Two Kinds of Volcanic Rock
Posted by: Groundspeak Premium Member Volcanoguy
N 44° 11.220 W 111° 19.702
12T E 473756 N 4892695
Quick Description: Sign about the geologic history of the Upper Mesa Falls area.
Location: Idaho, United States
Date Posted: 8/7/2011 4:28:20 PM
Waymark Code: WMC89F
Published By: Groundspeak Premium Member muddawber
Views: 3

Long Description:
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

Visit Instructions:
In your log, please say if you learned something new, and if you took any extra time to explore the area once you stopped at the historic marker waymark. If possible please post a photo of you OR your GPS at the marker location. Also if you know of any additional links not already mentioned about this bit of Idaho history please include that in your log.

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Recent Visits/Logs:
Date Logged Log User Rating  
Volcanoguy visited Two Kinds of Volcanic Rock 9/24/2010 Volcanoguy visited it