The History of Earth Relative Dating Absolute Dating Relative dating does NOT look for exact dates but instead looks for a sequence of events (order) – what came first… Absolute Dating Absolute dating determines and age or age range, using chemical and physical properties of an object Carbon based objects can be dated by using carbon based radioactive dating

Relative Dating : Which Came First? Earth Science Chapter 8.1

Objectives: Explain how relative dating is used in geology. Explain the principle of superposition. Describe how the geologic column is used in relative dating. Identify two events and two features that disrupt rock layers. Explain how physical features are used to determine relative ages.

Uniformitarianism Coined by 18th century Scottish physician and farmer James Hutton Made lots of observations on his farm and in his native land SO what is UNIFORMITARIANISM?!?!?! Big word that simply means – The geological events that occur today (volanoe eruptions, erosion etc.) were also happening in the past. Although the events are the same, the rate of occurrence is different

Game of Clues Geologists try to determine the order in which events have happened during Earth’s history. They rely on rocks and fossils to help them in their investigation. Relative dating The process of determining whether an event or object is older or younger than other events or objects.

The Law of Superposition Layers of sedimentary rock, such as the ones shown below, are stacked like pancakes. As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older. Superposition Law that states that younger rocks lie above older rocks, if the layers have not been disturbed. Theoretically – the fossil findings that are buried on the lower layers was on earth first

The Geologic Column Geologic Geologists use the geologic column to: an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest. Geologists use the geologic column to: Interpret rock sequences Identify the layers in puzzling rock sequences. Principle of Original Horizontality: Sedimentary rock left undisturbed will remain in horizontal layers – if its not that way, something disrupted/disturbed the land - now geologist must look for clues…..

Disturbing Forces – messes with Superpositioning Not all rock sequences are arranged with the oldest layers on the bottom and the youngest layers on top. Some rock sequences have been disturbed by forces within the Earth. These forces can: Push other rocks into a sequence Tilt or fold rock layers Break sequences into moveable parts.

Some of the Clues… Cross Bedding Graded Bedding Ripple Marks Size of the particles Bigger pieces are on the bottom If they are on the top, then the land was disturbed Shape of the Bedding Planes Sand deposits at angles & usually erode before new layer form Scientists look for the shape of the cross bed to figure out the original position Surface Waves Caused by wind and/or water If crests are not upwards, then there has been a disturbance

Disturbed Rock Layers Law of Crosscutting Relationships The fault or intrusion rock is younger than the rock above and below because it cut into the middle of something already there. The features must be younger than the rock layers because the rock layers had to be present before the features could cut across them.

Events That Disturb Rock Layers Four ways that rock layers may become disturbed. A fault break in the Earth’s crust along which blocks of the crust slide relative to one another. An intrusion molten rock from the Earth’s interior that squeezes into existing rock and cools. fault intrusion

Gaps in the Record -- Unconformities Missing Evidence Sometimes, layers of rock are missing, creating a gap in the geologic record. This is called... Unconformity break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

Types of Unconformities Most unconformities form by both erosion and non- deposition, but other factors may be involved. Geologists place them into three major categories: Disconformities Nonconformities Angular unconformities Disconformities exist where part of a sequence of parallel rock layers is missing. (layers are parallel)

Nonconformities Angular Unconformities exist where sedimentary rock layers lie on top of an eroded surface of non-layered igneous or metamorphic rock. Angular Unconformities exist between horizontal rock layers and rock layers that are tilted or folded.

Well that was confusing…

Rock-Layer Puzzles Rock-layer sequences often have been affected by more than one geological event or feature. For example, intrusions may squeeze into rock layers that contain an unconformity Determining the order events is like solving a jigsaw puzzle. piece together the history of the Earth.

What do you know? Why is it important or scientist to be able to understand the relative age of rocks? What is the difference between unconformity and non conformity? In which type of unconformity does the rock layers above and below the unconformity run parallel So they can understand the age and history of Earth Unconformity is a break in the rock record, non conformity is a type of unconformity where metamorphic and igneous rock are overlain by sedimentary rocks Disconformity

Which of the following statements best describes the relationship between the law of superposition and the principle of original horizontality? A. Both describe the deposition of sediments in horizontal layers B. Both conclude that Earth is more than 100,000 years old C. Both indicate the absolute ages of layers of rock D. Both recognize that the geologic processes in the past are the same as those at work now. A. Both describe the deposition of sediments in horizontal layers