page_title: Mount Garibaldi
section_title: Ancestral stages of eruptive activity
hierarchical_section_title: Mount Garibaldi / Geology / Ancestral stages of eruptive activity
caption_attribution_description: English: Mount Garibaldi in southwestern British Columbia, Canada. Red outline indicates the original volcano.
context_page_description: Mount Garibaldi is a potentially active stratovolcano in the Sea to Sky Country of British Columbia, 80 km north of Vancouver, British Columbia, Canada. Located in the southernmost Coast Mountains, it is one of the most recognized peaks in the South Coast region, as well as British Columbia's best known volcano. It lies within the Garibaldi Ranges of the Pacific Ranges.
  This heavily eroded dome complex occupies the southwest corner of Garibaldi Provincial Park overlooking the town of Squamish. It is the only major Pleistocene age volcano in North America known to have formed upon a glacier. Although part of the Garibaldi Volcanic Belt within the Cascade Volcanic Arc, it is not considered part of the Cascade Range.
context_section_description: The mountain grew in three phases. Garibaldi's first phase resulted in the creation of a broad composite cone made of dacite and breccia that has been potassium-argon dated to 250,000 years old. Parts of this "proto-Garibaldi" or ancestral volcano are exposed on Garibaldi's lower northern and eastern flanks and on the upper 240 m (787 ft) of Brohm Ridge. Around where Columnar Peak and possibly Glacier Pikes are now located, a series of coalescing dacite lava domes were constructed.  During the ensuing long period of dormancy, the Cheekye River cut a deep valley into the cone's western flank that was later filled with a glacier.
  After reaching its maximum extent the Cheekye Glacier along with part of the area's ice sheet were covered with volcanic ash and fragmented debris from Garibaldi. This period of growth began with the eruption of the Atwell Peak plug dome from a ridge surrounded by the several thousand foot ice sheet. As the plug dome rose, massive sheets of broken lava crumbled as talus down its sides. Numerous Peléan pyroclastic flows (consisting of a super-heated mix of gas, ash, and pumice) accompanied these cooler avalanches, forming a fragmental cone 6.3 cubic kilometres (1.5 cu mi) in volume and an overall slope of 12 to 15 degrees. (Erosion has since steepened this slope.) Some of the glacial ice was melted by the eruptions, forming a small lake against Brohm Ridge's southern arm. The volcanic sandstones seen today atop Brohm Ridge were created by ash settling in this lake. 
  Glacial overlap was most significant on the west and somewhat to the south. Subsequent melting of the ice sheet and its component glaciers initiated a series of avalanches and mudflows on Garibaldi's western flank that moved nearly half of the original cone's volume into the Squamish Valley. This series of debris flows carried 2.5 cubic kilometres (0.6 cu mi) of the mountain into the Squamish Valley where it covers 10 square miles (26 km²) to a thickness of about 300 feet (91 m). Gaps left by melting ice caused minor to moderate cone distortion where the ice sheet was thin and major distortion where it was thick. The ice was thickest in and thus cone distortion was greatest over the buried Cheekye valley.
  Soon before or after the buried ice had melted away, dacite lava quietly erupted from Opal Cone southeast of the Atwell Peak plug dome 10,700 to 9,300 years ago and flowed 20 km (12 mi) down Ring Creek on Garibaldi's southern and southwestern flanks without encountering any residual glacial ice. One of the lava flows traveled down a 30% to 35% grade over the landslide scar on the western flank. About 0.15 cubic miles (0.63 km³) of dacite erupted in Garibaldi's third period of activity. This lava forms a thin layer of solid rock on the southern and western sides of the volcano and contains well-defined lava flow margin levees. The Ring Creek lava flow is very unusual because lengthy lava flows are usually attained by fluid basalt flows, but the Ring Creek flow is dacite.
caption: Red outline indicates the ancestral volcano
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