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by Sandy Eldredge
An average of 270 inquiries about dome mountains
reach our Web site every month. Utah teachers who
address landforms often identify mountain types
formed by different processes. Up until the 1990s,
some curricula materials listed four types of mountains
(although there are more): fold, fault, volcanic,
and dome.
Teachers often defined a dome mountain
as forming from rising magma that pushed the
overlying rock layers upward to form a dome shape,
without the magma breaking through the surface.
However, geologists have a broader dome-mountain
definition that includes any region of flat-lying sedimentary
rocks warped upward to form a roughly circular
shape, as well as accumulations of lava that pile
up over a volcanic vent.
Adding to the confusion is
that many mountains that do not fit the formal definition
above are called dome mountains just because
of their rounded shapes. These mountains should be
referred to as just "dome-shaped" mountains.
To help unravel some of the ambiguities, let's take
a look at several dome features and how they are
formed.
What is a dome?
A dome is a circular or elliptical uplifted geologic feature
on which the rock layers slope gently downward
in all directions from a central high point. Generally
the term is used for any dome-shaped landform.
What are some of the dome structures?
Lava domes, salt domes, salt diapirs, dome mountains,
and laccoliths are several of many dome
features.
Southeastern Utah's laccoliths are the La Sal, Henry,
Abajo, and Navajo Mountains that rise above the
flat sedimentary rock expanses. The La Sals, Henrys,
and Abajos have aggregations of peaks composed
of the igneous rock now exposed after the overlying
sedimentary rock layers were eroded off. Navajo
Mountain, a solitary dome mountain, is different in
that the sedimentary rocks still cover the probable
underlying igneous intrusion.
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LAVA DOME
Lava (volcanic) domes are rounded, steepsided mounds built by the accumulation of viscous lava that typically does not move far from the volcanic vent before cooling and crystallizing.
 Elden Mountain is a lava dome in the San Francisco
volcanic field in Arizona. Photo courtesy of
U.S. Geological Survey.
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SALT DOME
Salt domes are formed
by upward-flowing salt
that warps the overlying
rock layers. Salt domes
are common along the
Gulf Coast where salt
beds are covered by a
thick sequence of sedimentary
rocks.
 The weight of the overlying rock layers forces the
salt to flow upward.
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SALT DIAPIRS
Salt diapirs are “pierced domes,”
where the overlying uplifted
rocks have been pierced or ruptured
by the rising salt core. A
Utah example is the Onion Creek
salt diapir (see the “GeoSights”
article in this Survey Notes issue).
Diapirs can also be formed by
flowing shale or magma.
The salt core breaks through the overlying rocks, and possibly the surface, creating a
salt diapir. |
DOME MOUNTAIN
Dome mountains form in
areas of flat-lying sedimentary
rocks that are warped up
to create a roughly circular
shape. A Utah example is
Navajo Mountain, which most
likely has a core of igneous
rock that arched up the overlying
sedimentary rock layers.
Navajo Mountain in San Juan County is a
dome mountain that was probably formed
by a laccolith. |
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LACCOLITH
Laccoliths are large bodies
of igneous rock that
were injected as magma
between sedimentary
rock layers, arching the
overlying layers into a
domelike form while
leaving the rock layers
below relatively flat.
Example of a laccolithic intrusion arching overlying
sedimentary rock layers that still cover the igneous
rock (similar to Navajo Mountain). |
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