MT. SHASTA: THE CASCADE’S LARGEST STRATOVOLCANO, BUILT AND REBUILT OVER 700 KA

Mt. Shasta is a ~350 km³ andesite-dacite stratovolcano in the Cascade arc, eclipsed in volume only by the sprawling rear-arc Medicine Lake volcano 50 km E and Oregon’s Newberry volcano. Shasta’s predecessor, the andesite-dacite Rainbow Mountain, was active ~1200-950 ka along the Cascade arc axis 15 km ENE, its eroded edifice now nestled among older and younger short-lived basaltic andesite edifices. Situated in northern California’s Shasta Valley W of the arc axis, Mt. Shasta was built, destroyed, and rebuilt in five Ar-dated episodes with central vents <3 km from the present summit. (1) The 700-400 ka Sand Flat (SF) edifice of lavas, domes and pyroclastic rocks, named for the intact lava ridge on the SW flank of Mt. Shasta, filled the southern portion of Shasta Valley. Preserved flank vents lie along a N-S trend through the current summit, parallel to Basin & Range faults abundant to the east. The SF edifice failed in a catastrophic debris avalanche with blocks preserved >40 km NNW of the source. (2) The 300-100 ka Sargents Ridge (SR) central vent volcano, vented 2.5 km south of the present summit and is named for a prominent ridge S of the vent. SR lavas are well-exposed to the SE, S and SW, but present in all sectors. Most lavas erupted 190-180 ka and 140-130 ka. The youngest lavas at the SR vent erupted at 125 ka, but SR flows to the east and north erupted ~115 and 100 ka, and may have erupted from the same SR vent. (3) The Misery Hill (MH) vent was established 50-35 ka ~500 m S of the present summit and erupted lavas to the E and W. Flank vents produced domes and flows along a N-S trend through the summit ~20 ka, and abundant undated pyroclastic deposits shed NW, NE and SE suggest a summit dome was present prior to 11 ka. (4) The Plinian Red Banks (RB) eruption at ~11 ka deposited pyroclastic flows in most sectors and pumice-fall ~50 km E, followed by the >15 km³ Shastina (ST) dacite dome eruption ~200 years later. (5) Lavas, domes and pyroclastic flows erupted from or N of the present summit in the Hotlum (HT) sequence, named for the large glacier NE of the summit. Though difficult to date due to youth, lack of incision or carbon, most HT lavas erupted ~10-6 ka. Similar to other arc volcanoes, Mt. Shasta’s long history is characterized by relatively short, productive growth episodes with intervening quiescence.