Law of Superposition Chapter 8 Lesson 2

Secrets of the Layers of Rock

Law of Superposition In layers of horizontal sedimentary rock, each layer is older than the layer above it and younger than the layer below it “Oldest on bottom, youngest on top”

Age of Rocks Pages 304-305 in Ch. 8 The relative age of a rock is its age compared to the ages of other rocks. Ex: You are a 6th grader. Most 6th graders are about 11 - 12 years old. The absolute age of a rock is the number of years since the rock formed. Ex: You are 11 years, 7 months old

Relative age Activity As a class you are going to line yourselves up by your relative age (from oldest by the door to youngest to the other side of the room)

Absolute age Now you are going to line yourselves up by your relative age (from oldest by the door to youngest to the other side of the room) Ex: You are 11 years, 7 months old

Intrusions: Magma pushes bodies of rocks below the surface. Fault: A break in the earths crust * They are always younger than the rock it is cutting through Intrusions: Magma pushes bodies of rocks below the surface. It cools and hardens into a mass of igneous rock * Intrusions are always younger than the rock layers it around it Extrusions: Lava that hardens on the surface and forms igneous rock *extrusions are always younger than the rocks below it

How Rock Layers Can Change Pages 308-309 in Ch. 8 Unconformities A gap in the geological time record. It shows were rock layers have been lost due to erosion Folding Layers are overturned completely. The youngest layer may end up as the top layer

How do fossils help us to tell time?

Index Fossils Fossils of widely distributed organisms that lived during only one short period. Example: ammonite

Methods for Dating the Ages of Rocks: Relative Age Index Fossils Extrusion (igneous) Intrustion (igneous) Law of Superposition Folding Uncomformity *The age of rock compared to the age of other rocks *Sedimentary rock ( usually) Absolute Age Radioactive decay Radioactive dating Half-life Carbon-14 dating *The number of years since the rock formed *Igneous Rock

Radioactive Decay Elements in rocks can break down, or decay, over time. Scientists study the amount of decay to discover the age of rocks. During radioactive decay, the elements of one atom break down to form atoms of a different element. This rate (or amount/speed) of decay is measured by the half-life (how long it takes half the atoms to decay).

Radioactive Dating Half-Life The half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay. What pattern do you see in the graph?

Radioactive Dating Studying radioactive decay (breakdown of rocks) to determine the age of a rock. That information helps scientists calculate how old a rock is. In Carbon-14 dating, scientists see how much of the element carbon-14 is left in a rock. When carbon-14 decays, it turns into Nitrogen-14, so how much carbon-14 is left will tell scientists how long it has been decaying, therefore telling them the age.

Radioactive Dating Elements Used in Radioactive Dating

Geologic Time The Geologic Time Scale: A record of the geological events and the evolution of life forms as shown in the fossil records The divisions of the geologic time scale are used to date events in Earth’s history. It helps make it easier to understand how things have changed over time, or evolved

Eras Divided time between the Precambraim Time and the present time into 3 long units Periods Eras are subdivided into units of geological time * The Precambrian Time covers about 88% of Earth’s History and it ended 542 million years ago. Very few fossils have survived this time period

How has Earth Changed over time? Natural processes such as weathering, erosion and plate tectonics have reshaped Earth’s surface. The distribution of land and water on Earth has changed over time. Pangea is the large landmass that use to be on earth. Overtime, the landmass broke up into different smaller landmasses that have become our continents Uniformitarianism – geologic processes that we have today we also had in the past. This helps scientists understand why and how the Earth changed. The Earth is always changing or evolving. By studying Earth’s changes we see in the present, scientists can make inferences about the past. (like changes in rocks, weather, etc)

Geologic Time Pangea The Evolving Earth The timeline shows how Earth has changed over time. Pangea The name of the large landmass when the continents were formed into one large landmass.