Using this principle, geologists were able to determine which rock layers were the oldest long before the technology existed to calculate the absolute ages of rocks. This now fundamental principle became known as the Law of Superposition, and it is considered a basic law of geology. That is when a Danish naturalist named Nichlaus Steno published his theory that older layers of sedimentary rocks are buried deeper in the planet than younger ones, which were laid down horizontally above the older strata. This seems like a simple and even obvious concept, but it was a new idea in 1669. Below are eleven others, in order, with the oldest issue-January-on the bottom and the youngest magazine-December-on top. Now when you look in the box, you will see only the December issue, which lies on top of the stack of magazines. This continues, month after month, until December. When you complete the February magazine you place it on top of the January issue. You read the January magazine and place it in a box when you are finished. Think about it like this: You have a magazine subscription and get one magazine in the mail every month. The oldest rock strata will be on the bottom and the youngest at the top. This principle states that layers of rock are superimposed, or laid down one on top of another. The xenoliths in meteorites were formed from collisions with other objects outside the Earths atmosphere.The law of superposition is one of the principles of geology scientists use to determine the relative ages of rock strata, or layers. Xenoliths have also been found in meteorites, or rocks from outer space that have crashed into Earth. Xenoliths can be a piece of rock trapped in a piece of sedimentary rock, but this is rare. Some of the features studied by geologists are temperature, pressure, construction, and movement within the Earth's surface. The information about the condition of the mantle at these depths would be impossible to understand without xenoliths and xenocrysts. Many xenocrysts were created hundreds of kilometers within the Earth, far below the deepest mines and wells. Scientists study the chemical properties of xenoliths to understandthe depth at which they were formed. Xenoliths and xenocrysts provide valuable information about the geology of the Earths mantle. The peridotite is usually yellow and dense, while the basalt is usually grey and light. A peridotite xenolith in a basaltic lava flow, for instance, means a chunk of the rock peridotite is embedded in basalt rock. Xenoliths and xenocrysts are often identified by the names of the two rock types involved. Metamorphic rock is rock that has changed from one form (sedimentary or igneous) to another. As it cools, the material may cease being a xenolith at all and become a metamorphic rock. A xenolith may lose its unique qualities if it melts into the surrounding magma. Xenoliths and xenocrysts are affected by temperature. Xenoliths can be as small as a grain of sand or as large as a football, and as long as several meters. They have a different color and density than the surrounding igneous rock. Lava cools fairly quickly, and various types of igneous rocks are formed. Crystals that are torn from the sides of magma pipes are called xenocrysts.Īs magma erupts or flows from the Earths surface, it is cooled by exposure to air or water. These bits and pieces, trapped in the magma but not melting into it, become xenoliths. As the molten material rises, it tears off bits and pieces of the magma pipe in which it is traveling. Magma rises to the Earths surface through these pipes between the Earths crust and mantle. Xenoliths are torn from deep cracks, or pipes, in the Earths surface. Xenoliths are different types of rock embedded in igneous rock. The rock that forms from cooled magma is called igneous rock. Magma is the molten rock beneath the Earths crust that emerges as lava during a volcanic eruption. Most of the time, a xenolith is a rock embedded in magma while the magma was cooling. A xenolith is a piece of rock trapped in another type of rock.
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