PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
  PUBLICATION_DATE    = 2009-11-15
  NOTE                = "Geologic setting description"
END_OBJECT            = TEXT
END

Geology of Fort Rock Tuff ring

Fort Rock tuff ring is a ring of tephra formed during a volcanic eruption that
was a result of interaction between abundant water and magma. Fort Rock is a
1.4 km-diameter structure in the Christmas Lake Basin southeast of Bend,
Oregon. The primary composition is palagonitized basaltic andesite [Heiken,
1971; Heiken et al., 1981]. Although the current environment of Fort Rock is
arid, sage bush country, a large lake filled the basin from the Late Pliocene
during the Pleistocene.  Fort Rock is one of 40 maars, tuff rings, and tuff
cones that formed by phreatomagmatic eruptions when abundant water was
present.  Subsequent wave action breached the Fort Rock tuff ring and eroded
all but the most lithified (palagonitized) tephra layers. Orange-brown
lapilli-tuff in graded beds 1-cm to 1-m thick make up the steep walls of Fort
Rock [Heiken, 1971].


References:
Heiken, G.H. (1971), Tuff rings:  Examples from the Fort Rock-Christmas Lake
Valley Basin, South-Central Oregon, J. Geophys. Res., 76, 5615-5626.

Heiken, G.H., Fisher, R.V., Peterson, N.V. (1981), A field trip to the maar
volcanoes of the Fort Rock-Christmas Lake Valley basin, Oregon, In:  Guides to
some volcanic terranes in Washington, Idaho, Oregon, and northern California,
U.S. Geological Survey Circular, p. 119-140.

Yingst, R.A., M.E. Schmidt, and R.C.F. Lentz (2009), Observations of a
potential Mars analog at the microscale using rover-inspired methods:  A
10-sol observation of Fort Rock tuff ring, J. Geophys. Res., in press.