The
coordinates take you to a turnout along Kolob Road. To the north is
Firepit Knoll and to the southwest is Spendlove Knoll. These are
two cinder cones that are approximately 200,000 years old. These
cinder cones are thought to be the source of the Grapevine Wash
basalt flow.
Kolob Road to the south is built on this flow. Cinder cones
often are formed near the end of an eruption. Any cones that are
created early in the eruption are likely to be destroyed by a later
episode in the eruption.
Cinder cones are formed from cooled chunks of lava
ejected from a single central vent. Gas-filled lava is blown into
the air where it breaks apart into small fragments and quickly
cools creating cinders. These cinders are typically air filled and
relatively light for rocks of their size. The cinders usually build
up around the central vent to create a circular or oval cone with a
crater at the summit.
Image Source:
http://pubs.usgs.gov/gip/volc/types.html
Basalt is one of the four major categories of volcanic rocks. It
has the least amount of silica of the four types of volcanic rocks.
(Silica is a common compound found in the earth’s crust).
Basalt also tends to be the hottest and most fluid of the volcanic
rocks. See
http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html for a
further description and graphics regarding types of volcanic
rocks.
Logging requirements:
Send me a note with :
- The text "GCZ5Z6 Firepit and Spendlove Knolls" on the first
line
- The number of people in your group.
- Comparing the current shape of the two cinder cones to the
idealized diagram presented in the discussion.
- Account for the similarities and differences. .
- Optional: Please post a picture of yourself or another member
of your party with one of the 'knolls' in the background.
The above information was compiled from the
following sources:
- Hamilton, Wayne L. 1987,
Geological Map of Zion National Park, Utah Miek, Robert F., et.
al.,
- Geology of Zion National Park, Utah in Geology
of Utah’s Parks and Monuments, 2003 Utah Geological
Association Publication 28 (second edition) D.A. Sprinkel, T.C.
Chidsey, Jr. and P.B. Anderson, editors
- USGS, Principal Types of Volcanoes,
http://pubs.usgs.gov/gip/volc/types.html
- USGS, Types of Igneous Rocks, Volcano Hazards
Program – Reducing volcanic risk,
http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html
Placement approved by the
Zion National Park