Thielsen's spire-like top is hit by lightning so often that some of its summit rocks have melted into a rare mineral called fulgurite and the mountain itself has earned the nickname "the lightening rod of the Cascades."
West of Thielsen is Diamond Lake and west of the lake is Mount Bailey (a much less eroded and younger stratovolcano). Physically, Thielsen is made of a shield volcano with a composite cone on top.
Thielsen's initial cone was made of pyroclastic material and probably reached a height of 1000 to 3000 feet (~300-900 meters). Around this cone there were peripheral eruptions of basaltic-andesite, which deposited scoria, cinders, and thin flows of lava. Thielsen's construction was later completed when a half mile wide plug of basaltic andesite was emplaced in the volcano's main vent.
Since then numerous glaciers have carried most of the mountain's volume away. Geology
It is not known how old Thielsen is but all its rock formations have normal magnetic polarity meaning that they all likely erupted within the last 700,000 years (it is possible, yet unlikely, that some of the earliest lavas flowed during a previous period of normal polarity).