Two Kinds of Volcanic Rock
Posted by: 
Volcanoguy
N 44° 11.220 W 111° 19.702
12T E 473756 N 4892695
Sign about the geologic history of the Upper Mesa Falls area.
Waymark Code: WMC89F
Location: Idaho, United States
Date Posted: 08/07/2011
Views: 5
This geologic history sign along walkways leading to Upper Mesa Falls and deals with the geology of the Upper Mesa Falls.
Marker Name: Two Kinds of Volcanic Rock
Marker Type: Roadside
Marker Text:
Two different kinds of rock can be seen from this viewpoint. Both types of rock were formed during eruptions that created the major landscape features of Park and the Yellowstone Plateau.
Tuff
The tuff, seen on the far side of the canyon, is a rock composed of volcanic ash and other materials expelled in violent eruptions. The hottest and densest ash flowed almost like molten rock. 1.3 million years ago, the Mesa Falls Tuff welded into a hard rock layer as it cooled. This hardened rock forms the ledge over which the Henrys Fork flows, creating Mesa Falls.
Basalt
Basalt, which forms the vertical cliffs on this side of the canyon, erupted in molten form approximately 200,000 years ago.
The nearly straight columns of rock you see along the canyon walls are known as columnar jointing. As the basalt flow cooled, the rock crystallized forming hexagonal patterns. The cooled rock shrank and fractured along parallel joint lines, creating six-sided rock columns. Freezing and thawing continue to accentuate these forms.
County: Fremont
Group Responsible for Placement: U.S. Forest Service
City: Not listed
Date Dedicated: Not listed
Marker Number: Not listed
Web link(s) for additional information: Not listed
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Visit Instructions:
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If possible please post a photo of you OR your GPS at the marker location.
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