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Geologic Time Relative Dating Any method of determining whether an event or object is older or younger than the events or objects around it. The word relative means “compared to.” How old is a rock layer compared to the rock layers around it? Relative Dating Scientists all over the world send in information about the rock layers in their area, these are analyzed and compared to the “geologic column” The geologic column is an ideal sequence of rock layers that contains all the known fossils and rock formations on earth. By comparing a sample to the geologic column, you can find it’s relative age and also whether some events or layers are missing. The Geologic Column Relative Dating In rock layers, we obey the rule of superposition which says... Younger over older. This means that the oldest rocks in an area are the deepest underground and the youngest are near the surface unless there has been a disturbance. A Geologic Sandwich In this assignment, you will use the small sandwiches on the menu of a sandwich shop to determine the order of the ingredients in a giant sandwich that the shop offers. The small sandwiches are like the samples of rock layers from all over the world and the giant sandwich is like the geologic column. Let’s take a look at the menu! A Geologic Sandwich This is the order of ingredients on the giant sandwich from the geology rock cafe! How did you do? A Geologic Sandwich Superposition The principle that older layers are on the bottom and younger layers are on the top, if there has not been a disturbance Remember: younger over older Scientists know that rock layers are undisturbed when they are horizontal. Possible Disturbances Fault Folding Intrusion Tilting Unconformity Fault A breaking and sliding of rock layers. A break in the Earth’s crust A fault is younger than the layers it has broken. Fold A Occurs when two plates slide towards each other. Rocks that are deeper fold instead of breaking. This is how mountains are made Intrusion When a lava flow interrupts rock layers. An intrusion is younger than the layers it disturbs. Unconformities A place in the rock record where layers are missing A gap in geological history Layers of rock have been deformed, weathered, or eroded away Erosion is a major cause of unconformities Usually indicated by a wavy line in a rock layer diagram Hints: A cross-cutting feature is always younger that the layers it cuts across. This is because a layer must exist BEFORE it can be disturbed. Tilting is the slanting of rock layers Find the Disruptions Find the Disruptions Find the Disruptions Find the Disruptions Find the Disruptions Divisions of Geologic Time Age of the Earth: 4.6 billion years Earth’s History is divided into sections. The largest chunk of time is called an Eon Eons are divided into smaller parts called Eras Eras are divided into Periods Some Periods are divided into Epochs Why Epochs? As we study the past that is closest to us, we get more information than we can about the distant past Information from the distant past may have been destroyed or be too far below the Earth’s surface to access. Our Place on the Scale Eon: Phanerzoic Era: Cenozoic Period: Quaternary Epoch: Holocene Important Events in History 4,600 mya - Planets formed 4,200 mya - oldest thing on Earth (Zircon Crystal) 3,500 mya - first life forms (single celled) 2,500 mya - Oxygen in the Atmosphere 800 mya - multicellular life 400 mya - life moves to land 248 mya - dinosaurs show up Important Events in History 65 mya - dinosaurs go extinct (probably because of climate change) - they were around for a total of 83 my 6 mya - humanoid life forms (walked upright) 2 mya - Ice Age 100,000 years ago - first appearance of species Homo sapiens (That’s us) Geologic Time Scale Geologic Time Scale Geologic Time Scale