Granite and Plutons

I have been avoiding dealing too much with absolute age in these early posts because geology is about looking at the earth and seeing differences in the rocks, soils, plants, and (yes) even weather. What does all of this have to do with a pluton/batholith?

Sediments are deposited in more-or-less horizontal form and thus we see younger over older. Intrusive rocks do not obey this rule (Steno’s Law) but they can appear anytime. However, unlike sediments, which are generally not directly datable using a proven scientific method, we can date various unstable radioactive isotopes in igneous rock because they melted and the minerals (mostly) participated in this operation.

I am not going to talk about radioactive dating but it allows a quantitative date to be assigned to igneous and metamorphic rocks under most circumstances. Plutons are emplaced from the lower crust and thus Steno’s Law does not apply.

When we go off road, we see a LOT of granite, which is an intrusive rock. Granite forms really nice gravel roads and it is quite popular in CO where granite allows a reduced road paving budget.

Here is a textbook example of a granite pluton, Mount Rushmore in the Black Hills of S. Dakota. The first image shows how it looks from space and the second shows colored geology over a relief map.

The granitic rock is the oval covered with trees because of its higher elevation. This is the Harney Granite (~1.6 billion). It can be seen in some close-up images that show how it melts the rock it is intruded into the surrounding sedimentary rocks at our family web page. This example is small enough to be viewed in its entirety from Harney Peak.

This beautiful pluton has had all of the overlying softer limestone eroded. It is also a great hike to the top at about 7300 feet.