geologictimepics

Geology and Geologic Time through Photographs

Archive for the tag “Sierra Nevada”

Cretaceous batholiths and roof pendants

The photos from the last posting were from the Sierra Nevada Batholith –called a “batholith” because it consists of many many smaller intrusive bodies that collectively define a much larger intrusive complex that doesn’t even have a well-defined root.  As it turns out, the Sierra Nevada are one of several really large batholiths that intruded the crust of the Pacific Margin during the Cretaceous Period, about 80-100 million years ago.

Granitic Batholiths of Cretaceous age in western North America.

And along the east side of the Sierra Nevada, we can see the original rock into which the granite of the Sierra Nevada intruded.  This original rock consists of older sedimentary and volcanic rock–that dates from the Cambrian Period through the Jurassic– much of which was metamorphosed by the heat from the intruding granite.  The photo below shows the Cretaceous granite below (light colored rock) and the dark-colored sedimentary (now metamorphic) rock above.  These older rocks that are intruded by the granite are called “roof pendants” because they show the roof of the batholith.

Cretaceous granite intruding Cambrian metasedimentary rock, Sierra Nevada Range.

And as far as geologic time goes, this photo shows us that the granite, discussed in previous posts, is younger than the sedimentary rock that overlies it.

And click here to see a photo of glaciated granite in Yosemite National Park.

 

Granite

That’s actually the moon at the end of the crack in this rock…

granite and moon, Sierra Nevada, California.

A typical exposure of granite --coarse grained with an interlocking, random assortment of crystals. Click here to search for geology pictures by keyword.

And the rock is a pretty typical example of granodiorite… which is a lot like granite, except it has a little less silica.  See yesterday’s post about igneous rocks if you’re interested.

It turns out that most of the Sierra Nevada Range in California, including Mt. Whitney (the conterminous US’s highest peak) is made out of granodiorite.  And if you consider that most of the magma cooled and crystallized at a depth of 10km, and now resides about 4km ABOVE sea level, we’re looking at millions of years to accomplish this uplift.

Here’s Mt. Whitney at sunrise… It’s the peak just left-of center.  From this view, you can see that the rock of this part of the Sierra Nevada Range is all pretty much the same: granodiorite.

Mt. Whitney and Sierra Nevada, California at sunrise. Mt. Whitney's elevation is 14, 505' above sea level, the highest spot in the conterminous US. The rock in this photograph is almost entirely granodiorite.

Post Navigation

<span>%d</span> bloggers like this: