MT. BAKER ERUPTIONS Mt. Baker’s volcanic cone (Fig. 2) is the

predominant landform in the North Cascades, standing above non–volcanic peaks as a ‘white sentinel’ visible throughout the Puget

Lowland. However, two older volcanic centers, the Black Buttes and Kulshan caldera, represent even larger eruptions, but their former edifices have collapsed or been eroded away. All of these eruptive sites are part of the larger Mt. Baker volcanic field, which includes.

(1) Pre–Kulshan caldera eruptions (1.3–1.15 million years ago). (2) Eruption and collapse of Kulshan caldera (1.15 million years ago). The Kulshan caldera is a large, collapsed crater filled with volcanic ash and ashflows just northeast of Mt Baker’s main summit cone. All of the once-present Kulshan volcanic cone is now gone as a result of collapse of the cone and subsequent erosion. (3) Post–Kulshan caldera eruptions (1.15–0.5 million years ago). These are eruptions that occurred in the vicinity of Kulshan caldera but are younger than collapse of the Kulshan volcanic cone. (4) Black Buttes eruptions (320–300,000 ago). A large volcanic cone was built just west of the present Mt Baker summit cone but has been deeply eroded by glaciers and is now known as the Black Buttes.

(5) Outlying eruptions (200–50,000 ago) occurred beyond the present volcanic cones. (6) Mt. Baker volcano eruptions 45,000–10,000 years ago built the present summit cone of Mt Baker (Fig. 3). (7) Schreibers Meadow cinder cone and Sulphur Creek lava flow (8,500 years ago) occurred at Schreibers Meadow on the south flank of Mt Baker. (8) Eruptions of volcanic ash from Sherman Crater south of the main summit occurred during the past 6,000 years.

ERUPTIONS 1.3–1.15 MILLION YEARS AGO THAT PRECEDED KULSHAN CALDERA

Scattered remnants of lava flows and dikes older than the Kulshan caldera (1.15 million years) occur outside the caldera NE of the main Mt. Baker volcanic cone.

Barometer Mtn. lava flow remnants between Wells and Anderson Creeks, 1.191 ± 0.094 million years old.

Lava flow along the east wall of Deadhorse Creek 1.151 ± 0.016 million years old

Dike between Swift Creek and Rainbow Creek, 1.180 ± 0.017 million years old.

Dike on southern Cougar Divide, 1.293 ± 0.016 million years old.

Figure 3. Mt. Baker

KULSHAN CALDERA Kulshan caldera (Figs. 4,5) is the remnant of

a former volcano that collapsed following the largest eruption in the Mt. Baker volcanic field 1.15 ± 0.01 million years ago. The explosive eruption of more than 12 cubic miles of lava and ash was followed by collapse of the volcano inward and filling of the caldera with about 3,000 feet of massive ash flows and air fall ash (Figs. 4,6). overlain by well–bedded, laminated, ash–rich sediments that

accumulated in the caldera depression. Tributaries of upper Swift Creek have eroded deep gorges, exposing about 3,000 feet of the ashy sediments. Abundant pumice in the deposits contains about 10–15% crystals of plagioclase, pyroxene, hornblende, and biotite. Isotope dating of pumice from the caldera yielded an age of 1.149 ± 0.010 million years.

Figure 4. Ashy sediments in Kulshan caldera in upper Swift Creek between Table Mt. and Mt. Baker.

Figure 5. Kulshan caldera (blue line) NE of Mt. Baker.

Figure 6. Ash flows and air fall ash (white exposures) in Kulshan caldera. View from Artist Point looking toward Table Mt.

Several dikes, tabular bodies of lava intruded into fractures, and irregular masses of lava intrude the caldera ash flows in the upper drainage of Swift Creek (Fig. 6), one of which was dated at 1.155 ± 0.038 million years. Three lava domes in the middle of the caldera were dated at 1.127 ± 0.012 million years, 1.110 ± 0.012 million years, and 1.008 ± 0.007 million years.

A lava flow remnant overlying the ash flows along the north rim of the caldera was dated at 1.063 ± 0.012 million years and two lava flows in the western part of the caldera were dated at 1.052 ± 0.016 million years and 0.992 ± 0.014 million years.

No ash from this eruption has been found outside the caldera in the Cascades, most likely because of erosion by huge Cordilleran Ice Sheets that overran the area multiple times during the Ice Age. However, about a foot of volcanic ash from this eruption (Fig. 7) blanketed the region as far south as Seattle and along Hood Canal where it is known as the Lake Tapps ash. It also occurs as rounded pumice balls in stream sediments at the southern tip of Camano Island where it has been isotope dated at 1.12 ± 0.156 and 1.14 ± 0.184 million years old. Westgate, Easterbrook, Naeser, and Carson (1987) measured the age of the Lake Tapps tephra at 1.0 million years and determined the

chemical composition of crystals and glass by electron microprobe, but these didn’t match any known eruptions in the Cascade Range. However, the age and chemical composition of

ash at Kulshan caldera are almost identical to the Lake Tapps tephra, so its source is now considered to be the Kulshan caldera.

Figure 7. Lake Tapps ash (white layer) in south Seattle (left) and Hood Canal (right). The chemical

composition is identical to ash in Kulshan caldera and both are a million years old.

Figure 8. Topography NE of Mt. Baker between Cougar Divide and Table Mt

LAVA FLOWS AND DIKES YOUNGER THAN KULSHAN CALDERA

Several post–caldera lava flows, dikes, and lava domes overlie or intrude the caldera ash flows both inside and outside the caldera margins. intrude ash flows inside the caldera in the basin of Swift Creek was dated at 1.155 ± 0.038 million years old, essentially the same age as the caldera ash flows. Three lava domes within the caldera were dated at:

1.127 ± 0.012 million years 1.110 ± 0.012 million years 1.008 ± 0.007 million years

A remnant of a lava flow overlying ashflows at the rim of the caldera was dated at 1.063 ± 0.012 million years. Two lava flows within the caldera were dated 1.052 ± 0.016 and 0.992 ± 0.014 million years.

Dikes are tabular, intrusive bodies of igneous rock that often act as feeders to surface lava flows. Many dikes have intruded the ashflows of the Kulshan caldera and are thus younger than the ashflows that fill the caldera. They occur in all parts of the caldera. Most are 2-13 feet thick and dip 70°–90.° A few pod-like intrusions are up to 160 feet thick.

LAVA FLOWS AND DIKES IN THE COUGAR DIVIDE–CHOWDER RIDGE AREA

At least six lava flow remnants and a number of dikes occur in the Cougar Divide area NE of Mt. Baker (Fig. 8). A 100–foot–thick lava flow north of Kulshan caldera was dated at 1.005 ± 0.017 million years and flow remnants on

Cougar Divide were dated at 1.015 ± 0.018 and 1.052 ± 0.016 million years.

A 230–foot thick lava flow near Chain Lakes was dated at 743,000 ± 34 years, a 400–foot–thick flow near Thompson Creek was dated at 878,000 ± 18,000 years, and a flow at Lookout Mt. was dated at 859,000 ± 14,000 years. A flow on the south wall of the Nooksack River canyon was dated at 202,000 ± 9,000 years.

Dozens of vertical dikes intrude rocks of the Nooksack Formation on Chowder Ridge and Dobbs Cleaver west of Kulshan caldera (Figs. 8, 9). Most are vertical, many are 3–26 feet thick, and a few are 60 feet thick. One dike was dated at 1.16 million years. A shallow intrusive dome on Cougar Divide was dated at 1.018 ± 0.008 million years (Figs. 10, 11).

Remnants of five lava flows occur along Cougar Divide. An extensive lava flow that caps the northern ridge of Cougar Divide is 600 feet thick and has been dated at 613,000 ± 8,000 years. This thick. ridge–capping lava flowed down an ancient stream valley whose sides have since been eroded away by streams and glaciers.

A 400–foot–thick lava flow along the east slope of Cougar Divide west side of Bar Creek has been dated at 334,000 ± 9,000 years (Figs. 10, 11). A small remnant of lava midway along the crest of Cougar Divide has been dated at 192,000 ± 8,000 years and a 400–foot–thick remnant of a lava along the west side of Bar Creek has been dated at 119,000 ± 23,000 years. The youngest lava flow along Bar Creek is a 200–foot–thick lava dated at 105,000 ± 8,000 years (Fig. 10).

Figure 9. Cougar Divide area NE of Mt Baker.

Figure 10. Remnant lava flows in the Cougar Divide area northeast of Mt. Baker.

my=millions of years, kyr=thousands of years (Geologic data from Hildreth et al., 2003)

Figure 11. Cougar Divide.

Figure 12. Topographic map of Lasiocarpa Ridge, Ptarmigan Ridge,

Coleman Pinnacle, Table Mt., and upper Swift Creek.

Figure 13. Lasiocarpa Ridge lava flow.

OTHER LAVA FLOWS OUTSIDE KULSHAN CALDERA A 100–foot–thick lava flow about half a mile outside the south rim of the caldera was dated

at 1.005 ± 0.017 million years. Near Thompson Creek, a 400–foot–thick lava-flow was dated at 878,000 ± 18,000 years and a lava flow on Lookout Mountain was dated at 859,000 ± 14,000. A lava flow near Chain Lakes that forms a 1,500–foot–long ridge was dated at 743,000 ± 34,000 years (Fig. 10).

Lasiocarpa Ridge lava flow

Lasiocarpa ridge is a high, prominent ridge nearly two miles long (Figs. 12,13) made up of 300 feet of coarse flow breccia

overlain by a massive 400–foot–thick lava flow dated at 515,000 ± 8,000 years. The lava thickens northwestward to 600–800 feet thick. The lava originally flowed down a deep valley, but the valley walls have now been completely eroded away, leaving the more resistant lava as a ridge. The flow straddles the rim of Kulshan caldera. At the upper part of the flow inside the caldera, the ridge is a high, steeply sloping pinnacle and basal breccia lies upon older lava, lake sediments, and hydrothermally altered ash flows. The lower part of the ridge beyond the caldera margin flattens out considerably and the lava lies directly on Mesozoic marine bedrock.

Ptarmigan Ridge and Coleman Pinnacle

Ptarmigan Ridge is a long linear ridge of lava between Table Mt. and Coleman Pinnacle. (Figs. 14, 15). It consists of (1) 100 feet of basal lava overlain by lava making the base of eastern Coleman Pinnacle, (2) a 200–foot high ridge of lava banked against the southwest side of Coleman Pinnacle; and (3) several dikes and pods that cut the lava of Coleman Pinnacle. Several dikes that are 15–60 feet thick and two that are 150–300 feet thick and may have fed large eruptions lava that covered the floor of the caldera but have since been eroded.

A trail from the parking lot at Artist Point follows the base of Table Mt. to the divide separating the upper Swift Creek basin and upper Wells Creek Basin. The trail then splits, the right one leading into Chain Lakes at the base of Table Mt. and the left one traversing Ptarmigan Ridge to Coleman Pinnacle. Spectacular views of Mt. Baker and other alpine scenery may be seen from this trail.