BH250-243
Title
Subject
Description
Mineralogy Comments: fiamme
The ignimbrite (BH250-243) exposed along Charlie Brown Road near Shoshone is a great example of a rock formed during an explosive volcanic eruption. It is part of the Miocene Resting Spring Pass Tuff and belongs to a large volcanic event that spread ash and pumice across much of eastern California and Nevada. Later stretching of the Earth’s crust in the Basin and Range Province tilted and exposed these rocks, giving us a rare cross-section through a volcanic deposit.
This rock formed from a pyroclastic density current (also called a nuée ardente), which is a fast-moving, ground-hugging cloud of hot ash, pumice, crystals, and gas. When this material was deposited, it was still very hot, so it compacted and welded together to form solid rock. Because of this, the ignimbrite contains a mix of ash, pumice fragments, and pieces of older rock (lithics) in a matrix that was originally volcanic glass.
One of the most important things to notice at this outcrop is the vertical layering caused by cooling. In the middle is a dark, glassy layer called a vitrophyre (BH250-243b), which formed where the deposit was hottest and most strongly welded. Around it are lighter-colored zones of welded tuff where pumice has been flattened into lens-shaped features called fiamme. Toward the top and bottom, the rock becomes less welded (BH250-243c) and more porous, showing that these parts cooled faster and were not as compressed. This vertical pattern tells us how temperature changed within the flow after it was deposited.
In the field, you can also see that the flattened pumice (fiamme) are aligned, forming a texture called eutaxitic fabric. This gives the rock a streaked or layered look and shows the direction the material flowed and how it compacted. In some places, the alignment suggests the flow moved across a sloping surface rather than settling in a flat area.
The composition of the ignimbrite is silicic (rich in silica), similar to rhyolite or dacite. It contains minerals like quartz, feldspar, and biotite. Much of the original volcanic glass has changed over time into very fine crystals, a process called devitrification.
Even though pyroclastic flows like this can travel very far—sometimes tens to over a hundred kilometers—the features at this outcrop help us estimate how far this deposit is from its source. The strong welding, the presence of the vitrophyre (BH250-243b), the large flattened fiamme, and the thickness of the deposit all suggest that this ignimbrite formed relatively close to the volcanic source (proximal to medial) rather than far away.
We cannot see the original volcano or caldera today because it has been eroded or buried, but these field clues allow us to infer that the source was likely a nearby Miocene volcanic center in the southern Death Valley–eastern California region, probably within a few tens of kilometers.
Overall, this outcrop is an excellent place to learn how geologists use textures, layering, and composition to understand how volcanic rocks form and how far they travel from their source.
Coverage
Nearby Geographic Feature: Death Valley
GPS Coordinates: 35.997303, -116.219606