Earth’s History

Warm Up/DO NOW What causes geologic change (changes to the land) on earth? What are the 3 types of plate boundaries? Which plate boundary causes sea floor spreading?

LEARNING WITH A FOCUS Student Learning Map Unit 1: Earth’s History Concept 3: Dating the Earth What information do humans get from rock layers, index fossils, and ice cores in order to paint a picture of Earth’s History?

Student Objectives By the end of this lesson students will be able to…. Identify the difference between absolute and relative age Determine the relative age of a rock using the law of superposition and index fossils Explain how ice cores help scientists study how Earth’s climate was in the past

Vocabulary Relative Age Absolute Age Law of Superposition Igneous Inclusion Index Fossil Unconformity

Agenda Do Now (15 min.) LEQ #3 Assessment (15 min.) Activator: Student Line (8 min.) Absolute vs. Relative T-Chart (12 min.) Sandwich Demo (10 min.) Group Challenge (10 min.) Law of Superposition Notes (15 min.) Ticket Out (5 min.)

Activator: Student Line Up I need 7 volunteers What is the exact position of _______ in the line? What is an estimate of _______ in the line? What is the difference between exact and estimate?

SOOO….. Looking at our list of vocabulary words, which do you think means exact? Which of our vocabulary words do you think means estimate? Relative Age Absolute Age Law of Superposition Igneous Inclusion Index Fossil Unconformity

Absolute vs. Relative Age (Draw a T Chart in your Notebook) Determining the age of an object in relation to other objects “Estimate” Index Fossils, Rock layers Example: A friend tells you she has 2 brothers – one that’s older than her and one that’s younger Absolute Age Determining the actual age in years of an object “Exact” Radiometric dating, half life, radioactive decay Example: I have one brother who is 8 years old and one who is 17 years old

Absolute = exact Relative = estimate Mnemonic Memory Help When you hear these two terms, think… Absolute = exact Relative = estimate

How old is old? Scientists use several strategies to determine age of rocks and fossils. Absolute Age: actual age of an object or how long ago an event occurred by using radioactive dating. Radioactive Dating: measures age by comparing the amount of radioactive element isotope present to how much is normally present in that material Relative Age: age of an object compared to another- “I am older than you”

Absolute Age Half life; the amount of time (years) it takes for 1/2 of the radioactive isotope to decay by 1/2, indicates the age of the object.

Radiometric Dating

What does it mean when we say an atom has “decayed”? At the end of each half-life, at approximately what percentage are the atoms “decaying”? 3. Does the amount of time you shake the box affect the outcome? 4. Do the number of atoms you start with affect the outcome? Explain. 5. How do scientists use radioactive decay to date fossils and artifacts?

What is the difference between relative dating and absolute dating? HOT Q 1: 4 Corners What is the difference between relative dating and absolute dating? Relative dating provides the exact age of a rock while absolute dating provides a rough estimate of the rocks age. Relative dating can estimate a rocks age only in relation to other objects and events while absolute dating can provide the actual age Relative dating uses several types of radioactive decay to determine an objects relative age while absolute dating only uses the half of uranium 235 Relative dating and absolute dating can only determine the rocks age in relation to other objects or events

So… Why Do We Care? Relative and Absolute Dating tell us… The age of the Earth as well as its history When the dinosaurs roamed the earth How the Earth was and where it is going Possibly Skip this slide? Need Transition

DO NOW (5 min) 1. List these eras in order from oldest to youngest (the present!): Mesozoic, Precambrian, Cenozoic, Paleozoic. 2. When did life first appear on Earth? During what era did this occur?

Now that we know how to date the earth, lets start dating! Sandwich Demonstration Each of you will be in charge of “a piece” of the sandwich As soon as I place your piece in the sandwich, start counting (silently)

The youngest layer is at the top Sandwich Demo Cont’d Drawing: Which ingredient is the oldest? Which ingredient is the youngest? How do you know? So basically what you are saying is that the oldest layer of the sandwich is on the bottom AND The youngest layer is at the top Draw the time in seconds next to each layer

Which layer do you think is the oldest and youngest? Law of Superposition Says: “ The rocks at the bottom are OLDER than the rocks above”

The Position of Rock Layers It can be difficult to determine a rocks absolute age. So… scientists use the law of superposition. According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Cornell Notes What is the Law of Superposition? In an undisturbed rock layer, the layer of rock on the bottom is OLDER than the layers above it. Makes Sense: Older sediments must be laid down before the younger ones can pile up on top What is the Law of Superposition?

Let’s Practice Together! Which rock layer is the oldest? Which rock layer is the youngest? Which is older, layer M or layer R? How do you know Put them in order from oldest to youngest Which is younger R or T? How do you know?

Think*Write*Pair*Share Is dating rocks using the law of superposition a form of relative dating or absolute dating? Explain your answer

Remember… Law of Superposition is a form of relative dating Relative dating is based on the rocks that are surrounding it, NOT the exact age We know which is older but we do not know HOW old exactly

TICKET OUT! Which rock is younger, limestone or basalt? How do you know? Which of the labeled rocks is oldest? How do you know? Which of the labeled rocks is youngest? How do you know? Name the rocks in order from oldest to youngest.

Do Now What is the Law of Superposition? Which rock layer is the OLDEST? How do you know? Which rock layer is the YOUNGEST? How do you know

Do Now What is the Law of Superposition? Which rock layer is the OLDEST? How do you know? Which rock layer is the YOUNGEST? How do you know

D A T I N G R O C K

Other Clues to Relative Age Clues From Igneous Rock Lava that cools at the surface is called an extrusion. Rock below an extrusion is always older. Magma that cools beneath the surface is called an intrusion. An intrusion is always younger than the rock layers around an beneath it.

Other Clues to Relative Age Faults (a break in the rock) are always younger than the rock it cuts through! Unconformities: An unconformity is a gap in the geological record that can occur when erosion wears away rock layers and other rock layers form on top of the eroded surface.

Fossils Fossils are usually found in sedimentary rocks. When an organism dies, its soft parts often decay quickly leaving only the hard parts to fossilize. Ex. Bones, Shells, Teeth, or Seeds

Using Fossils to Date Rocks! Scientists use index fossils to match rock layers. An index fossil must be widely distributed and represent a type of organism that existed only briefly. They are useful because they tell the relative ages of the rock layers they are found in.

HOT Q 2 Which layer is oldest? How do you know? Which layer is the youngest? Explain your answer in at least 2 sentences.

(write) Geological Time Scale Review Earth formed 4.6 bya (billion years ago) Precambrian=no plants or animals yet– mainly prokaryotes in ocean. Paleozoic Era= Animals/plants develop. Fish in the ocean and fern forests on land. Extinction at end. Mesozoic Era= Dinosaurs rule! Birds, mammals appear. Extinction at the end. Cenozoic Era= Age of mammals. This is our era! Most eras ended with a mass extinction (most species died)!!

Fossils give us clues about. . . How old a rock is. What life existed in the past. When big events happened. Climate in the past.

FOSSILS HELP FIND ROCK AGES Geologists can use fossils to estimate the age of the rock around it. This is called RELATIVE DATING. Relative Dating means finding the age of an object in relation to other objects.

INDEX FOSSILS (write) Fossils used to help date rock are called INDEX FOSSILS. GOOD INDEX FOSSILS ARE: Found around the world Around a short time

Index Fossils (write) Organisms that lived during a specific time IDENTIFY THE AGE OF ROCK THEY ARE FOUND IN INDEX FOSSILS MUST BE WIDE SPREAD They also help compare ages of rocks layers around the world

Fossils and Climate (write) Climate=average conditions like temperature, humidity, wind, and rainfall. Fossils show us how climate changes over time.

Ice Cores (write) Ice forms layers similar to that of rock Also follow law of superposition Contain elements that help describe climate of Earth Scientists drill down and pull ice cores up An ice core from Antarctica 3km long showed 740,000 years of history and 8 ice ages!!!

Set Up Your Notes First! (write) What is an igneous inclusion? What is an ice core? Definition 2 Facts Picture

Set Up Your Notes First! (write) What is an index fossil? What is an unconformity? Definition 2 Facts Picture

EOG PRACTICE Which statement BEST explains why Greenland, which is ice-covered and located in the Arctic region, has fossils of tropical plants? Greenland was once located in the tropics and had a tropical climate. Birds carried plant seeds from the tropics to Greenland. Plants used to be adapted to grow in ice-covered regions. Ocean currents carried the fossils to Greenland.

Reading about Earth’s History You are now going to read an article called “Dating the Earth: Rocks, Fossils, and Ice Cores” This tells you key points the EOG always ask its 8th graders As you read you will be required to take notes cornel style This information will be on your independent practice as well as the assessment….SO DO A GOOD JOB!