The Kulshan Caldera is the predecessor to the Mount Baker Volcanic Field in northern Washington State. It was recognized relatively recently in 1990. We posted about this part of the world and neighboring Glacier Peak to the south in 2016. Kulshan and Baker are the next volcanic system to the north.
The Kulshan Caldera and Mount Baker Volcanic Field are located some 48 km E of the coastal city of Bellingham, WA. Baker is the fifth tallest volcano in the Cascades at 3,268 m. It is also the most heavily glaciated of all Cascades volcanoes. And it is this glacial coverage that removed evidence that would have earlier identified the Kulshan Caldera. It is one of the snowiest places in the world, with 29 m of snow at Mount Baker Ski area 15 km N in 1999. It is visible as far away as Seattle. The name Koma Kulshan or simply Kulshan is derived from Native names for Mount Baker in two languages.
Mount Baker is surrounded by the Mount Baker – Snoqualmie National Forest, one of the most visited forests in the US. It is located on the western side of the Cascades between the Canadian Border and Mount Ranier National Park. This string of national parks and forests lies to the east of the heavily populated coastal cities of Olympia, Tacoma, Seattle, Everton and Bremerton, WA. Mount Baker is visible from Seattle, though not as close as Mount Rainier. Visitors enjoy camping, hiking, winter sports, and fishing. Local wildlife is abundant, as is precipitation.
The largest city downstream from Mount Baker is Bellingham, some 50 km W. Bellingham has a population of at least 92,000. While Bellingham is not located in any of the volcanic hazard zones (lahar) for Mount Baker, the zone curves around it to the N and goes through neighboring Burlington and Mount Vernon to the S.
Monitoring of Mount Baker was increased due to the 1975 – 1976 activity. Much of it has been dismantled over the last 45 years. Current monitoring is now considered to be insufficient based on the threat posed on surrounding communities.
Activity in the region began some 4.0 Ma, with injection of a magma body that eventually created the Hannegan caldera. The 7 km diameter Hannegan caldera, some 20 km ENE of Baker, formed in a massive eruption 3.7 Ma. It is around 45 km2 in area. Post-caldera activity lasted until 2.96 Ma and produced rhyodacite ignimbrites, intracaldera breccias, numerous dikes and post-caldera intrusions. These are visible on the south face of Hannegan Peak, Ruth and Icy Peaks. Volcanic activity migrated to the SW in steps over the last 4.0 Ma. There were 3-4 small post-caldera plutons intruded after its collapse.
A second large body of magma pushed into the crust to form the Lake Ann stock around 2.8 Ma. This is midway between the Hannegan and Kulshan calderas. It is unclear if anything erupted from this intrusion due to severe glacial scouring from nearly 30 ice age cycles over the last 2.8 Ma (average ice age is around 100 ka). About 7 km2 of this body has been unroofed by the ice.
The next large known eruption took place some 1.15 Ma at the Kulshan caldera. The VEI 7 eruption was similar in size to the Crater Lake (Mazama) caldera forming eruption and produced a pale ash flow tuff. Multiple ice ages produced glaciers that scoured almost everything from that eruption outside the rim of the caldera proper. It also lowered the rim of the caldera by hundreds of meters. The Kulshan silicic magma body is the youngest of dozens of granitoid plutons emplaced 32 – 2.7 Ma creating the Chilliwack batholith.
The scope of the eruption was put into perspective with the discovery of a 20 – 30 cm thick layer of the Lake Tapps tephra some 200 km S of Kulshan. A second 15 cm thick tephra horizon at Washtucna some 300 km from the volcano was sampled in 1993. Both tephra horizons are chemically similar to the rhyodacite tuff remaining within the caldera proper.
The most recent SW shift in magmatic focus was underway some 750 ka, growing an andesitic stratovolcano cluster from 500 ka – present. The oldest of these was the Black Buttes stratovolcano, dating 495 – 288 ka. It is located 9 km SW of the caldera and off the SW flank of current day Mount Baker. Black Buttes was larger than the current Mount Baker. The Black Buttes – Mount Baker cluster has erupted over 60 km3 of andesite in the last 0.5 Ma.
Baker is the youngest stratovolcano. It is an andesitic cone located some 25 km SW of the Hannegan Caldera, and 9 km SW of the Kulshan Caldera that dates back 140 ka. The main pulse of activity from Baker took place 40 – 12 ka. There are smaller vents throughout the field. It appears that the magmatic system never fully shut down between main pulses. The current Black Buttes – Baker – satellite system may be similar in extent and longevity to the previous three pulses (Hannegan, Lake Ann and Kulshan). The difference today is that there has not been enough glacial activity to remove evidence of the newer vents as yet.
The most important thing about this field is the glacial ice and its erosive work on soft volcanic structures and debris. Ice quickly removes tephras and pyroclastic deposits. Most of the surviving volcanic outcrops are lava flows. Much of the activity was initially phreatomagmatic, as eruptions progressed through some thickness of ice cover.
The story of the Kulshan Caldera is one of fire and ice, with the movement of the Cordilleran Ice sheet out of British Columbia every 100 k years covering all but the highest peaks in the region. This scouring by glacial ice cap has removed all the soft material from caldera formation, including several hundred meters from the rim of the caldera itself.
There are two main ways to discover a caldera. One is to find a large hole and try to figure out what came out of it. The other is a reverse process, find deposits and figure out where they came from. Sufficiently large unexplained pyroclastic tuffs and fall deposits generally lead to the identification of a previously unknown source, usually a caldera. The action of multiple advances and retreats of the Cordilleran Ice Sheet from British Columbia meant that Kulshan was discovered based on exterior fall deposits and interior pyroclastic deposits.
Most calderas are associated with individual stratovolcanoes or clusters of them. Kulshan is unique with no record of precaldera volcanic edifice.
10 volcanic units so far are identified around the periphery of the caldera. Most of these were emplaced before the caldera collapse 1.15 Ma. There was no large cone preceding the caldera, rather, scattered dike-fed lava flows. Four of the units are rhyodacites, compositionally similar to the caldera-filling ignimbrite and early post-collapse lavas. These may be leaks from the precollapse silicic reservoir. The other six units are andesite-dacite dikes and dacites lavas just outside the caldera rim. There is little remaining of these units mostly due to glacial action. Each one may have fed a sizable eruptive unit.
The caldera-forming eruption is the largest eruption of the Mount Baker volcanic field, releasing over 50 km3 of rhyodacitie from a shallow reservoir. Eruptive products may be larger than 120 km3 from this eruption. A layer over 1 km thick of intracaldera tuff fill a steep-walled 30 km2 caldera with no bottom identified as yet. The Lake Tapps tephra 200 km south is near the southern limit of the Cordilleran Ice Sheet advances. None of the tephras or extracaldera ignimbrites survived ice sheet advances after the eruption. The top layer of the tuff is at best weakly welded and covered by wall collapse breccias and lake sediments. Strongly welded tuff is found deeper.
Eruptions following caldera formation remained predominantly rhyodacitic for 160 ka, mostly emplacing silicic domes, lava flows, dikes and shallow intrusions inside the caldera. There are at least two postcaldera rhyodacites 3-4 km W of the caldera in the Chowder Ridge – Cougar Divide area. There are several precaldera rhyodacite dikes and lavas in this area. This suggests that a long-lived silicic magma reservoir was more extensive than the area of the roof collapse and fed activity in both the caldera and Chowder Ridge. After 0.99 Ma, these two became discrete centers of andesite – dacite magmatism, active into the late Pleistocene.
Around 1.00 Ma, volcanic activity having erupted most eruptible material from a strongly zoned magma chamber started switching to a mostly andesitic composition. There are more than 60 mostly andesitic dikes that cut the material filling the caldera. These are the only known record of post-caldera eruptions that likely built a large andesitic volcanic pile completely removed by the ice. There is no rhyodacite younger than 0.99 Ma in or near the caldera.
Caldera-forming pyroclastic eruptions always produce significant fall deposits. Generally, this is at least 20% of the total volume erupted, at times as large or larger than associated ignimbrites. The problem with Kulshan is that the proximal tephras and ash-flow deposits did not survive multiple advances and retreats of the Cordilleran Ice Sheet out of British Columbia to the N. This glacial action lowered the landscape by hundreds of meters.
Volcanologists compare the Kulshan eruption 1.15 Ma with the Mazama (Crater Lake) eruption 7.7 ka. Both are similar in size and left similarly sized calderas. The logic is that a pyroclastic caldera forming eruption from Kulshan distributed fall deposits at least as far and as wide as the Mazama eruption did. The 1981 discovery of a 30 cm ash layer at Lake Tapps east of Tacoma and another site in the Olympic Peninsula was the first of these deposits. They were compared with Kulshan tuffs and found to be chemically similar. A second 15 cm layer another 90 km farther out was also similar. These were both south of the limits of glaciation and found in lake sediments.
The ashfall was deposited in two pulses, though no break in eruption is apparent. The break was likely due to a change in prevailing winds. The Lake Tapps tephra was deposited over glacial outwash / till. None of the Kulshan tephra has been found to the north, likely due the action of ice. Tephras were deposited on an extensive ice cap and removed by subsequent advances and retreats of ice. North winds off an ice cap might also explain the southward distribution of the tephra.
There are some oddities about the tuff that fills the caldera that can be explained by an eruption progressing through an ice cap which would include a meltwater lake or a collapsing glacial lid that was destroyed in steps rather than all at once. These include a general lack of welding, pervasive fine vesicularity of pumice, and gross upward grading in the upper 200 m of the remaining intracaldera tuff. To get this sort of behavior, huge amounts of water quenching the erupting pyroclastics are required, meaning either a pre-caldera lake or an eruption beneath an ice cap.
Mount Baker is the youngest stratovolcano in the field, built upon the older Black Buttes. Much of its early activity was eroded away during the last ice age. Ice sheets filled neighboring valleys surrounding the volcano. Most of that ice disappeared by 14 ka, leaving nearly 2 km3 of thick glacial cap on the volcano itself.
The most recent vent on Baker is Sherman Crater, with ridges of lava and hydrothermally altered rock. Eruptions are not typically highly explosive. It is not frequently active, with four periods of magmatic eruptions recently recognized. Eruptions have produced tephras, pyroclastic flows, and lava flows from summit vents and the Schriebers Meadow Cone. The most destructive and frequent events have been lahars and debris avalanches. These are not necessarily all magmatic events.
Baker was built by stacks of lava flows and volcanic breccias before the end of the most recent glaciation. There are two craters on the mountain. Carmelo Crater, under the summit ice dome, is the source for the most recent cone building eruptions. The highest point on Baker, Grant Peak is on the exposed SE rim of Carmelo Crater. The more recent Sherman Crater is S of the summit, with an ice-covered floor some 300 m below the summit ice dome. This crater is the source of all Holocene eruptive activity and has hundreds of fumaroles.
Lava flows from the summit erupted 30 – 10 ka, with final lava flows being blocky pyroclastic flows. The volcano continues to have a vigorous hydrothermal system.
Around 6.6 ka, was a series of eruptions that ended with the largest post-glacial tephra (magmatic) eruption. Activity began with a collapse of the Roman Wall, creating a lahar down the Nooksack River, draining 48 km to the Fraser River to the N. Some lahar deposits from this event approach 100 m thick. The collapse and lahar were followed by a small hydrothermal eruption from the Sherman Crater, which triggered a second flank collapse E of the Roman wall. This collapse created another lahar that followed the path of the first for at least 32 km. The final event was a tephra eruption that put ash at least 64 km downwind to the N and E.
There were several eruptions from Sherman Crater during the 19th Century. A possible eruption 1792 was observed by Spanish explorers. There was another eruption in 1843 that left ash, burned forest, and lahars in the rivers. A Nov 1860 eruption was visible from a steamer on the Pacific to the W. Another large debris avalanche sent nearly 15 km3 down the side of the volcano, creating a lahar that traveled some 10 km.
Activity in the 20th century declined, though debris avalanches continued. Fumarole activity increased in 1975 melting snow in the Sherman Crater, leading to concern that an eruption was near. Heat output rose by a factor of ten in some places for months. Baker Lake was constructed earlier in the 20th Century downstream from the volcano. Possible seiches due to lahars from the 1975 activity led to the operator lowering the lake level by 10 m to minimize downstream danger.
Debris flows (lahars) are the most predictable hazard from Mount Baker. They have moved down all drainages that head on Baker. Small flows with volumes less than 0.01 km3 are the most frequent and travel up to a few kilometers from the source. They are only a hazard to climbers or hikers / campers in the drainages. These are generally caused by rainfall or small landslides.
There are two active fumarole fields on Mount Baker. The first is in Sherman Crater, just south of the summit. The second dis Dorr Fumarole Field, high on the N flank, 2 km N of Sherman Crater. Before 1975, Sherman Crater had an energetic fumarole field with clusters of boiling-point fumaroles along its W and NW margins. There were ice pits in the SW, NW and E parts of the crater glacier. Acidic meltwater drains through the crater’s East Breach, under Boulder Glacier, into Boulder Creek and into Baker Lake, a hydroelectric reservoir some 13 km down slope. The Dorr Fumarole field was about a third of the size of the Sherman Crater field. Seismic monitoring since 1972 came from a single instrument on the W flank of the volcano, 6 km from the crater. A second instrument was installed Sept 2009.
In 1975, Baker had the largest observed change in thermal activity since the phreatic explosions in the 19th Century. Initial observations were increased heat discharge, ejecta from fumaroles, chemical changes in fumarole emissions and surface water. On Mar 10, 1975, staff at Upper Baker dam reported unusually large, dark vapor plumes from Sherman Crater. These were widely reported as were multiple airborne observations of increased steam emission from the crater, snow and ice melting, new crevasses in crater glaciers, and thin layers of newly deposited dust next to the crater. There was not a magmatic component to the ashfall material, so it was referred to as “fumarole ejecta.” Mud streams flow from the new main fumarole at the same time as the airborne ejecta. This continued at low levels to Sept 1975.
The following video comes courtesy David Hirsch of a vigorous fumarole in the Smiley Group in Sherman Crater, July 2007.
The area of newly exposed heated ground continued to increase. New snowfall did not last long. By mid-April, a 40 m plug melted out of the central part of the crater glacier creating a shallow lake. Increased acidic runoff from the lake and fumarole clusters spilled acidifying Boulder Creek downstream. Through the summer, more fumaroles were discovered and the snow-free thermal area in Sherman Crater tripled in size. There was no observed change in the Dorr Fumarole field over this time.
The snow-free area in Sherman Crater has not returned to its pre-1975 area since the event. Fumarole temperatures remained high for years. By 1994, the new main fumarole was plugged with debris. By 2006, fumaroles in the W part of the crater were back at pre-1975 temperature and output levels.
Initial conclusion was that the activity was not due to magma movement at depth. It was blamed on a change in the hydrothermal plumbing system. After 30 years of observations following the event, those conclusions were change to active magmatism beneath the volcano. A small magma intrusion took place in 1975 and the volcano and hydrothermal system have been recovering from this in the years afterwards. There were two episodes of deflation since 1975, 1981 – 1983 and 2006 – 2007. There are shallow, low frequency earthquakes attributed to glacial movement. Deeper long-period events may be connected to magma presence or movement into the mid-crust in 1975 or earlier. There may also be an opening of the conduit to a deep volatile source releasing a pulse of magmatic gasses. The cause of the 1975 activity is more likely a connection to a deep magmatic source rather than a stalled, shallow intrusion.
We took an initial look at tectonics of this part of Washington State and the Northern Cascades with our Glacier Peak post in 2016.
Volcanism in the modern Cascades over the last 7 Ma is related to the subduction of the Juan de Fuca Plate under the western coast of North America. The Juan de Fuca Plate is the last remaining chunk of the long-subducted Farallon Plate, that has been driving volcanism in western North America for over 30 Ma. The Juan de Fuca Plate is in the midst of fragmenting, splitting off the northern chunk, the Explorer Plate along the Nootka Fracture Zone some 7 – 5 Ma. The southern chunk, the Gorda Plate to the west of the Oregon – California Border has not yet separated. When the fragmentation carving off the Explorer Plate was complete, magmatism returned to the Cascade Arc and uplift of Cascade and Olympic ranges began.
The whole mass is subducting slowly beneath the North American Plate along the Cascadia subduction zone off the west coast of North America. This subduction has provided magma to power the modern Cascade volcanic arc and pushed up the modern Cascade and Olympic mountain ranges parallel with the coast. Note that volcanism along the Cascade arc has an extensive history dating back at least 37 Ma. The original volcanoes were completely eroded, leaving only plutons and batholiths.
As the magmatism began, there were some changes along the northern end of the historic volcanic arc. Earlier volcanism extended generally northward from the location of modern-day Glacier Peak. Today, it is now generally oriented NW toward Mount Baker and the Garibaldi belt into British Columbia. This is thought to be related to a rollback of the subducting Juan de Fuca Plate. Following the fragmentation, the Juan de Fuca started subducting in a more easterly direction.
The 20 – 25 km magmatic progression from the Hannegan caldera to Mount Baker (5-6 mm/yr) is one eighth of the convergence rate of the Juan de Fuca Plate beneath North America. Hildreth et al has speculated that this trend might be related to an increase rollback of the Juan de Fuca Plate after detachment from the buoyant Explorer Plate from its northern edge some 4 Ma, about the time activity in this region ramped up.
Comparison of the three most recent Cascades caldera show that they erupted similar volumes of magma, 50 km3. The Mazama and Kulshan calderas are of similar size (25 and 30 km2). Rockland is 5-6 km in diameter, likely with similar eruptive volume. The difference is that Mazama and Rockland erupted through thick piles of andesites and silicic lavas. Kulshan erupted through basement rocks. Kulshan is the only eruption with phreatomagmatic involvement due to ice coverage. It did not have a pre-collapse stratocone. All three calderas erupted silicic lavas into the newly formed caldera.
The question is why are these the only three Quaternary calderas along the entire 1100 km Cascades arc? A larger question is why calderas are relatively uncommon in the Cascades? Only 10 are known as of July 1996. What processes or properties of the Cascades make it different from other Pacific volcanic arcs where young calderas are common? The Cascade crust is 40 – 45 km thick, thicker than most, but not quite as thick as the Central Volcanic Zone of the Andes. Plate convergence is relatively slow at 4.3 cm/yr, but recent volcanic activity is not noticeably limited. Some of the most productive long-lived Quaternary volcanic districts (Adams, Rainier, Jefferson, Three Sisters, Shasta) do not have calderas.
The Mount Baker Volcanic Field is the most recent manifestation of active magmatism in the area over the last 4 Ma. This has created the Hannegan and Kulshan calderas. One of the magma intrusions beneath Lake Ann between the two calderas does not appear to have erupted anything. Magma packets are still injected below the system, the most recent beneath Mount Baker in 1975. There is no reason to believe that this will be the last magma injection. While volcanism has migrated from Kulshan, it is active beneath the Baker field. That activity coupled with nearly two kilometers of ice on the peak and a history of debris avalanches and flows, make this system a very dangerous one today and for the foreseeable future to those living downstream.