DATE: Thursday, March 27th
SPEAKER: Wade Aubin, Colorado Mesa University
TITLE: What does paleomagnetism tell us about large pyroclastic flows?
LOCATION: Vallecito Room, Student Union Building, Fort Lewis College, Durango, CO
TIME: 5:00-6:30, Social hour and dinner / 6:30-7:30 Society business and presentation
COST: $25 pp
RSVP: Using the following link by Monday, March 24th: https://form.jotform.com/240926885399072
ZOOM LINK: https://fortlewis.zoom.us/j/96161104005?pwd=QkVk0RY3SdX5PylUs2a2Pdbymq5c5A.1
ZOOM PASSCODE: 733336
BSTRACT:
The caldera-forming eruption at Crater Lake, OR, produced large-volume, fast-moving
Pyroclastic Density Currents (PDCs) that blanketed the surrounding terrain with thick deposits.
The PDCs were extensively erosive, stripping the slopes of Mt. Mazama of earlier fall and flow
deposits for 10s of kilometers from the vent, and depositing a thick sheet of lithic breccia as
proximal deposits. They traveled at high speeds, surmounting barriers as high as 200 m and
depositing meter-sized lithic blocks over 15 km away. The mechanisms of air entrainment and
cooling, and flow and sedimentation of large volcanic eruptions and their associated PDCs are
not well understood. To investigate relationships between these phenomena we studied the PDC
deposits of the Crater Lake eruption using the paleomagnetism of pumice and lithic clasts
(accidental rock fragments) in the deposits. We collected samples in all directions from the
caldera, at multiple distances from the caldera rim. The samples were progressively
demagnetized thermally and/or by alternating field (AF). The remanent magnetization of
samples was measured after each demagnetizing step. Demagnetization vectors are stable, but
are however, randomly oriented in pumice and in all lithic clasts. This dictates that the pumice
and lithic clasts cooled prior to their final deposition(s). This requires that the final PDC
deposits are a collection of material that was deposited at relatively cool temperatures. Some
medial deposits are incipiently welded, and previous paleomagnetic studies showed that
proximal lithic breccia were deposited while still hot. Considering these facts with the abundant
evidence of scouring and entrainment by the culminating eruption PDCs, it is likely that early
pyroclastic fall and flow deposits that had cooled were scoured and entrained by later PDCs that
deposited them at farther distances from the caldera. These scouring PDCs were generated by
collapse from tall eruptions columns that cooled the material. Sedimentation in the PDCs was
retarded by their high rates of speed, allowing them to travel 10s of km from the caldera. The
thick sheets of lithic breccia at Crater Lake are the proximal deposits of these intensely scouring
PDCs.