Chapter 5-Intepreting Earth's History
Relative Age Determination. superposition stihotvorenia.info ( bytes), Superposition of rock units is a very simple and straightforward method of relative age the sedimentary rocks had to be there prior to emplacement of the igneous . is the “age” of a rock or strata compared to the ages of other rocks or strata. Extrusion. Kubu Rock – Botswana, Africa. Intrusions. are igneous rocks that form when magma Hints: use your timescale, and the definition of index fossils. Relative Age – The age compared to the ages of other rocks. (older than When magma cools on the surface of the Earth it's called an EXTRUSION. Extrusions.
Weathering and erosion occurred, forming a layer of soil on top of layer A.
Unconformities in Rock Layers[ edit ] Steno discovered the rules for determining the relative age of rock beds, but he did not have a good understanding of how long it would take for these rock formations to form. At the time, most Europeans believed that the Earth was around 6, years old, a figure that was based on the amount of time estimated for the events described in the Bible.
One of the first to question this time scale was a Scottish geologist named James Hutton Often described as the founder of modern geology, Hutton formulated a philosophy called uniformitarianism: The present is the key to the past. According to uniformitarianism, the same processes we see around us today operated in the past as well.
For example, if erosion and deposition occur slowly now, they probably have always occurred slowly. Hutton discovered places where sedimentary rock beds lie on an eroded surface.
Such a formation is called an unconformity, or a gap in rock layers, where some rocks were eroded away. Hutton reconstructed the sequence of events that led to this formation. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland Figure Hutton's Unconformity on the coast of Scotland. Based on figure 15, at least nine geological events can be inferred: A series of sedimentary beds is deposited on an ocean floor.
The sediments harden into sedimentary rock. The sedimentary rocks are uplifted and tilted, exposing them above the ocean surface. The tilted beds are eroded by rain, ice, and wind to form an irregular surface. A sea covers the eroded sedimentary rock layers. New sedimentary layers are deposited.
The new layers harden into sedimentary rock. These layers are tilted. Uplift occurs, exposing the new sedimentary rocks above the ocean surface.
- High School Earth Science/Relative Ages of Rocks
Hutton realized that an enormous period of time was needed to account for the repeated episodes of deposition, rock formation, uplift, and erosion that led to the formation of an unconformity, like the one at Siccar Point. Hutton realized that the age of Earth should not be measured in thousands of years, but millions of years. Matching Rock Layers[ edit ] Superposition and cross-cutting are helpful when rocks are touching one another, but are useless when rocks are kilometers or even continents apart.
Three kinds of clues help geologists match rock layers across great distances. The first is the fact that some sedimentary rock formations span vast distances, recognizable across large regions.
A second clue could be the presence of a key bed, or a particularly distinctive layer of rock that can be recognized across a large area. Volcanic ash flows are often useful as key beds because they are widespread and easy to identify.
Probably the most famous example of a key bed is a layer of clay found at the boundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct Figure This thin layer of sediment, only a few centimeters thick, contains a high concentration of the element iridium.
Iridium is rare on Earth but common in asteroids. Ina team of scientists led by Luis Alvarez and his son Walter proposed that a huge asteroid struck Earth about 66 million years ago, causing forest fires, acid rain, and climate change that wiped out the dinosaurs.
The Relative Age of Rocks
White layer of clay that marks the Cretaceous-Tertiary Boundary. For example the processes that created oceans, volcanoes, land, etc. Superposition - The principle of superposition is simply the fact that the lowest layer of a rock formation is the oldest layer while the youngest layer is towards the top.
Geologic layers are of course deposited in order.
Chapter 5-Intepreting Earths History
Original Horizontality - The deposition of sediments occurs horizontally; for example, the deposition of sediments within lakes, rivers,oceans is horizontal. Processes such as tilting, folding, faulting, and intrusions of igneous rocks can distort the original strata.
Tilted geologic layers were originally horizontal. Sediments will fill in uneven geologic layers. Geologists observe the tilted layer and try to visually reposition it horizontally. Igneous rocks can intrude or cut through pre existing sedimentary rock layers. There are two types of intrusions: Intrusive- molten rocks that cuts through the pre existing rocks. When a observer or geologist come across sedimentary layers with an intrusion, the intrusion is the youngest event to have occurred.
The dashed lines indicate contact metamorphism. Remember, contact metamorphism is changes in a rock due to contact with magma or lava. Think of it as a boundary layer between igneous rocks and sedimentary rocks.
Relative Age Determination
Faults- breaks in the rock where movements has occurred. Fossils can help geologists determine the age of geologic strata. For example, if you have one type of organism that lived 10 million years ago and it is found within one geologic layer, then you have a better clue of the age of that layer.
Such fossils are called Index Fossils. Index fossils- Organisms that have lasted a very short period of geologic time but found over large portions of Earth.
Index fossils are also used to correlate or match up geologic layers. In June, it is almost a certainty that you will have to answer questions that asks you the order of geologic events. The geologic formation that you may see will be complicated as the illustrations below.