Glacier Peak - Geology

Geology

Remnants of past, prehistoric lava domes are main components of the summit of the volcano, in addition to its false summit, Disappointment Peak. Past pyroclastic flow deposits are easily visible in river valleys near the volcano, likely caused by lava dome collapse, along with ridges found east of the summit consisting of ash cloud remains. On its western flank, the volcano also has a lahar, or mudflow deposit, which runs for about 35 kilometres (22 mi) into the White Chuck River Valley around 14,000 years ago. 10 other pyroclastic flow deposits are visible, all identified as relatively 10,000 years old. There is also a considerably newer mudflow, about 5,500 years old, which covers an area of 15 kilometres (9.3 mi) between the same river valley, along with two small incidents both under 3,000 years old. Another lahar, of unidentified age, was rich in oxyhornblende dacite; and continued for 30 kilometres (19 mi) into the Sauk River.

There are also ash cloud deposits on the opposite eastern flank of the volcano. Studies of the mountain have to date been unable to find any correspondence with pyroclastic flows, but several past mudflows have been identified. In the Dusty Creek, located by the mountain, there is a lahar at least 6 kilometres (3.7 mi) thick, containing pyroclastic flow deposits and other mudflows. However, this large mudflow is part of a 300 metres (980 ft) thick concentration of past incidents at the volcano that spans the Dusty and Chocolate Creek. In the area at least ten cubic kilometers of lithic debris are contained. Tephra deposits are for the most part constrained to the left flank of the volcano, and at least nine past incidents have been identified. These form several layers of tephra constructing the mountain. Smaller eruptions involving tephra occurred between 6,900-5,500 years ago, 3,450–200 years ago, and as recent as 316–90 years ago.

On the mountain, three additional cinder cones, Dishpan Gap, Indian Pass, and White Chuck, are located about 1,800 m (5,900 ft) up. The volcano has also caused such thermal events such as hot springs. There were three hot springs on the mountain: Gamma, Kennedy, and Sulphur, but Kennedy Hot Springs was destroyed and buried in a slide.