breakage, volcanic activity, or imminent hazard. There was some slumping along the shore of Becharof Lake. There was also a slight change in the color of the crater lake of the East Maar that was thought to be disturbed sediment.
The location of this swarm was 10 – 20 km NW of the Ukinrek Maars on the SW shore of Lake Becharof. The quakes were shallow no deeper than 7 km. These earthquakes were monitored from instruments at Katmai and Aniakchak. The swarm was at the southern end of the Bruin Bay fault and are thought to be tectonic in nature. The problem with this interpretation is that there were no clear aftershocks to the larger earthquakes, which indicated a volcanic component. Intrusion of magma associated with tectonic movement is possible. Over the years, five seismic webcorders were placed around the complex.
There were 414 events recorded between May and October 1998, with a cumulative moment of an M 5.8 earthquake.
The volcanic center like all volcanoes along the Alaska Peninsula and Aleutians is subduction – driven with the Pacific Plate subducting underneath the North American Plate. It is about 320 km to the west of the Aleutian trench. It is some 110 – 120 km above the top of the Benioff zone with the Pacific plate dipping 35 – 40 degrees to the NNW underneath it. The plate initially subducts at a shallow angle for the first couple hundred miles of travel under the North American Plate. It then dives at a 45 degree angle under the region of the line of back arc volcanoes along the Alaska Peninsula. The Pacific Plate subduction zone is at least 150 km below the volcanic center.
The volcanic center sits at the SW end of a 200 km long segment of subduction volcanoes that extends through the Katmai region to Douglas volcano. It is some 60 km to the nearest volcano to the NE and 65 km to the nearest volcano to the SW.
Basement rocks beneath the center are marine and continental sedimentary rocks, siltstones, and mudstones. The center is 13 km east of the NE trending Bruin Bay Fault which may provide a weak point for magma to make its way to the surface. The problem with this conclusion is that there is no surface sign of the southern end of the fault that would be an obvious source of magmas for the Lone basalt, Blue Mountain dacite, Ukinrek Maars, and The Gas Rocks. The plumbing systems for the dacite and basalt eruptions do not appear to be connected with that of Ugashik – Peulik.
The major fault system in the areas is the Bruin Bay Fault System (BBFS). It is generally NNE to SSW with the southern end of it in the vicinity of the western end of Becharof Lake and the northern end in the vicinity of the east flank of Redoubt Volcano. Most its activity was ancient, with the most recent activity in the Oligocene, perhaps 30 Ma. It is not defined by a single plane but by a system of steeply diving faults 6 – 8 km wide. While no longer active, it does represent a line of weakness along the Alaska Range which melt from Pacific Plate subduction has exploited. Its southern end in the vicinity of Becharof Lake is not well defined. The 120 km wide gap between major volcanic centers straddling Ugashik – Peulik may be due to the location of the end of the fault system.
Interesting area, with three different types of magma being erupted over the last 632,000 years. The major volcanic complex is still active with a well-defined low velocity zone some 6 – 7 km below Peulik. Both Ugashik and Peulik have multiple domes, flank collapses, debris avalanches, lava flows, and associated eruptive debris. It is also recently active, with an inflation episode 1996 – 1997.
The oldest activity took place at the Blue Mountain dome complex with dacite domes and lava flows. The next oldest was a basalt eruption at Lone basalt. There is at least one other basalt in the vicinity.
The Gas Rocks were built some 24,000 years ago, once again out of dacites. It also has vigorous CO2 emissions.
The basalt eruptions that created the Ukinrek Maars took place in 1977.
The area is still tectonically active with the last earthquake swarm during the summer of 1998 centered under the western end of Becharof Lake.
This region is still active and should continue to be so for the foreseeable future.
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