Abstract

During the night of March 1 and 2, 1951, an inconspicuous group of hot springs and small mud volcanoes in northeastern California burst into spectacular eruption, unequalled by other known mud volcanoes. The eruption cloud of steam, gases, and mud particles rose several thousand feet in the air and distributed fine debris to the southeast for a distance of at least 4 miles. More than 20 acres of the hot-spring area was intensely disturbed and greatly modified by the eruption, estimated to involve at least 6 million cubic feet or 300,000 tons of mud. Several days after the eruption, the area was barely active. The eruption appears to be unique in the history of the springs.

The hot-spring system is in deep fine-grained clastic sediments immediately east of the Surprise Valley fault bounding the Warner Range. The sediments of the spring area are saturated with near-neutral hot saline water. Previous temperatures and geothermal gradient of the area were probably high.

Mud volcanoes exist in similar physical environment near Gerlach in Washoe County, Nevada, and on the southeast shore of Salton Sea, Imperial County, California. Other mud volcanoes occur in acid thermal areas and are characterized by abundant volcanic gases and near-surface alteration by sulfuric acid; their eruptions involve only surficial material and not underlying competent bedrock.

Eruptions in deep fine-grained basin sediments are attribured to unstable or metastable temperature-depth relations existing in many high-energy thermal systems. Vapor pressure at depth may equalor exceed hydrostatic pressure. Great energy is stored in a thermal system of this type, but ordinarily is released slowly.

A mud-volcano origin is possible for some eruption deposits classed as phreatic or cryptovolcanic. Although near-boiling hot springs are considered phases of volcanism, true volcanic eruptions are distinct from mud-volcano eruptions. The former derive their energy directly from new volcanic rocks or magma, but the latter are caused by sudden release of energy stored in near-surface hydrothermal systems and do not involve direct release of energy from new volcanic magma. The energy of true volcanic eruptions, however, may be increased by release of energy from previously existing hydrothermal systems, for example in the Rotomahana phase of the great Tarawera eruption of 1886 in New Zealand.

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