PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

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

Hole-in-the-Ground maar

Hole-in-the-Ground maar (low-relief, broad volcanic crater formed by shallow
explosive eruptions associated with interaction with groundwater) is part of
the Fort Rock volcanic complex of central Oregon. As its name implies, it is a
1600 m diameter crater with a floor about 150 m below the original ground
level and a rim ranging from 35-65 m above the original ground level. Only 10%
of the ejecta that resulted from the eruption that formed Hole-in-the-Ground
is juvenile basalt material; the rest is disrupted coutry rock. Most of the
ejecta is fine-grained, but some of the clasts are several m in diameter.

Hole-in-the-Ground is the result of an explosive volcanic eruption that
occurred between 13,500-18,000 years before present (Pleistocene), when Fort
Rock Basin was a lake, and the location of the eruption was near the shore,
with access to groundwater. It formed when groundwater 300-500 m deep came
into contact with hot basaltic magma rising from below. The resulting steam
blew out overlying rock, as well as a small amount of juvenile basalt. Four
major explosive events occurred, driven by slumping and subsidence along the
ring fault that closed the vent, or by changes in the supply of groundwater.


References:

Heiken, G.H., R.V. Fisher and N.V. Peterson (2006), A Field rip to the Maar
Volcanoes of the Fort Rock-Christmas Lake Valley Basin, Oregon, Geol. Surv.
Circ. 838, USGS.

Peterson, N.V., and J.R. McIntyre (1970), The reconnaissance geology and
mineral resources of eastern Klamath County and western Lake County, Oregon,
Ore. Dept. Geol. Min. Indust. Bull, 66, 70 pp.

Wood, C.A., and J. Kienle, eds. (1992), Volcanoes of North America:  United
States and Canada, Cambridge Univ. Press, p. 205.