You cannot download interactives. The rock cycle is a web of processes that outlines how each of the three major rock types—igneous, metamorphic, and sedimentary—form and break down based on the different applications of heat and pressure over time. For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss.
If it is heated further, the rock will melt completely and reform as an igneous rock. Empower your students to learn about the rock cycle with this collection of resources. The fossil record helps paleontologists, archaeologists, and geologists place important events and species in the appropriate geologic era. It is based on the Law of Superposition which states that in undisturbed rock sequences the bottom layers are older than the top layers.
Therefore, some discovered fossils are able to be dated according to the strata, a distinct layer of rock, that they are found in. Another common way that fossils are dated, is through radiocarbon dating. The development of this type of dating, in the s, transformed paleontology and enhanced the accuracy of the fossil record.
With every new fossil discovery, our understanding of the environment in a particular time becomes richer. Use these resources to teach middle schoolers more about the fossil record and radiocarbon dating. In the mids, Charles Darwin famously described variation in the anatomy of finches from the Galapagos Islands.
Alfred Russel Wallace noted the similarities and differences between nearby species and those separated by natural boundaries in the Amazon and Indonesia. Independently they came to the same conclusion: over generations, natural selection of inherited traits could give rise to new species.
Use the resources below to teach the theory of evolution in your classroom. Where did we come from? Humans continue to search for the answer to this fundamental question.
Over the years, we've turned to both religion and science to explain where our species came from. Innovators of their time, Charles Darwin and Alfred Russel Wallace, used science to explain where humans came from, posing the theory of evolution. Then, Mary and Louis Leaky explored the fossil record to see if they could piece together the story of humans.
Evolutionary science and archaeology continue today. Use these materials in your classroom to teach your students about the origins of the homo sapien. Lake Turkana in Kenya has only been around for the past , years, but the expanding and receding shores of the lake have provided food and water to organisms for millions of years. Today, scientists study the stratigraphy of the Turkana basin to better understand the age of fossils discovered there.
Discoveries at Lake Turkana reveal information about the history of human evolution. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. How has tectonic activity influenced discoveries at Turkana?
What is one technique that scientists use to date the fossils they find? In , Danish naturalist Nichlaus Steno put forth the idea that horizontal layers of sedimentary rock represent a time sequence of Earth's geologic history.
He realized that older layers of sedimentary rocks are deeper in the earth, and younger ones build on top of them. In geology, this is called the law of superposition.
For example, revisions to a method called electron spin resonance allow scientists to date rare fossils, like hominin teeth, because they can directly date the fossil without visibly damaging the specimen. In the Afar, scientists are attempting to date the actual layers from which the fossils erode, rather than relying on the presence of volcanic ash.
This would more accurately tell us the age of fossil-bearing rocks, and open new field sites for exploration that lack such layers. Fossils span geologic time from hundreds to even billions of years and are discovered in many rock types and settings. Selecting a suitable dating technique is a critical step to obtaining a meaningful and accurate age. Scientists have dated fossils found in South African caves to between , to , years old using several different geochronometers, including optically stimulated luminescence , a tool that allows us to calculate the last time cave sediments were exposed to light.
Another common method, uranium-lead dating, relies on the radioactive decay of uranium and can be used to date rocks containing the oldest known fossils on Earth—older than 3. To put that in context, the age of the Earth is 4. The 18th-century geologist James Hutton recognized that geologic processes require long timescales, a concept central to what we mean by the term deep time. Erin DiMaggio is an assistant research professor of geosciences at Penn State University, where she studies sedimentary and volcanic rocks in Africa to learn about ancient environments.
Alka Tripathy-Lang is a visiting scientist at the Berkeley Geochronology Center, where she studies the time-temperature history of rocks to understand how they came to the surface of the earth.
The Afar region has layered sedimentary rocks, containing thin white volcanic ash that we could use to date the fossil jawbone. Erin DiMaggio, Penn State University Early on, before we had more precise means to date fossils, geologists and paleontologists relied on relative dating methods. Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.
This is the principle of cross-cutting relationships. The principle states that any geologic features that cut across strata must have formed after the rocks they cut through Figures 2 and 3. Figure 3: The sedimentary rock layers exposed in the cliffs at Zumaia, Spain, are now tilted close to vertical. According to the principle of original horizontality, these strata must have been deposited horizontally and then titled vertically after they were deposited. In addition to being tilted horizontally, the layers have been faulted dashed lines on figure.
Applying the principle of cross-cutting relationships, this fault that offsets the layers of rock must have occurred after the strata were deposited. The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks.
Each fossil species reflects a unique period of time in Earth's history. The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks Figure 4.
Figure 4: The principle of faunal succession allows scientists to use the fossils to understand the relative age of rocks and fossils. Fossils occur for a distinct, limited interval of time. In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence — when it went extinct.
Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species i. For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed.
In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box.
Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box.
Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. Because they are often rare, primate fossils are not usually good index fossils.
Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly. Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time Figure 4. If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time.
Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas. All elements contain protons and neutrons , located in the atomic nucleus , and electrons that orbit around the nucleus Figure 5a. In each element, the number of protons is constant while the number of neutrons and electrons can vary. Atoms of the same element but with different number of neutrons are called isotopes of that element.
Each isotope is identified by its atomic mass , which is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon 12 12 C , carbon 13 13 C , and carbon 14 14 C Figure 5a. Figure 5: Radioactive isotopes and how they decay through time. C 12 and C 13 are stable. The atomic nucleus in C 14 is unstable making the isotope radioactive.
Because it is unstable, occasionally C 14 undergoes radioactive decay to become stable nitrogen N The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope.
Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14 C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both. This change is called radioactive decay.
For example, unstable 14 C transforms to stable nitrogen 14 N. The atomic nucleus that decays is called the parent isotope. The product of the decay is called the daughter isotope. In the example, 14 C is the parent and 14 N is the daughter. Some minerals in rocks and organic matter e.
The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. Attempting to date one directly would yield a false result—much older than the organism itself. Because fossils are usually found in sedimentary rock layers, paleontologists can date them by examining the minerals above or below the sedimentary rock.
Zircon, a mineral commonly found in igneous rocks, proves particularly useful. As zircon forms in cooling magma, its crystal structure contains uranium but no lead. Thus, any lead present in a sample of zircon must have formed via radioactive decay of uranium. This feature allows geologists to date volcanic ash flows that are interspersed with layers of sedimentary rock like a prehistoric layer cake.
Any fossils found in the sedimentary rock must be younger than the ash below and older than the ash above. So far, so good. But what if there is no ash layer close to the fossils? And then we can get an estimate for how old or young something is, based on the relative position. To understand how fish anatomy changed over time, Gibson depends on the results of uranium-lead dating, magnetostratigraphy and index fossils.
Because of the difficulties in determining the age of rock layers, however, she does not perform radiometric dating herself. In fact, researchers released a major update of the radiocarbon calibration curves this year.
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