How To Tell The Age Of Rocks – Minnesota is home to some of the oldest rocks in the world; Sections of the Morton Gneiss in western Minnesota have been dated at 3.5 billion years old. Rocks this old or older are rare on Earth because geological processes on and within our active planet recycle old rocks and produce younger ones.
Only Minnesota, Michigan, northwestern Canada, Greenland, Siberia, South Africa and Australia have preserved rocks older than 3.5 billion years. The oldest mineral grains so far identified on Earth are about 4.4 billion years old; In Australia, they have been found in rocks that represent sediments recycled from even older rocks. Rocks brought by astronauts from the Moon and meteorites that fell to Earth are about 4.5 billion years old. Since the moon, earth, and meteors probably formed at the same time (at the same time as the rest of the solar system), we can conclude that the earth itself is about 4.5 billion years old.
How To Tell The Age Of Rocks
How do we know that morton gneiss is older or younger than other rocks? How do we know the age of a stone? There are two types of questions about geological age: relative age (is this rock older or younger than this other rock?) and numerical age (how many years ago did something form or happen?). Relative age allows geologists to show that a particular unit of rock is older than another unit of rock without knowing how old either is in calendar years. They understand the processes by which rocks are formed and have developed logical rules based on relationships in observed fields to determine the relative ages between rock units.
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Numerical ages are determined by counting a physical event (eg, a calendar year is one revolution of the Earth around the Sun) that occurs at a constant rate. Although we may not be used to thinking about them in this way, calendars and clocks are simply convenient devices for counting orbital revolutions, or the rotation of the Earth. The calibration of human history depends on humans systematically counting and recording orbital revolutions. However, for the vast majority of geologic time, humans were not around to follow astronomical calendars and clocks. Therefore, we must use other types of calendars or clocks based on other types of constant degrees to date geologic events.
Relative dating of rocks determines the order in which geological units were deposited or formed. Geologists use superposition, physical properties and relationships of rocks, biostratigraphy, section relationships, magnetostratigraphy, and chemostratigraphy to determine the relative age of rocks.
The principle of superposition states that rocks deposited on the Earth’s surface are deposited in order of age, with the oldest (deposited first) at the bottom. This principle applies to sedimentary rocks and lava flows. A related principle of primordial horizontality (physical properties and relationships) states that sedimentary and volcanic rocks are deposited in near-horizontal layers (Figure 1). These principles make it possible to distinguish the order of deposition and also to determine when originally flat-lying rocks were deformed by tectonic forces. Tectonism tilts rock layers by folding or bending them and can even turn them upside down. In the latter case, geologists must look for primary rock features, such as B. Wave marks or cross-strata, which prove which way was once upward.
Figure 1. Rock layers within two different cliffs along the Mississippi River illustrate the principles of superposition and faunal assemblage. The Shakopee Formation contains the oldest rocks. Fossils identified in rocks on the left are correlated with rocks containing similar fossils on the right (Southwick and Lusardi, 1997, Fig. 1).
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Fossils are strong indicators of relative age. Previous generations of geologists noted that fossil assemblages in thick sedimentary rock sequences changed with depth; This means that there were different fossils in the lower (older) rocks than in the higher (younger) rocks. The principle of faunal collections (biostratigraphy) was derived from this observation; It states that similar fossil assemblages are of similar geological age and report similar ages for the rocks they contain. Fossils, which are easily distinguishable, widespread, and limited in geologic time in which the original organism lived, are an excellent tool for correlating, or matching, rock sequences of similar age from one location to another (Figure 1).
To determine the relative age of intrusive igneous rocks such as granite or gabbro, geologists rely on the principle of cross-sectional relationships. Intrusive rocks form when molten rock (magma) seeps into other rocks and fills cracks in other rocks, then cools and crystallizes in place. The rock intruded (or “cut”) by the magma was there first and therefore older than the intrusive rock (Figure 2).
Figure 2. Principle of cross-cutting relationships (units numbered from oldest to youngest; Southwick and Lusardi, 1997, Figure 2).
Magnetostratigraphy is a technique for dating sedimentary and volcanic rocks that uses information about the residual magnetization in the rock, which correlates with the polarity of the Earth’s magnetic field at the time the rock was formed. The polarity (or direction) of the Earth’s magnetic field has changed over time; When the polarity coincides with today’s magnetic north pole, it is called “normal polarity” and when it is rotated 180°, it is called “reversed polarity”. When a rock (either igneous or sedimentary) is formed, the magnetic minerals align with the Earth’s magnetic field. Because geologists have good constraints on the timing of past magnetic polarity reversals, they can be used to correlate rock units with numerical ages, as well as to correlate rock units with each other.
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Chemostratigraphy uses the different chemical compositions of certain rocks to determine stratigraphic relationships. Both bulk chemical analysis and stable isotope geochemical techniques can be used to better delineate the correlation of stratigraphic units. This work can be performed on rocks of any age and used to correlate units from the local scale to the global.
All of these techniques are extremely useful on their own, but when two or more techniques are combined, they prove to be extremely powerful tools for determining the age and stratigraphic relationships of rocks. Of course, these dating methods provide the relative ages of rock sequences. To estimate the numerical age of a rock, geologists must use radiometric dating, or natural radioactive “clocks” that indicate geologic time.
Radiometric dating uses known information about the small amounts of radioactive atoms in a mineral’s structure to determine how long ago that mineral formed. A chemical element is made up of atoms made up of protons, neutrons and electrons. Protons and neutrons together form the nucleus of an atom. The number of protons determines the type of element; the number of neutrons determines the isotope of that element. For example, the element carbon has 8 different isotopes, all of which have 6 protons. The number of neutrons can vary from 3 to 10. The carbon-14 isotope has 6 protons and 8 neutrons. Isotopes of the same element have slightly different chemical properties.
There are many naturally occurring stable and unstable isotopes. A radioactive isotope is unstable and will spontaneously change to a more stable isotope at a measurable, constant rate. The original isotope is called the parent, and the resulting stable isotope is called the daughter; the conversion from parent to daughter is called radioactive decay. Because the decay rate for a given isotope is constant, a geologist can measure the amount of daughter isotopes present in a rock and determine how long it took for that amount to accumulate.
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The time it takes for half of the parent material to convert to the daughter material is called the half-life. For example, potassium-40 has a half-life of 1.3 billion years. Over 1.3 billion years, half of the potassium-40 has changed to its daughter isotopes argon-40 or calcium-40. After two half-lives, or 2.6 billion years, 75 percent of the original potassium-40 is gone. The amount of daughter isotopes increased by the same amount. Some isotopes have short half-lives, from hours to days. However, isotopes useful for dating geological events have long half-lives (Table 1).
Potassium-40 is found in many common minerals in igneous rocks (Table 1). As the magma cools and crystallizes, potassium-40 binds to the mineral grains of the newly formed rock (Fig. 3A, B). Argon-40, a gas, does not penetrate mineral crystals and will escape until the system cools below a certain temperature. When this temperature is reached, the clock is set; Argon-40, produced by the radioactive decay of potassium-40, begins to accumulate in the mineral and will continue to accumulate until the rock is heated. From the measured ratio of argon-40 to potassium-40, the time since the igneous rock was last cooled below the “threshold temperature” of argon-40 can be calculated (Fig. 3C). The age obtained may be close to when the igneous rock first formed, or it may record a later heating event. This can be interpreted by a trained geologist taking into account other geological information.
Figure 3. Schematic representation of mineral formation and radioactive decay (Southwick and Lusardi, 1997, Figure 3).
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