Landscape Evolution at an Active Plate Margin - Biological Science ...

Day 2
Theme: Glaciers and snow where the hot
springs flow
with contributions from: D. R. Jessey, Steve Lipshie, R. E. Reynolds,
Janet Westbrook, and Tom Schweich
Start Day 2
What We Will See: Our 130 mile route north of Owens
Valley through Round Valley and into the Long
Valley Caldera will reach elevations of 7,000 feet,
as we travel parallel to the Sierra crest with average
heights of 12,000 feet. The Long Valley Caldera contains
numerous hot springs, evidence of fumaroles,
and signs of hydrothermal alteration. Timber, minerals,
rock commodities, and water—hot and cold—
have been exploited over the last 160 years. Our route
crosses eastern Sierra canyons where evidence of the
powerful forces of Ice Age glaciers remains.
0.0 (0.0) Line St. in Bishop.
0.5 (0.5) Visitor center at Yaney Street. Move to the
right lane.
0.8 (0.3) CONvENE at Giant Sequoia Tree and the
3 Flags at the intersection of highways 395 and 6.
Sequoias were planted in 1918. There are seven in Big
Pine, more than 25 in Bishop, three in Independence,
and at least one in Lone Pine.
Bishop, CA, founded in 1864, has a population of approximately
4000. The town was named after Bishop
Creek; the creek was named after Samuel Addison
Bishop, a cattle rancher in the Owens Valley. The city
of Bishop calls itself the “Mule Capital of the World”
and hosts a festival called Mule Days over the Memorial
Day weekend.
BEAR RIGHT at traffic light and PROCEED
NORTH on Highway 6 when it intersects with Highway
395.
1.8 (1.0) Bear right on Hwy 6 at Dixon Lane.
2.1 (0.3) Continue past Five Bridges Road on Hwy 6.
4.6 (2.4) Cross the Owens River. Pass Silver Canyon
Road, which leads to the Laws Railroad Museum, on
the right.
6.1 (1.5) Continue past Jean Blanc Road. Highway 6
bears north.
8.5 (2.4) Prepare for left turn.
8.7 (0.2) TURN LEFT on Rudolph Road to the Chalfant
Quarry. If you pass a Mono County sign, you’ve
D. R. Jessey and R. E. Reynolds
gone too far.
9.5 (0.8) BEAR RIGHT as road forks (NW).
9.6 (0.1) BEAR LEFT to the quarry.
9.8 (0.2) STOP 2-1. Chalfant Quarry (N 37°27’36.1”;
W 118°21’58.3”). The Bishop Tuff, dated at 760 ka,
was produced by a 6-day-long eruptive event from
the Long Valley Caldera (Hildreth and Wilson, 2007).
Air-fall ash remnants have been found from western
California to Nebraska, an area of 1 x 10 6 mi 2 .
Pyroclastic flows spread 40 miles to the southeast
down Owens Valley, 20-30 miles east to the White
Mountains and 30–50 miles north into Mono Basin.
The caldera collapsed into an 8 mi x 14 mi elliptical
depression. It has been was subsequently enlarged
by slumping and erosion to its current 11 mi x 20 mi
dimension.
The Bishop Tuff consists predominantly of biotite–
plagioclase–quartz–sanidine high-silica rhyolite. Ash
fall deposits are plinian pumice lapilli and crystalrich
ash. Ash flows, as thick as 500 ft, range from
nonwelded to eutaxitic, and from unconsolidated, or
welded vitric zones to fully devitrified zones (Hildreth
and Wilson, 2007). Eruptive volume is not well
known but Hildreth and Wilson (2007) estimate that
150 mi 3 of magma was erupted to produce the Bishop
Tuff.
Two main major units of the Bishop Tuff deposit
are visible here (Fig. 2-1). The lower 12 feet
of the section consists of the poorly-sorted airfall
tuff that was deposited downwind from the
Figure 2-1. Two units of the Bishop Tuff exposed at the Chalfant
Quarry. The lower 12 feet of the section consists of airfall pumice,
and the upper 15-18 ft of the section is comprised of the basal
portion of a thick pyroclastic flow. The dark layers just below
the contact are manganese oxide stains caused by groundwater
circulation. [Source: http://lvo.wr.usgs.gov/]
30 2009 Desert Symposium