Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How do geologists obtain an isotopic date? What does the age of an igneous rock mean? What does the age of a metamorphic rock mean? (E)

Short answer

Geologists obtain an isotopic date by measuring the abundance of certain isotopes in rock samples and comparing it to the abundance of other isotopes, based on radioactive decay rates. The age of an igneous rock refers to the time elapsed since the rock solidified from magma, representing the cooling and solidification process. The age of a metamorphic rock represents the time elapsed since the rock underwent its most recent metamorphic event, involving significant changes in temperature, pressure, or both.

Step-by-step solution

Explain isotopic dating

Isotopic dating is a technique used by geologists to determine the age of rocks. This method involves measuring the abundance of certain isotopes (different forms of an element) in the rock and comparing it to the abundance of other isotopes present. This comparison can be used to calculate the age of the rock, based on the radioactive decay rates of the isotopes involved.

Discuss obtaining an isotopic date

To obtain an isotopic date, geologists first take samples from the rock they are studying. Then, they use specialized equipment in a laboratory to analyze the concentrations of various isotopes in the samples. One particularly useful isotope for dating igneous and metamorphic rocks is uranium-238 which decays to lead-206 over time. By measuring the ratio of uranium-238 to lead-206, geologists can determine the age of the rock, since the rate of decay is known.

Explain the meaning of the age of an igneous rock

The age of an igneous rock refers to the time elapsed since the rock solidified from molten material or magma. When magma cools and crystallizes, elements within the molten rock are incorporated into mineral structures, and some of these elements have radioactive isotopes. As soon as the rock solidifies, the radioactive isotopes within it begin to decay at a consistent rate. So, the age of an igneous rock represents the time it took for the magma to cool and solidify, and the time elapsed since that moment.

Explain the meaning of the age of a metamorphic rock

The age of a metamorphic rock represents the time elapsed since the rock underwent its most recent metamorphic event. Metamorphic events involve significant changes in temperature, pressure, or both, often due to the burial of the rock in the Earth's crust. During these events, the chemical composition and structure of the rock can be altered, and new minerals may form, some of which contain radioactive isotopes. As with igneous rocks, the decay of these isotopes allows geologists to determine the age of the metamorphic event. However, it's important to note that this age does not necessarily represent the original formation of the rock, as it could have undergone multiple metamorphic events throughout its history.

Key concepts

Understanding Igneous Rock Age

The age of igneous rock is key to understanding the geological history of our planet. Igneous rocks form when molten material, known as magma, cools and crystallizes. This process traps various elements, including radioactive ones, within the minerals of the rock.

Since the radioactive isotopes decay at a predictable rate, known as a half-life, the age of the igneous rock corresponds to the time it has been since the rock solidified from magma. This 'clock' starts ticking as soon as the rock cools enough for the minerals to form, freezing the isotopic 'starting material' in place. So, when a geologist talks about the age of an igneous rock, they are referring to the time elapsed since these rocks cooled and solidified beneath or on the Earth's surface.

Metamorphic Rock Age Demystified

Metamorphic rocks are the transformers of the geological world. They originate from pre-existing rocks that have undergone profound changes due to high temperature, high pressure, or both, deep within the Earth's crust. The age of a metamorphic rock does not signify its original formation. Instead, it marks the moment of its most recent metamorphosis.

This age is determined by analyzing radioactive elements that were produced or reconfigured during the metamorphic event. These changes reset our isotopic clock. Consequently, the isotopic date obtained for a metamorphic rock pinpoints the timing of the conditions that led to its current state, rather than the age of the original parent rock.

Radioactive Decay Explained

Radioactive decay is a fundamental concept central to isotopic dating. It is the process by which an unstable atomic nucleus loses energy by emitting radiation. Over time, this results in the transformation of a parent isotope into a stable daughter isotope at a rate that's characteristic for each element, known as the half-life.

The half-life is the amount of time required for half of the quantity of a radioactive isotope to decay. This predictable rate allows scientists to use isotopes as natural clocks. By measuring the ratio of parent to daughter isotopes in a rock sample and knowing the half-life, geologists can calculate the age of the rock. This is because as the parent isotopes decay over time, they produce a proportional amount of daughter isotopes, providing a time-stamped chemical signature.

The Role of Uranium-Lead Dating

Uranium-lead dating is a premier method for determining the ages of geological materials. This technique relies on the decay of the radioactive isotope uranium-238 into the stable isotope lead-206.

The process has an impressive half-life of approximately 4.5 billion years, making it particularly suitable for dating ancient rocks. Scientists measure the ratio of uranium-238 to lead-206 in a rock to estimate how long the decay process has been occurring. Since this ratio changes at a constant and known rate, the analysis yields a precise age for the igneous or metamorphic rock in question. The accuracy of uranium-lead dating makes it an invaluable tool for piecing together the Earth's formative episodes and the geological timeline.