Lava Flows

Lava flows come in a variety of styles depending upon the composition of erupted material. Two major categories of lava flow in Colorado areas basalt (magnesium and iron rich) flows and rhyolite (silica rich) flows.

Distribution of late Tertiary basalt flows

Distribution of late Tertiary basalt flows.

Cinder Cone Flows

A cinder cone or scoria cone is a steep, straight-sided conical hill of pyroclastic material called tephra (volcanic debris) that accumulates around and downwind from a volcanic vent.[1] Cinder cones range in size from tens of meters to 300 meters tall.[1] The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles “frozen” into place as magma exploded into the air and then cooled quickly.[1] Many cinder cones have a bowl-shaped crater at the summit.During the waning stage of a cinder-cone eruption, the magma has lost most of its gas content. This gas-depleted magma does not fountain, but oozes quietly into the crater or beneath the base of the cone as lava.[2] Lava rarely issues from the top (except as a fountain) because the loose, un-cemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent.[1] Because it contains so few gas bubbles, the molten lava is denser than the bubble-rich cinders.[2] Thus, it often burrows out along the bottom of the cinder cone, lifting the less-dense cinders like a cork on water, and advances outward, creating a lava flow around the cone’s base.[2] When the eruption ends, a symmetrical cone of cinders sits at the center of a surrounding pad of lava.[2] However, if the crater is fully breached, the remaining walls form an amphitheater or horseshoe shape around the vent.

Cinder cones are commonly found on the flanks of shield volcanoes, stratovolcanoes, and calderas.[1] Colorado’s youngest cinder cone volcano is located near Dotsero, Colorado and has been dated to be about 4,150 +/- 300 years old. More information is available in the Colorado Geological Survey Open File Report OF-08-14, Geologic Map of the Dotsero Quadrangle, Eagle and Garfield Counties, Colorado report.

Dotsero cinder cone and flow

The small dark-colored (Dotsero) cinder cone is near the top middle (north end) of the photo and the lava flow extends southerly.

cinders in Dotsero crater

Cinder deposition can be seen in the crater wall near Dotsero, Colorado.

References:

  1. [1] Public domain material from the United States Geological Survey document: “Photo glossary of volcano terms: Cinder cone”.
  2. [2] Public domain material from the United States Geological Survey document: Susan S. Priest, Wendell A. Duffield, Nancy R. Riggs, Brian Poturalski, and Karen Malis-Clark (2002). “Red Mountain Volcano—A Spectacular and Unusual Cinder Cone in Northern Arizona”. USGS Fact Sheet 024-02.

Flood Basalts

A flood basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalt provinces are often called traps, which derives from the characteristic stair-step geomorphology of many associated landscapes. Eleven distinct flood basalt episodes occurred in the past 250 million years, resulting in large volcanic provinces, creating plateaus and mountain ranges on Earth.[1] The Columbia River Plateau of western North America is the nearest to Colorado and occupiesWashington, Oregon, Idaho, Nevada, and California.

During the middle to late Miocene epoch, the Columbia River flood basalts engulfed about 163,700 km2 (63,200 sq mi) of the Pacific Northwest, forming a large igneous province with an estimated volume of 174,300 km3 (41,800 cu mi). Eruptions were most vigorous from 17–14 million years ago, when over 99 percent of the basalt was released. Less extensive eruptions continued from 14–6 million years ago.[2]

References:

  1. [1] Rampino,Michael R. and Stothers, Richard B. (1988). “Flood Basalt Volcanism During the Past 250 Million Years”. Science 241 (4866): 663–668. doi:10.1126/science.241.4866.663. PMID 17839077
  2. [2] Carson, Robert J. and Pogue, Kevin R. (1996). Flood Basalts and Glacier Floods: Roadside Geology of Parts of Walla Walla, Franklin, and Columbia Counties, Washington, Washington State Department of Natural Resources (Washington Division of Geology and Earth Resources Information Circular 90),p.2.