In this series of articles, Utah artist J. Brad Holt talks about what artists are seeing as they look at the landscape. Holt studied geology in college and is attentive to what the rocks suggest in the scenes he paints.
Lead Image: Brad Holt painting at Cedar Breaks National Monument in July
Geologists sometimes talk as though you already know at least half of what’s going on. I’ve realized as I’ve glanced back through some of these articles that it might be good to double back and clarify some terms and methodologies. The lead image shows a painting I recently did at Cedar Breaks National Monument. The rock layers of Cedar Breaks are composed of colorful lake-deposited limestones, called the Wasatch or Claron Formation. They were laid down in the Miocene Epoch, roughly 15 million years ago. This is considered young in geological terms. But how do we know the age of these, or of any other rocks?
Time and dating in geology are of two types: relative dating and numerical dating. Relative dating is simply putting in context the sequence of formation and events with regard to one another. It tells us that formation A is older than formation B, but not when either happened on the calendar. Relative dating is done through a series of laws and principles. The law of superposition applies to underformed sedimentary layers and states that the layer below will be older than the layer above. This rule also applies to surface-deposited extrusive igneous rocks, such as basalt flows and ash deposits.
The principle of original horizontality means that sedimentary rocks are horizontal when they are laid down. Any tilting or deformation of layers must happen long after the formation has been deposited. The principle of cross-cutting relationships shows that anything that intrudes, or breaks across a layer, must be younger than the layer it disrupts. This is simple in theory, but it can become very complex in the field. The principle applies to intrusive igneous dikes, sills, laccoliths, and batholiths, as well as faults and folds. Often intruding structures will have inclusions of the rock into which they intrude. Obviously, inclusions will be of greater age than the rock they are trapped in.
Throughout the history of the earth, there have been interruptions in the orderly accumulation of sedimentary layers. These interruptions leave an imprint on the rock record, and they are called unconformities. There are three types of unconformity. An angular unconformity is where a sequence of sedimentary layers has been tilted, then eroded, then newer layers are laid down on the surface. The unconformity is easy to see, as the older tilted layers will meet the younger horizontal layers at an angle.
It was a particularly conspicuous angular unconformity along the coast of Scotland that led James Hutton to the musings that would ultimately result in the modern science of geology. A disconformity is common, but much harder to spot, as the newer rock is laid down on older horizontal layers. Usually a disconformity represents a lesser period of erosion, but not always. There may be many millions of years of missing time across a layer that appears to be nothing more than a regular bedding plane.
Nonconformities separate older igneous and metamorphic rock from younger sedimentary strata. Nonconformities are easy to see in most cases, but sometimes care must be taken to be sure that a given nonconformity is not an example of an igneous intrusion. An intrusion, even a very thick sill that has intruded between horizontal planes, will have inclusions of the country rock. In a nonconformity, the sedimentary layer just above the break will have inclusions of the eroded igneous or metamorphic rock below.
Sedimentary layers do not continue laterally forever. Depositional basins were not uniform. Over distances, formations change in thickness and character. This is called facies change. Eventually formations tend to peter out, and other formations begin. How do earth scientists correlate rock of similar age across a continent, or between continents? The answer is through the study of index fossils. Index fossils are organisms that were widespread geographically, but were limited in the length of time the species endured.
The principle of fossil succession states that fossil organisms succeed one another in a determinable order, making it possible to recognize any time period by its fossil content. In addition, fossils provide important clues to what was going on geologically, and the nature of the environment in which they lived.
Numerical dates are obtained through the study of radioactive decay ratios in the sample. Every element has an atomic number and a mass number. The atomic number is determined by the number of protons in the nucleus. Change the atomic number, and you change the element. The addition of the number of neutrons in the nucleus yields the mass number.
The mass number of an element can vary up or down a few neutrons. These variations in an element are called isotopes. The nuclei in certain isotopes are unstable, and they want to break apart, or decay. This process is called radioactivity. There are three primary types of radioactive decay: alpha emissions, beta emissions, and electron capture. An alpha particle is composed of two protons and two neutrons, so when this is emitted, the atomic number is reduced by two, and the mass number by four. In other words, it is now a different element.
Beta particle decay is where the unstable nucleus sheds an electron. Because the composition of a neutron is actually one proton and one electron held together, the loss of an electron results in the default addition of a proton to the nucleus, raising the atomic number by one. Sometimes a nucleus captures an electron, which will combine with one of the protons to create a new neutron, reducing the atomic number by one, and leaving the mass unchanged. The original unstable isotope is called the parent. The isotopes resulting from decay are called the daughter products. The amount of time that it takes for half of the sample to be reduced to daughter products is called the half-life. When the half-life of an isotope is known, and the ratio of parent to daughter products is measurable, a numerical date for the sample can be obtained.
The isotopes of choice for dating rock samples have been the decay of potassium-40 into argon-40. This is because potassium is a component of feldspars, and is present in almost every rock sample. K-40 has a half-life of 1.3 billion years. Because argon is an inert gas, it will not readily combine to form new compounds. The drawback is that argon may easily escape, spoiling the ratio. More recently, scientists have found that if they bombard a potassium sample with a stream of neutrons, it will decay into the unstable daughter product argon-39. Next, a mass spectrometer is used to measure the amount of Ar-39 against the amount of stable Ar-40 in the sample, which has accumulated there over the millennia in the process of natural decay. The size of the sample can be a single grain of potassium from deep inside a crystal. This argon-argon method yields much more precise dates, with much less chance of contamination.
Using radiometric dating methods, earth scientists have been able to determine that the oldest formation on earth is a metamorphic gneiss found in the area of Great Slave Lake in Northern Canada. Rocks nearly as old have been found in Michigan, Minnesota, Greenland, South Africa, and Western Australia. Radiometric dating only tells us when a crystal was formed, making it valuable for assigning ages to igneous and metamorphic formations. We can measure the age of individual grains in a sedimentary rock, but that does not tell us when that sediment was laid down; it gives us the age of the original rock that the grain eroded out of. Zircon crystals yielding a date of 4.3 billion years old have been found in younger sedimentary rocks in Western Australia.