Hannegan caldera

Hannegan caldera is a 3.72 million year old volcanic collapse structure in the North Cascades of the U.S. state of Washington.

[3][4] The caldera is filled with 55-60 km3 of tuff consisting of ignimbrite,[5] wall-rock breccia,[6] and post-caldera sedimentary rocks.

[7] Rocks filling the caldera are lumped into a stratigraphic unit called the Hannegan volcanics.

[1][15][16] Around 4 million years ago large volumes rhyolitic magma rose high into the crust.

Over 10s or 100s of thousands of years, this upward deformation eventually resulted in a semicircular fracture, or ring fault to form in the brittle crust.

[14] Though no longer preserved due to millions of years of intense erosion, searing pyroclastic flows must have swept for 10s of kilometers down river valleys beyond the margins of the caldera, incinerating everything in their path.

As the surface subsidized a kilometer or more during eruption, volcanic ash filled the resulting horseshoe-shaped caldera.

At some time, a lake formed in the depression, and fine grained sediment was deposited on its floor, preserved on the northern flank of Ruth Mountain today as shale and sandstone.

Two granodiorite magma bodies invaded the Ruth Mountain ignimbrite, and are exposed today as a pair of plutons on Icy Peak and the eastern wall of Nooksack Cirque.

[18] Dikes and small rhyolite pods intruded the intracaldera tuff after the caldera collapse was complete.

One sequence of three lava flows exposed on the ridge crest between Ruth Mountain and Chilliwack Pass has a preserved thickness of 240 meters and is dated to 2.96 +/- 0.30 million years old.

Following the end of magmatism in the Hannegan area, the focus of magma intrusion and volcanism migrated to the southwest, and sequentially emplaced the Lake Ann Stock (2.75 million years old),[19][20] Kulshan caldera (1.15 million years old),[21] and the numerous vents in the Mount Baker Volcanic field,[22] including the currently active Mount Baker itself.

South face of Hannegan Peak from Hannegan Pass showing intracaldera structures. A 10-m breccia block is circled.
Outline map of Hannegan caldera in the North Cascades. Postcaldera plutons intrude the southwest portion of the caldera. [ 1 ]
The contact between the darker ignimbrite of Ruth Mountain and the pale ignimbrite of Hannegan Peak 1.4 km northeast of Ruth Mountain (skyline) in the Hannegan caldera. [ 2 ] [ 1 ]
Welded tuff, or ignimbrite, on Hannegan Peak. Knife points to fiamme , pumice clasts flattened by compaction of hot pyroclastic flows.
Fossil Pliocene leaf, 8-cm-long, in intracaldera lake sediment in Hannegan caldera.
Volcanic clasts characterize the ignimbrite of Ruth Mountain. The gray matrix is fine grained volcanic ash erupted during the second collapse of the Hannegan caldera. This outcrop is on the east shoulder of Ruth Mountain. The apple is for scale.
This 240-m thick stack of andesite lava west of Chilliwack Pass in Hannegan caldera is dated to 2.96 million years old. It is the youngest dated rock in the Hannegan volcanics inside the Hannegan caldera. Note geologist in lower left. [ 2 ]