THE PHREATOMAGMATIC ORIGIN OF HOME PLATE, GUSEV CRATER
[*James W. Rice, Jr.*] (School of Earth and Space Exploration, Arizona State University,
PO Box 876305, Tempe, AZ 85287-6305, Ph. 480-965-3205, FAX 480-965-1787,
jrice@asu.edu); Nathalie Cabrol (NASA Ames Research Center, Space Science Division,
MS 245-3, Moffett Field, CA. 94035-100); Timothy McCoy (Dept. of Mineral Sciences,
National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave,
NW, Washington, D.C. 20560-0119); Mariek Schmidt (Dept. of Mineral Sciences,
National Museum of Natural History, Smithsonian Institution, 10th and Constitution Ave,
NW, Washington, D.C. 20560-0119); Steven W. Squyres (Dept. of Astronomy, Space
Sciences Bldg, Cornell University, Ithaca, NY 14853); R. Aileen Yingst (Space Grant
Center, Department of Natural and Applied Sciences, University of Wisconsin-Green
Bay, 2420 Nicolet Drive, Green Bay, WI 54311-7001); and the MER Athena Science
Team
Terrestrial phreatomagmatic eruptions occur when ascending magma contacts ground
water, ice and or wet sediments resulting in an explosion and forming one of the
following volcanic edifices; tuff cones, tuff rings and maars. Tuff cones and tuff rings
form by shallow explosions and tend to have finer grained deposits with better sorting
than maars which are formed by deeper more powerful eruptions [Fisher and Schmincke,
1984]. Deposits of phreatomagmatic eruptions are characterized by well developed beds
typically ranging in thickness from a few millimeters to several tens of centimeters,
however most are less than ten centimeters thick. This profusion of numerous thin beds is
the result of a large number of short eruptive pulses. Bedding varies from plane parallel
to cross bedded. Lapilli and bomb sags are also commonly associated with
phreatomagmatic deposits.
The Mars Exploration Rover Spirit continues to investigate a layered feature called Home
Plate. These deposits may contain the first known example of extraterrestrial
phreatomagmatic eruptions. Home Plate, a roughly circular shaped plateau structure 2 to
4 m high and 90 m diameter, is located on the floor of the Inner Basin of the Columbia
Hills [Squyres et al, 2007]. The Inner Basin is an amphitheater shaped lowland which
opens westward toward the basaltic plains of Gusev crater. Spirit reached Home Plate on
sol 744 and found this feature to be composed of inward dipping layered rock capped
with scoriaceous basaltic rocks. Home Plate is composed of two major rock units. The
lower most unit called Barnhill is a coarser grained wavy undulating, laminated to
massive, ribbed rock with alternating coarse and fine layers. Home Plate was found to
have a composition very similar to nearby scoriacious basalts, but with markedly higher
abundances of Cl, Br, Ge, and Zn [Schmidt, et al this meeting ]. The Barnhill unit
contains subrounded to rounded coarse granules up to several mm in size and rounded
voids. These granules are interpreted to be accretionary lapilli, spherical balls of volcanic
ash that form from a wet nucleus falling through a volcanic ash cloud, and the voids are
interpreted to be vesicles formed by the entrainment of vapor in wet ash or possibly
cavities created when lapilli erdoded out of the rock. However, the most impressive
feature in this lower unit is a bomb sag. The sag was created by the ballistic emplacement
of a clast into sediments with a volume of pore water ~15-20% based on terrestrial
observations of similar features [Waters and Fisher, 1971]. All of these features suggest
that Home Plate may have been created by hydrovolcanic explosions forming a tuff
ring/maar. The Barnhill unit represents base surge deposits from these events. The upper
unit called Rogan is a finer grained, moderately sorted, finely laminated, matrix
supported cross bedded clastic rock. Rogan has been interpreted to have been reworked
by aeolian processes or may represent the distal facies of a base surge deposit.
References:
Fisher, R.V. and Schmincke, H.U., 1984, Pyroclastic Rocks, 472p.
Schmidt, M. et al, 2007, Magma-brine interaction to produce Home Plate, Gusev Crater,
Volcano-Ice Interactions on Earth and Mars abstract (this meeting).
Squyres, S.W. et al., 2007 in revision, Mars Exploration Rover Results at Home Plate,
Gusev Crater, Science.
Waters, A. C. and Fisher, R. V., 1971, Base surges and their deposits: Capelinhos and
Taal volcanoes, J. Geophys. Res., 76:5596-5614.
ORAL
CORRESPONDING AUTHOR: JAMES W. RICE, JR. (NOT A STUDENT)