USGS Science⌕ Search

SEARCH · USGS Science

Results for “Evolution”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 829 records · Page 46Linked to original sources
USGS1994

Water fact sheet; evolution of sediment yield from Mount St. Helens, Washington, 1980-1993

The most enduring geological consequence of the eruption of Mount St. Helens, Washington, on May 18, 1980, and the most costly single element in the recovery effort, has been the persistent downstream sedimentation caused by erosion of the approximately 3 cubic kilometers (km 3 ) of sediment deposited on the landscape surrounding the volcano. Most of the sediment was associated with the emplacement of a 2.8 km 3 debris avalanche in the upper part of the watershed of the North Fork Toutle River, and debris flows in the channels of the South Fork Toutle River, Pine Creek, Swift Creek, and Muddy River. An additional 0.2-0.3 km 3 of volcanic material was emplaced by pyroclastic flows, blasts, and ash fall. Part of this vast quantity of volcaniclastic sediment has been subsequently eroded by runoff and streamflow. This brief report summarizes the changes in sediment yield at five locations around Mount St. Helens in the first 13 years following the May 18, 1980 eruption.

Washington↗
USGS1998

Rotational and accretionary evolution of the Klamath Mountains, California and Oregon, from Devonian to present time

The purpose of this report is to show graphically how the Klamath Mountains grew from a relatively small nucleus in Early Devonian time to its present size while rotating clockwise approximately 110°. This growth occurred by the addition of large tectonic slices of oceanic lithosphere, volcanic arcs, and melange during a sequence of accretionary episodes. The Klamath Mountains province consists of eight lithotectonoic units called terranes, some of which are divided into subterranes. The Eastern Klamath terrane, which was the early Paleozoic nucleus of the province, is divided into the Yreka, Trinity, and Redding subterranes. Through tectonic plate motion, usually involving subduction, the other terranes joined the early Paleozoic nucleus during seven accretionary episodes ranging in age from Early Devonian to Late Jurassic. The active terrane suture is shown for each episode by a bold black line. Much of the western boundary of the Klamath Mountains is marked by the South Fork and correlative faults along which the Klamath terranes overrode the Coast Range rocks during an eighth accretionary episode, forming the South Fork Mountain Schist in Early Cretaceous time.

California, Oregon↗