Rock layers extend laterally, or out to the sides. They may cover very broad areas, especially if they formed at the bottom of ancient seas. Erosion may have worn away some of the rock, but layers on either side of eroded areas will still match up.
Relative age hookup of rocks worksheet Grand Canyon in Figure Its a good example of lateral continuity. You can clearly see the same rock layers on opposite sides of the canyon. The matching rock layers were deposited at the same time, so they are the same age. Sediments were deposited in ancient seas in horizontal, or flat, layers.
If sedimentary rock layers are tilted, they must have moved after they were deposited. Rock layers may have another rock cutting across them, like the igneous rock in Figure Relative age hookup of rocks worksheet Which rock is older? To determine this, we use the law of cross-cutting relationships.
The cut rock layers are older than the rock that cuts across them. Geologists can learn a lot about Earths history by studying sedimentary rock layers. But in some places, theres a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity. Look at the rock layers in Figure They show a feature called Huttons unconformity. The unconformity was discovered by James Hutton in the s. Hutton saw that the lower rock layers are very old.
The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent "Relative age hookup of rocks worksheet" layers were deposited. Huttons discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow.
He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually. The study of rock strata is called stratigraphy. The laws of stratigraphy can help scientists understand Earths past. The laws of stratigraphy are usually credited to a geologist from Denmark named Nicolas Steno.
He lived in the s. The laws are illustrated in Figure Refer to the figure as you read about the laws below. Superposition refers to the position of rock layers Relative age hookup of rocks worksheet their relative ages. Relative age means age in comparison with other rocks, either younger or older. The relative ages of rocks are important for understanding Earths history.
New layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. This is the law of superposition. You can see an example in Figure The largest blocks of time on the geologic time scale are called eons. Eons are split into eras.
Each era is divided into periods. Periods may be further divided into epochs. Geologists may just use early or late.
An example is late Jurassic, or early Cretaceous. Index fossils are commonly used to match rock layers in different places. You can see how this works in Figure. Phanero- zoic means visible life. During this eon, rocks contain visible fossils.
Before the Phanerozoic, life was microscopic. The Cenozoic Era means new life. It encompasses the most recent forms of life on Earth. The Cenozoic is sometimes called the Age of Mammals. Before the Cenozoic came the Mesozoic and Paleozoic.
The Mesozoic means middle life. This is the age of reptiles, when dinosaurs ruled the planet. The Paleozoic Relative age hookup of rocks worksheet old life. Organisms like invertebrates and fish were the most common lifeforms.
The geologic time scale may include illustrations of how life on Earth has changed. Major events on Earth may also be shown.
These include the formation of the major mountains or the extinction of the dinosaurs. It shows how Earths environment and life forms have changed. When rock layers are in the same place, its easy to give them relative ages. But what if rock layers are far apart? What if they "Relative age hookup of rocks worksheet" on different continents? What Relative age hookup of rocks worksheet is used to match rock layers in different places?
Some rock layers extend over a very wide area. They may be found on more than one continent or in more than one country. For example, the famous White Cliffs of Dover are on the coast of southeastern England. These distinctive rocks are matched by similar white cliffs in France, Belgium, Holland, Germany, and Denmark see Figure It is important that this chalk layer goes across the English Channel.
The rock is so Relative age hookup of rocks worksheet that the Channel Tunnel connecting England and France was carved into it! Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic Relative age hookup of rocks worksheet scale. So, many of the names of the time periods are from places in Europe. Like index fossils, key beds are used to match rock layers.
A key bed is a thin layer of rock. The rock must be unique and widespread. For example, a key bed from around the time that the dinosaurs went extinct "Relative age hookup of rocks worksheet" very important. A thin layer of clay was deposited over much of Earths surface. The clay has large amount of the element iridium. Iridium is rare on Earth but common in asteroids. This unusual clay layer has been used to match rock up layers all over the world.
It also led to the hypothesis that a giant asteroid struck Earth and caused the dinosaurs to go extinct. To create the geologic time scale, geologists correlated rock layers. Stenos laws were used to determine the relative ages of rocks. Older rocks are at the bottom and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late s changed that.
Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about million years ago. It lasted for about 55 million years. Determining the Age of Rocks and Fossils.
By: Frank K. McKinney. VOCABULARY. Aboslute age dating. Fossil. Geologists. Half-life. Relative age dating. Topic: Relative age dating of geologic cross sections. oldest rock (usually located near the bottom) in the diagram below and work your way up. Your. Fossils and relative dating worksheet answer key - How to get a good woman. May already given on the relationship to correlate distant rock layer formed first answer hook up in this earth science 10 unit 6 project worksheet 8 answer key.