1Earth Science 12.1 Discovering Earth’s History: Geologic Time
2Studying Earth’s History For geologists today, one goal of geology is to interpret Earth’s history. Geologists do this by studying the rocks of Earth’s crust; especially the sedimentary layers.When studying the Earth’ s history, geologists make use of three main ideasThe rock record provides evidence of geologic events and life forms of the pastProcesses observed on Earth in the present also acted on it in the pastEarth is very old and has changed over geologic time
3Studying Earth’s History Scientists in Europe and the British Isles began to develop these ideas during the 1700s. They wondered about the processes that the Earth’s landscape features and they noticed that sedimentary rocks were laid down in layers. They thought about how much time it must have taken for these layers to form.In the late 1700s, James Hutton published his Theory of the Earth. In this work, Hutton put forth the idea of uniformitarianism which simply states that “the physical, chemical and biological laws that operate today also operated in the geologic past.”
4UniformatarianismUniformatarianism means that the process that we observe today have been at work for a very long time; hundreds of millions of years.To understand the geologic past, we must first understand present day processes.It is important to remember that although many features of our landscape may seem to be unchanging within our lives; when viewed over a scale of millions of years, they are actually constantly changing.
5Principals of Relative Dating In relative dating, geologists follow several principals:the law of superposition,the principal of original horizontality,and the principal of cross-cutting relationships.These principals help scientists determine the sequence in which geologic events occur.
6Law of Superposition: Law of Superposition Nicolaus Steno, a Danish anatomist, geologist and priest,( ) made observations that are the basis of relative dating.Based on his observations, Steno developed the Law of Superposition.The law of superposition states that in a sequence of sedimentary rock layers, each layer is older than the layer above it, and younger than the layer below it.
7Law of Superposition: Law of Superposition Although it may seem obvious that a rock layer could not be deposited unless it had something older than it for support, it was not until 1669 that Steno stated the principal.Applying the law of superposition to the rock layers exposed in a section of the Grand canyon, we can easily place the layers in their correct geological order.
8Principal of Original Horizontality: Law of SuperpositionPrincipal of Original Horizontality:Steno also developed the principal of original horizontality.The principal of horizontality states that layers of sediment are generally deposited in a horizontal position.If you see rock layers that are flat, they are undisturbed and are still in their original position.If you see layers that are tilted vertically or are bent and folded; they have undergone change since they were originally formed.
9Principal of Cross-Cutting relationships Later in time, geologists developed another principal used in relative dating.The principal of cross-cutting relationships states that when a fault cuts through rock layers, or magma intrudes into other rocks and hardens, than the fault or intrusion is younger than the rocks around it.Simply, a fault that splits layers of sedimentary rock must have happened after the layers were first formed.
10Reading the Rock Record Reading the Rock Records:Today, geologists apply Steno’s principals to interpret or read the order of rock layer geological events.Geologists also determine how the rocks in one area are related to similar rocks in other places.Methods that geologists use to interpret the rock record include the study of inclusions and unconformities.Geologists also correlate rock layers at different locations to compare evidence.
11Reading the Rock Record Reading the Rock Records:By studying rocks from many different places worldwide, geologists can construct a model of the rock record called a geologic column.A geologic column is made up from rocks arranged according to their relative edges.The oldest rocks are at the bottom of the column and the youngest rocks are at the top.
12Inclusions: Inclusions Sometimes, the study of inclusions can help the rock dating process.Inclusions are pieces of one rock unit that are contained within another rock unit.The rock unit next to the one containing the inclusion must have been their first in order to provide the rock fragments.Therefore, the rock unit containing the inclusions is the younger of the two.
13Unconformities: Unconformities Throughout Earth’s history, the deposition of sediment has been interrupted again and again. Nowhere is Earth’s rock record complete.A surface that represents a break in the rock record is called an unconformity.An unconformity indicates a long period during which deposition stopped, erosion removed previously formed rocks, and than deposition resumed.Unconformities help geologists identify what intervals of time are not represented in the rock record.
14Unconformities: 3 Types There are three basic types of unconformities:Angular unconformitiesDisconformitiesNonconformities
15Unconformities: 3 Types In an angular unconformity, layers of sedimentary rock form over older sedimentary rock layers that are tilted or folded.
16Unconformities: 3 Types In a disconformity, two sedimentary rock layers are separated by an erosional surface.
17Unconformities: 3 Types Younger strata test upon older, metamorphic or igneous rocks (Precambrian-Cambrian contact - Alexander Bay, NY)In a nonconformity, an erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks.strata rest upon older, metamorphic or igneous rocks (Precambrian-Cambrian contact - Alexander Bay, NY)
18Correlating Rock layers Interpreting unconformities helps geologists read the rock record in one location.Geologists use correlation to match rocks of similar age in different locations.
19Correlating Rock layers Geologists often correlate layers by noting the position of a distinctive rock layer in a sequence of layers.If geologists find a distinctive rock layer in another location, they may infer that the same layer once covered both locations.
20Correlating Rock layers By correlating the rocks from one place to another, it is possible to create a more complex view of the geographical history of a region.For example, by correlating the data from two sites on the Colorado Plateau (one in southern Utah and one in Arizona) scientists can piece together the geologic history of the plateau by cross examining the information from each site.Correlation reveals a more complete and complex picture of the rock record than can be found from a single site.