Introduction to the Precambrian
Some of the oldest rocks found anywhere in the world occur in the Beartooth Mountains. Rocks as old as 3.96 billion years (3,960 million years or 3,960,000,000 years) have been determined by zircon dating of a quartzite sample from Hell Roaring Plateau. These quartzite rocks were derived from preexisting igneous rocks that must have been even older than this date. The age of the earth is generally accepted as being 4.6 billion years. Therefore these Beartooth rocks represent some of the first rocks formed after the initially molten earth cooled. The “Precambrian” is the period of time from the origin of the earth (4.6 billion years ago) to the beginning of the Cambrian Period (see legend).
Grizzly Peak stands at an elevation of 9,416 feet. It forms the highest part of the Red Lodge Ski area. Its name is derived from the occasional grizzly bear that was sighted in its foraging out of the Yellowstone area. A grizzly sighting, although quite rare, does still occur in this region. Most such reports turn out to be brown-colored black bears.
Isotopic Age Dating Techniques
There are a number of elements whose nuclei spontaneously emit particles that change in form and produce new elements. This process is known as radioactivity. Early in the 20th century it was discovered that minerals containing radioactive isotopes could be used to determine the age of the minerals contained in some rocks. We are most familiar with radio-carbon dating. The carbon-bearing material may be wood, bone, ivory, or charcoal, for example. This technique has proven very useful for items no older than about 60,000 years.
Radioactive decay occurs when one element (the parent isotope) changes to another element (the daughter isotope) at a measurable rate. We call this rate the half-life of that decay process. That is the time required for half of the nuclei in a sample of an element to decay to its daughter isotope. After one “half-life” there will be found in the sample only one-half the amount of the original parent isotope and there will also be one-half the amount of the newly formed daughter isotope. After another “half-life” the ratio will be one-quarter parent isotope and three-quarters daughter isotope. After the third “half-life” the ratio will be 1/8 parent isotope and 7/8 daughter isotope. This process continues until there is so little of the original parent isotope that it becomes impossible to detect by current techniques.
The half-life of uranium-238 is 4,510,000,000 years (or 4.51 x 1010 years). When uranium-238 decays lead-206 is formed. Therefore the ratio of uranium-238 to lead-206 in a sample depends on the length of time decay has been going on. By measuring this ratio, we can determine what is called the absolute age of that rock. (The term “absolute” differentiates that age from the relative age of a rock or structure, which is determined merely by determining that one rock or structure is simply older or younger than another such rock or structure.) There are a dozen or so such radioactive isotopes that have been discovered and used for age dating. It should be noted that when several different age dating techniques have been used on a single rock sample, there is amazing correlation between such ages. This suggests that many such radioactive dates are valid estimates of the actual rock age.
variety of rocks found in the Beartooth Mountains are Precambrian in age. Here
on Grizzly Peak are some granites, granite gneisses and schists. Elsewhere occur
these same rock types as well as granodiorites, migmatites, gabbros, basalts,
ultramafics, and amphibolites. All these igneous rock names come from the fact
that they contain varying amounts of the major rock forming minerals – quartz
and feldspars (plagioclase and potassium feldspar), iron-magnesium minerals such
as biotite, amphiboles, and pyroxenes. One such classification of igneous rocks
is illustrated here.
Studies of the Precambrian “basement” rocks of the Beartooth Mountains have revealed several episodes of igneous intrusion of dikes cutting the older rocks. The earliest intrusive event appears to have been approximately 3600 to 3700 million years ago. A major mountain-building episode, called the “Beartooth Orogeny” occurred about 2800million years ago, with intrusion of the Long Lake granite and granodiorite and associated metamorphism of the adjacent rocks. This was the first in a series of four age groupings of such dikes – 2800-2500 m.y., 2200-2100 m.y., 1300 m.y. and 740 m.y. These dikes occupy fracture sets with a variety of orientations. The 2800-2500 m.y. old group shows the most diverse set of orientations. The 2200-2100 m.y. group are all generally oriented north-south. The 1300 m.y. old set is oriented 30 degrees west of north. The 740 m.y. old set is oriented 75 degrees west of north.