MT. BAKER ERUPTIONS (5) Outlying eruptions (200–50,000 ago) Mt. Baker’s volcanic cone (Fig. 2) is the occurred beyond the present volcanic cones. predominant landform in the North Cascades, (6) Mt. Baker volcano eruptions 45,000– standing above non–volcanic peaks as a ‘white 10,000 years ago built the present summit sentinel’ visible throughout the Puget cone of Mt Baker (Fig. 3). (7) Schreibers Meadow cinder cone and Lowland. However, two older volcanic Sulphur Creek lava flow (8,500 years ago) centers, the Black Buttes and Kulshan caldera, occurred at Schreibers Meadow on the south represent even larger eruptions, but their former flank of Mt Baker. edifices have collapsed or been eroded away. All (8) Eruptions of volcanic ash from Sherman of these eruptive sites are part of the larger Mt. Crater south of the main summit occurred Baker volcanic field, which includes. during the past 6,000 years. (1) Pre–Kulshan caldera eruptions (1.3–1.15 million years ago). ERUPTIONS 1.3–1.15 MILLION (2) Eruption and collapse of Kulshan caldera YEARS AGO THAT PRECEDED (1.15 million years ago). The Kulshan caldera is a large, collapsed crater filled with KULSHAN CALDERA volcanic ash and ashflows just northeast of Scattered remnants of lava flows and dikes Mt Baker’s main summit cone. All of the older than the Kulshan caldera (1.15 million once-present Kulshan volcanic cone is now years) occur outside the caldera NE of the main gone as a result of collapse of the cone and Mt. Baker volcanic cone. subsequent erosion. Barometer Mtn. lava flow remnants between (3) Post–Kulshan caldera eruptions (1.15– Wells and Anderson Creeks, 1.191 ± 0.5 million years ago). These are eruptions 0.094 million years old. that occurred in the vicinity of Kulshan Lava flow along the east wall of Deadhorse caldera but are younger than collapse of the Creek 1.151 ± 0.016 million years old Kulshan volcanic cone. Dike between Swift Creek and Rainbow (4) Black Buttes eruptions (320–300,000 Creek, 1.180 ± 0.017 million years old. ago). A large volcanic cone was built just Dike on southern Cougar Divide, 1.293 ± west of the present Mt Baker summit cone 0.016 million years old. but has been deeply eroded by glaciers and is now known as the Black Buttes.

Figure 3. Mt. Baker

KULSHAN CALDERA Kulshan caldera (Figs. 4,5) is the remnant of accumulated in the caldera depression. a former volcano that collapsed following the Tributaries of upper Swift Creek have eroded largest eruption in the Mt. Baker volcanic field deep gorges, exposing about 3,000 feet of the 1.15 ± 0.01 million years ago. The explosive ashy sediments. Abundant pumice in the eruption of more than 12 cubic miles of lava and deposits contains about 10–15% crystals of ash was followed by collapse of the volcano plagioclase, pyroxene, hornblende, and biotite. inward and filling of the caldera with about Isotope dating of pumice from the caldera 3,000 feet of massive ash flows and air fall ash yielded an age of 1.149 ± 0.010 million years. (Figs. 4,6). overlain by well–bedded, laminated, ash–rich sediments that

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 No ash from this eruption has been found into fractures, and irregular masses of lava outside the caldera in the Cascades, most likely intrude the caldera ash flows in the upper because of erosion by huge Cordilleran Ice drainage of Swift Creek (Fig. 6), one of which Sheets that overran the area multiple times was dated at 1.155 ± 0.038 million years. Three during the Ice Age. However, about a foot of lava domes in the middle of the caldera were volcanic ash from this eruption (Fig. 7) dated at 1.127 ± 0.012 million years, 1.110 ± blanketed the region as far south as Seattle and 0.012 million years, and 1.008 ± 0.007 million along Hood Canal where it is known as the Lake years. Tapps ash. It also occurs as rounded pumice A lava flow remnant overlying the ash flows balls in stream sediments at the southern tip of along the north rim of the caldera was dated at Camano Island where it has been isotope dated 1.063 ± 0.012 million years and two lava flows at 1.12 ± 0.156 and 1.14 ± 0.184 million years in the western part of the caldera were dated at old. Westgate, Easterbrook, Naeser, and Carson 1.052 ± 0.016 million years and 0.992 ± 0.014 (1987) measured the age of the Lake Tapps million years. tephra at 1.0 million years and determined the chemical composition of crystals and glass by ash at Kulshan caldera are almost identical to the electron microprobe, but these didn’t match any Lake Tapps tephra, so its source is now known eruptions in the Cascade Range. considered to be the Kulshan caldera. However, the age and chemical composition of

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 Cougar Divide were dated at 1.015 ± 0.018 and YOUNGER THAN KULSHAN 1.052 ± 0.016 million years. CALDERA A 230–foot thick lava flow near Chain Lakes was dated at 743,000 ± 34 years, a 400–foot– Several post–caldera lava flows, dikes, and thick flow near Thompson Creek was dated at lava domes overlie or intrude the caldera ash 878,000 ± 18,000 years, and a flow at Lookout flows both inside and outside the caldera Mt. was dated at 859,000 ± 14,000 years. A flow margins. intrude ash flows inside the caldera in on the south wall of the Nooksack River canyon the basin of Swift Creek was dated at 1.155 ± was dated at 202,000 ± 9,000 years. 0.038 million years old, essentially the same age Dozens of vertical dikes intrude rocks of the as the caldera ash flows. Three lava domes Nooksack Formation on Chowder Ridge and within the caldera were dated at: Dobbs Cleaver west of Kulshan caldera (Figs. 8, 1.127 ± 0.012 million years 9). Most are vertical, many are 3–26 feet thick, 1.110 ± 0.012 million years and a few are 60 feet thick. One dike was dated 1.008 ± 0.007 million years at 1.16 million years. A shallow intrusive dome A remnant of a lava flow overlying ashflows on Cougar Divide was dated at 1.018 ± 0.008 at the rim of the caldera was dated at 1.063 ± million years (Figs. 10, 11). 0.012 million years. Two lava flows within the Remnants of five lava flows occur along caldera were dated 1.052 ± 0.016 and 0.992 ± Cougar Divide. An extensive lava flow that caps 0.014 million years. the northern ridge of Cougar Divide is 600 feet Dikes are tabular, intrusive bodies of igneous thick and has been dated at 613,000 ± 8,000 rock that often act as feeders to surface lava years. This thick. ridge–capping lava flowed flows. Many dikes have intruded the ashflows of down an ancient stream valley whose sides have the Kulshan caldera and are thus younger than since been eroded away by streams and glaciers. the ashflows that fill the caldera. They occur in A 400–foot–thick lava flow along the east all parts of the caldera. Most are 2-13 feet thick slope of Cougar Divide west side of Bar Creek and dip 70°–90.° A few pod-like intrusions are has been dated at 334,000 ± 9,000 years (Figs. up to 160 feet thick. 10, 11). A small remnant of lava midway along LAVA FLOWS AND DIKES IN THE the crest of Cougar Divide has been dated at COUGAR DIVIDE–CHOWDER RIDGE 192,000 ± 8,000 years and a 400–foot–thick AREA remnant of a lava along the west side of Bar Creek has been dated at 119,000 ± 23,000 years. At least six lava flow remnants and a number The youngest lava flow along Bar Creek is a of dikes occur in the Cougar Divide area NE of 200–foot–thick lava dated at 105,000 ± 8,000 Mt. Baker (Fig. 8). A 100–foot–thick lava flow years (Fig. 10). north of Kulshan caldera was dated at 1.005 ± 0.017 million years and flow remnants on 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.