The rate at which this occurs depends only on the number of atoms around, so it follows exactly the same function as that described above. We can plot the numbers of parent and daughter atoms as we did for unpopped and popped kernels of popcorn:. The decay constant of a particular parent can be measured in the laboratory by counting the number of times particles decay per second.
If the decay constant of the parent is known, the age of a particular rock sample can be determined by comparing the ratio of parent to child, assuming there was no child in the sample to begin with, and none has been lost in the mean time.
Decay rate is related to the half-life as you saw above. All radioactive elements decay in the same way, just some take a long time and some decay very rapidly.
For a material to be useful to geologists, it has to have a half-life on the order of geologic processes and be around. Here is a list of commonly used isotopes and their half-lives:. Because of the requirement that no child product be incorporated in the material to begin with, the minerals that are favored separate parent from child. Igneous rocks do this pretty well by excluding gases like argon and separating rubidium from strontium these partition into different minerals during crystallization.
Dating minerals in sediments generally will give you the age when the mineral formed - not the sedimentary rock, so geologists favor igneous rocks for dating purposes.
Most of the isotopes used for dating were made billions of years ago in a super-nova explosion, like the rest of the stuff we are made of. However, please observe the short half-life of for example, carbon This could not possibly have survived from before the birth of the Earth and in fact is made in the upper atmosphere by bombardment of cosmic rays.
Carbon is also different in that it is incorporated into organic material. It is used for dating things like trees, fires, cloth, soils, corals, etc.
Over the last few decades, geologists have found datable material in lots of useful places. We now know pretty well how long various parts of the rock cycle take. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland Figure Based on figure 15, at least nine geological events can be inferred:.
Hutton realized that an enormous period of time was needed to account for the repeated episodes of deposition, rock formation, uplift, and erosion that led to the formation of an unconformity, like the one at Siccar Point.
Hutton realized that the age of Earth should not be measured in thousands of years, but millions of years. Superposition and cross-cutting are helpful when rocks are touching one another, but are useless when rocks are kilometers or even continents apart. Three kinds of clues help geologists match rock layers across great distances.
The first is the fact that some sedimentary rock formations span vast distances, recognizable across large regions. A second clue could be the presence of a key bed , or a particularly distinctive layer of rock that can be recognized across a large area.
Volcanic ash flows are often useful as key beds because they are widespread and easy to identify. Probably the most famous example of a key bed is a layer of clay found at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct Figure This thin layer of sediment, only a few centimeters thick, contains a high concentration of the element iridium.
Iridium is rare on Earth but common in asteroids. In , a team of scientists led by Luis Alvarez and his son Walter proposed that a huge asteroid struck Earth about 66 million years ago, causing forest fires, acid rain, and climate change that wiped out the dinosaurs.
A third type of clue that helps scientists compare different rock layers is index fossils. Recall that index fossils are the remains of organisms that were widespread but only existed for a relatively short period of time.
If two rock units both contain the same type of index fossil, their age is probably very similar. As scientists collected fossils from all over the world, they recognized that rocks of different ages contain distinctive types of fossils. Each era, period, and epoch of the geologic time scale is defined by the fossils that appeared at that time. For example, Paleozoic rocks typically contain trilobites, brachiopods, and crinoid fossils. The presence of dinosaur bones indicate that a rock is from the Mesozoic era, and the particular type of dinosaur will allow the rock to be identified as Triassic, Jurassic, or Cretaceous.
The Cenozoic Era is also known as the Age of Mammals, and the Quaternary Period represents the time when the first humans spread across Earth. Nicholas Steno first formulated the principles that allow scientists to determine the relative ages of rocks in the 17th century.
Steno stated that sedimentary rocks are formed in continuous, horizontal layers, with younger layers on top of older layers. A century later, James Hutton discovered the law of cross-cutting relationships: a fault or igneous intrusion is younger than the rocks that it cuts through. Hutton also was the first to realize the vast amounts of time that would be needed to create an unconformity, a place where sedimentary rocks lie above an eroded surface.
Other methods come into play when comparing rock layers that are separated by a large distance. Many sedimentary rock formations are large and can be recognized across a region. Distinctive rock layers, called key beds, are also useful for correlating rock units. Article Navigation. Research Article October 01, Google Scholar.
GSA Bulletin 63 10 : — Article history received:. Abstract The age of igneous rocks is determinable by a method based on the included accessory minerals. This content is PDF only. Please click on the PDF icon to access. First Page Preview. Close Modal. You do not currently have access to this article. You could not be signed in. Librarian Administrator Sign In. Buy This Article.
0コメント