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All the United States California Basalt Plug

Basalt Plug

This large chunk of dark rock is a relic from an ancient volcanic eruption.

Mono City, California

Added By
slgwv
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Plug seen looking southeasterly, close-up.   slgwv / Atlas Obscura User
Plug seen looking southeasterly, close-up.   slgwv / Atlas Obscura User
Plug viewed from the road to Sinnamon Cut.   slgwv / Atlas Obscura User
The plug, closer up.   slgwv / Atlas Obscura User
Plug seen looking southeasterly. Mono Lake in the background.   slgwv / Atlas Obscura User
Plug viewed from the road to Sinnamon Cut. This is the easiest access point.   slgwv / Atlas Obscura User
Granite boulder, about a foot across, in the basalt.   slgwv / Atlas Obscura User
Plug from the southwest.   slgwv / Atlas Obscura User
Plug viewed from the south.   slgwv / Atlas Obscura User
Granite chunks in the basalt. Largest pieces are several inches across.   slgwv / Atlas Obscura User
Granite fragments in the basalt. Note the large rounded boulder (about 1 ft across) left of center.   slgwv / Atlas Obscura User
Granite pieces in the basalt.   slgwv / Atlas Obscura User
View of the wall of the plug, showing the included granite fragments.   slgwv / Atlas Obscura User
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About

For a volcanic eruption to occur, magma must make its way up to the surface, which implies there must be conduits through the rock below. Usually these conduits aren't exposed. They're either covered by the erupted lava, or, if erosion has removed the lava, are so small relative to the size of the erupted material that they're likely to have been eroded away too. In any case, they are easily missed. But not far from Mono Lake, there is an unusual case where the "pipe" that fed an eruption at the surface is both exposed and readily recognizable.

This so-called "plug" is composed of dark basalt, which contrasts strikingly with the surrounding light granite. Up close, the basalt can be seen to be full of granite fragments broken out of the surrounding rock. They are typically rounded; not only was the molten basalt above the melting point of the granite but it is out of chemical equilibrium with it, so the edges of the granite fragments were not just melting, but dissolving.

Based on its age and chemical composition, this plug is thought to have been the feeder for basalts that are now exposed along US-395 a couple of miles north of Conway Summit and some 600 feet higher. This height gives an estimate of how much erosion has taken place. The plug may well have fed a cinder cone on the surface, as those are a typical result of small basalt eruptions, but it has long since eroded away. 

Related Tags

Geological Oddities Volcanoes Rock Formations Geology

Know Before You Go

The most straightforward access to the plug is from the east. Turn east on Conway Ranch road, a graded road that intersects US-395 1.8 miles north of California SR-167, and go 1.1 miles to Rattlesnake Gulch road. This intersection is probably unmarked but is at about 38.071065 N, 119.151853 W. Then go north (left) here 1.4 miles to where a ridge comes in from the left (west), at about 38.08789 N, 119.1434 W. At this point you can see the plug as a dark ragged outcrop, about two hundred yards away, contrasting with the nearer light-colored granite.

While in the area, Sinnamon Cut is another 0.6 miles up this road.

Community Contributors

Added By

slgwv

Published

October 14, 2022

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Sources
  • David A. John, Edward A. du Bray, Stephen E. Box, Peter G. Vikre, James J. Rytuba, Robert J. Fleck, and Barry C. Moring, Geologic Map of the Bodie Hills, California and Nevada, USGS Scientific Investigations Map 3318, 2015.
Basalt Plug
Mono City, California, 93541
United States
38.085797, -119.14484

Nearby Places

Sinnamon Cut

Bridgeport, California

miles away

Black Point Fissures by Mono Lake

Lee Vining, California

miles away

May Lundy Mine

Lundy, California

miles away

Explore the Destination Guide

Photo of California

California

United States

Places 1,494
Stories 135

Nearby Places

Sinnamon Cut

Bridgeport, California

miles away

Black Point Fissures by Mono Lake

Lee Vining, California

miles away

May Lundy Mine

Lundy, California

miles away

Explore the Destination Guide

Photo of California

California

United States

Places 1,494
Stories 135

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