Bell Mountain Rocks!
By Dinah Shumway
February 5, 2006
Conical hills and mountains are not unusual in the geological landscape, but Bell Mountain in north Apple Valley is an outstanding example of the conical form. Many residents and visitors to the High Desert have no trouble in understanding how Bell Mountain got its name as it retains its conical form from whichever direction it is viewed. The volcanic shape of Bell Mountain is a result of many geological events and displays evidence of past geological environments of our high desert.
Bell Mountain looks like a volcano but is actually an erosional remnant. Bell Mountain consists primarily of granitic rocks of about middle Mesozoic age (200 million or so years before present). The mountain is capped however by older and more resistant Sidewinder Volcanics (also middle Mesozoic in age). It is these more resistant volcanic rocks that allow Bell Mountain to develop the familiar volcano-like shape.
The volcanic rocks that cap Bell Mountain are not the rocks of a volcanic neck but the rocks of broad volcanic extrusive products like andesitic “flows” or welded tuffs. These volcanic rocks were later intruded by the granitic rocks that make up most of Bell Mountain. When the surrounding rocks eroded away, the hard resistant volcanic andesitic rocks kept the top of Bell Mountain from eroding, thus promoting the familiar pointy “bell” shape we see today.
Andesites are volcanic rocks that are lighter in color than their darker brothers, the basalts. Andesites are usually associated with volcanic terrains that are located over subduction zones, or places on the earth where the tectonic plates are diving under other plates. Japan is the classic location of andesitic volcanic rocks where the Philippine and Pacific plates are diving under the Eurasian plate and where Japan has formed as islands over the subduction zone. Subduction zone volcanic chains produce andesitic flows and volcanic ash that metamorphoses into very hard rocks.
Unlike volcanic rocks which do not display individual crystals, granitic rocks are composed of several different types of crystals and consequently are less resistant than the harder andesites. The granitic rocks at Bell Mountain weather readily, but the cap of volcanic rocks protects the top and forms the volcano-like erosional remnant.
So, although Bell Mountain is not really a volcano, the rocks there do help to tell part of the geological tale about volcanism: about 200 million years ago there was a chain volcanoes located off the western shore of the continent. These volcanoes were located over a subduction zone where oceanic crust was diving east under the American plate over a subduction zone. Andesitic flows and ash were distributed over a wide area. Later these volcanic rocks were intruded by slightly younger and less resistant granitic rocks.
Photo Caption: Bell Mountain looking southwest from Dale Evans Parkway.