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Earth is 4. To put this into perspective, if the average human lifespan is 80 years, the Earth has been around for 57,, lifetimes. Constantly writing out millions and billions of years is time-consuming, so when geologists talk about ages, they use a few abbreviations. The symbols ka thousands , Ma millions , and Ga billions refer to points in time like a date. For example, the dinosaur extinction occurred at 66 Ma. Geologists also use other abbreviations for lengths of time, including ky, kya, kyr, and k. All four varieties of abbreviations mean the same thing in this case.
Here, you would say the dinosaurs have been extinct for 66 myr. There is a debate amongst geologists , and other sciences, over the notation used for geologic time. Fun fact: The Tyrannosaurus rex was one of the last dinosaurs to evolve about The first dinosaurs evolved about Ma a specific date , myr a length of time before T-rex evolved. We are closer in time to the T-rex than the T-rex is to its earliest dinosaur ancestor! See the difference in abbreviations yet? Since 4.
The commonly accepted time scale comes from the International Commission on Stratigraphy Figure 3. It is continually revised as new research fine-tunes s between time scale divisions. The one in Figure 3. The divisions on the time scale are often based on ificant events that have taken place tectonically, biologically, or climatically, and the numerical ages are derived from radiometric dating of rocks, minerals, and fossils.
Geologic time is first divided into eon s; these are the Hadean, Archean, Proterozoic, and Phanerozoic. The eons are subdivided into eras, and eras are subdivided into periods, and periods into epochs, and epochs into ages. For the purposes of this lab manual, we will refer to this nomenclature.
The difference between the two is discussed here , but it is not important for our purposes. Throughout this lab manual, you will mainly see us referencing periods of geologic time. Look at the time scale in Figure 3. The far-right column goes from 4. The other three columns make up the remaining myrs. Geologists use abbreviations to refer to the different portions of geologic time Table 3.
So, be careful! The first is called relative dating, meaning how events relate to each other in time, or more plainly, they figure out the sequence of events what came first, second, third, etc. Relative dating has no regard for numerical ages. The second method is absolute dating, where geologists use radioactive isotopes to figure out the numerical age of a rock or mineral. There are several principles geologists use for relative dating. The first four principles were developed in the 17th century by an early geologist named Nicolas Steno , three of which pertain to sedimentary rocks.
The first is the law of superposition, which states that in layers of horizontal sedimentary rocks, the oldest rock layer is at the bottom, and the youngest is at the top Figure 3. The second rule is the principle of original horizontality, which says that layers of sediment are originally deposited horizontally Figure 3.
So, any tilting or folding of the rock occurred after it was deposited Figure 3. The third principle states that layers of sedimentary rock are continuous, and anything that interrupts the layer like a river or canyon happened after the rock formed. This is called the principle of lateral continuity Figures 3. This principle is used when other geologic events cut through sedimentary rocks, like an igneous dike or a fault.
This principle basically states that when a geologic event cuts across another, the event doing the cutting is younger than the one being cut Figure 3. The same can be said of a fault that cuts through any rock; the fault has to be younger because the rocks had to exist first to be faulted. Some years later, the fifth principle of relative dating was developed by Charles Lyell called the principle of inclusions. This principle explained that a clast , or a different-looking rock that is contained inside of another rock, is older than the rock that contains it Figure 3.
How can this happen? Originally, a mafic magma was cooling quickly, producing the finer-grained mafic rock in the middle of Figure 3. Then, something happened to change the chemistry of the magma to felsic and slowed the cooling rate to produce the surrounding, coarse-grained granite. The mafic rock formed first, and then the felsic rock formed around it.
Simply speaking, an unconformity is a pattern that you look for in a group of rocks that tells you erosion has taken place. This material is then transported away by wind, water, or ice, a process known as erosion. Many people use weathering and erosion interchangeably, but they do mean different things: weathering is the breakdown of rocks, while erosion removes the broken down material. There are four types of unconformities, and each forms in a slightly different way Figure 3. A disconformity Figure 3. This type of unconformity typically forms when horizontal layers of sedimentary rock are deposited in a shallow marine environment; then sea level lowers to expose these rocks and allows erosion to occur; then sea level rises again, and new horizontal layers of sedimentary rock are deposited.
Erosion removed some of the original rock, creating a large age gap between the rocks above and below the erosional surface. This age gap is the disconformity and is located at the contact point between the older rock and younger rock. Oftentimes the erosion process leaves behind evidence of river channels or soil development, which provide clues to geologists to locate the unconformity in what looks like a continuous succession of sedimentary layers. A nonconformity Figure 3. The unconformity is where the bedrock meets the sedimentary rock. For example, when a mountain belt is eroded below sea level, and afterward sediments are deposited on top of the igneous or metamorphic rock, the contact is a nonconformity.
An angular unconformity Figure 3. The most famous angular unconformity is from Siccar Point in Scotland. Figure 3. For this to occur, sedimentary rocks deposited in the marine environment are lifted above sea level by an orogeny or similar event. The orogeny causes the sedimentary rocks to become tilted or folded. Since these rocks are exposed above sea level, erosion takes place. The rocks can either be eroded below sea level, or sea level can rise, which would allow new, horizontal layers of sedimentary rock to be deposited on top of the titled ones.
This creates an angle between the younger, horizontal layers on top and the older, tilted layers below. A paraconformity Figure 3. We know these occur because sediment above and below the paraconformity have been radiometrically dated and reveal a large gap in time. The images in Table 3.
In the blank spaces provided next to the images, create a sketch for each unconformity, label where you think the unconformity is located, and identify the type of unconformity. Below are relative dating outcrop diagrams that represent sections of rock. Each letter represents the deposition of a different layer of sedimentary rock or geologic event.
The symbols used to represent common types of rocks are standard USGS symbols. You will only see conglomerate, limestone, sandstone, shale, granite, and gneiss in this exercise. The subscript letters stand for igneous dikes D , faults F , and unconformities U. The colors for each unit are from the geologic time scale shown in Figure 3. Hint: it is easier to start with the oldest event and work your way forward through time. The web program Visible Geology lets you create block models with a complicated geologic history.
An example of a diagram made with this website is in Figure 3. Answer the following questions using Figure 3. Create your own block model using the web program Visible Geology. It is done by sending sound waves into the ground or ocean. As these sound waves move through different layers of rock or sediment, some of the waves are reflected back toward the surface and recorded by geophones. Different types of sediments or rocks change the characteristics of the wave, such as its velocity.
The als recorded are called the seismic amplitudes, a measure of the difference in rock properties between two layers. Seismic data is commonly converted to impedance, or hardness this is not the same as Mohs hardness. The relative hardness can be positive, negative, or the same.
Thus, many seismic sections will use three colors to better distinguish different strata. The continental slope off of the north island of New Zealand is called the Hikurangi margin. Geoscientists wanted to explore this area to better understand the plate boundary by drilling a sediment core.
As the Pacific plate traveled over numerous hot spots, the basement of the Gulf of Alaska became riddled with seamounts. The seamounts have a variety of ages and sizes, as you can see in Figure 3. These are partially covered by a thick blanket of sediments up to 1.
The principles of relative dating allow geologists to compare seemingly similar groups of rocks separated by some distance. On a larger scale of kilometers to hundreds of kilometers, it can be comparing sets of sedimentary rocks that have similar patterns.Activity 8.3 absolute dating of rocks and fossils answers
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Overview of Relative and Absolute Dating