Unit 4 Section 4: The Relative Age of Rocks

​Science Root of the Day:

DO NOW: Write what you think the example words mean in your lab manual.

Extra Credit: Find three additional words that use this root and write them and their definition in your lab manual (6 Points Max)

​Lesson 6.2: The Relative Age of Rocks

Dropdown

An organism is​

Select answer

if it no longer exists and will never live again.

Multiple Choice

Scientists study the ________ to learn about the history of life, past environments on Earth, and how different groups of organisms have changed over time.

• Amber

• Fossil Reccord

• Fault lines

• Carbon Films

Multiple Choice

Which type of fossil can tell us about the activities of an organism?

• Mold/Cast

• Trace Fossils

• Petrified Remains

• Carbon Films

​Key Questions

• Describe how geologists determine the relative age of rocks

• Understand how rock layers can be changed by activities like folding

• Define radioactivity

• Describe what happens during Radioactive decay

• Identify what can scientists learn from radioactive dating

​Why Age Rock Layers?

• We can learn a lot about an organism just by looking at a fossil

  • Body structure

  • Activities

  • Diet

• A fossil on its own can not tell us when an organism lived

  • Need to study the rock layers around a fossil

​Relative and Absolute age

• Earth is always and has always been changing

  • Need to look at patterns that reflect changes

• Two Methods of expressing the age of a rock

  • Relative Age: its age compared to the other rocks around it

  • Absolute age:​ the number of years that have passed since a rock has formed

    • May be impossible to know in some situations

​Rock Layers

• Fossils are mostly found in sedimentary rock

• Geologists use the law of superposition ​to determine the relative age of each layer

  • In undisturbed horizontal ​layers, the oldest layer is on the bottom and the youngest layer is on the top

​Clues From Igneous Rock

• Lava hardens into igneous rock in several different ways

• Extrusion: Lava hardens on the surface, and is younger than the rocks below it

• Intrusion: Magma pushes rock layers back down below the surface and hardens inside the rock

  • Is younger than the rocks around it and below it

​Clues From Faults

• Faults break up the crust and move rocks

• A fault is always younger than the rock it cuts through

• Scientists can use when a fault was formed to age the rocks around it or can use the age of the surrounding rocks to determine the age of the fault​

Dropdown

The number of years that have passed since a rock has formed is the rock's ​

Select answer

Multiple Choice

The youngest layer shown here is an

• metamorphic boundary
• sedimentary layer
• igneous intrusion

Multiple Choice

Which of the following does the law of superposition tell scientists?

• Absolute age
• Relative age
• Exact age
• How long a Rock will last

Multiple Choice

Geologists use the law of __________ to determine the relative ages of sedimentary rock layers.

• Relative aging

• Fossilization

• Superposition

• Intrustion

Multiple Choice

A mass of igneous rock below the surface is called a(n)

• intrusion

• extrustion

• fossil

• index fossil

​Clues from Fossils

• Geologists often try to determine the relative age of one layer

  • They can then match other layers based on that one

• Index Fossil: a fossil that is widely distributed and existed for a short period of time

  • Tell the relative age of rock layers in which they occur​

  • Used to compare different areas

​How Can Rock Layers Change?

• Most of the geologic record has been lost to erosion

  • As layers erode a way, newer layers are exposed and have sediment deposited on them

• Unconformity: a place where new rock formation meets much older rock formations

  • Creates a gap in the geologic record​

​How Can Rock Layers Change?

• Some forces fold layers of the Earth so much that layers move to new locations

  • Older rock moves on top of newer rock

Multiple Choice

True or False: Most of the geologic record has been preserved

• True

• False

Multiple Choice

True or False: A unconformity is a fossil of an organism that existed for a short time over wide area of land

• True

• False

Multiple Choice

True or False: Folding can cause much older rock to sit on top of newer rock

• True

• False

Multiple Choice

A hollow area in sediment in the shape of a an organism is called a

• mold

• trace fossil

• cast

• carbon film

Multiple Choice

A gap in the geologic record that occurs when sedimentary rocks cover an eroded surface is called a(n)

• intrusion

• extrusion

• fault

• unconformity

What is Radioactive Decay?

• Radioactivity: the slow breakdown of certain elements

  • Not all elements are radioactive

  • Isotopes are forms of normal elements that are radioactive

• Radioactive decay: The Process by which an element releases particles and energy as it breaks down

  • An element breaks down and forms atoms of another element

What is a Half Life?

• The rate that an element decays never changes

• Half-Life: the amount of time it takes for half of a radioactive element to decay

Fill in the Blanks

Multiple Choice

True or False: The rate of Decay of a radioactive element changes frequently

• True

• False

Multiple Choice

True or False: Most elements do not change

• True

• False

What is Radioactive Dating?

• Igneous Rocks naturally contain some radioactive elements

• Scientists can measure the amount of radioactive elements in a rock and compare it to the half-life of those elements

Potassium-Argon Dating

• How scientists often date rocks

  • Used due to a very long half-life

• When Potassium-40 breaks down, it forms argon-40

  • has a half-life of 1.3 Billion Years

Carbon-14 Dating

• Carbon-14 is a radioactive form of carbon

  • Has a half life of only 5,730 years

• All plants and animals have carbon

  • After a plant or animal dies, Carbon-14 decays into nitrogen-14

• Only good for fossils younger than 50,000 years old

  • after that the amount of carbon is too small to be measured

Multiple Choice

The time it takes for half of a radioactive element's atom to decay is its

• era

• relative age

• absolute age

• half-life

Multiple Choice

Scientists use the method of _________ to find the absolute age of a rock

• radioactive dating

• relative dating

• Carbon-dating

• lava dating

Multiple Choice

True or False: Carbon-14 is used for dating fossils because it has a short half life

• True

• False