North Sister and Middle Sister

North Sister (left) and Middle Sister (right).

North Sister

North Sister is a shield volcano made of basaltic andesite. The shield is about 5 miles (8 km) in diameter. The summit cone is made of cinders, lava flows, dikes, sills, and a central plug. The flanks of North Sister have been cut by Pleistocene and Holocene glaciers.

Pleistocene: A epoch in Earth history from about 2-5 million years to 10,000 years ago. The term also refers to the rocks and sediment deposited in that epoch.

Holocene: The time period from 10,000 years ago to the present. The term also refers to the rocks and deposits of that age.

Middle Sister

Middle Sister formed after North Sister. Basalt was erupted from fissures on the flanks of the volcano. Andesite lava flows were erupted from a vent near the base of the volcano. Domes and flows of dacite and rhyodacite also make part of Middle Sister. The most recent eruptions were from vents high on the north and south sides of the volcano. The produced long dacite lava flows. Middle Sister has also been eroded by glaciers.

This picture was taking from the Dee Wright Observatory.