Chapter 8: The Rock Record

Chapter 8- Pretest 12 questions

Section 1: Determining Relative Age

James Hutton 18th century geologist Physician and farmer Observed geologic changes that occurred on his farm Same changes that he observed on his farm also changed the Earth’s surface in the past. Studying the past we can learn about the future.

James Hutton observed small changes on his farm over a long period of time Suggesting the Earth is older than he thought. Earth is about 4.6 Billion years old.

Principle Of Uniformitarianism Uniformitarianism: a principle that geologic processes that occurred in the past can be explained by current geologic processes Basic foundation of geology!! “The present is the key to the past” – Charles Lyell

Age of Earth Even though the processes of the past and present are the same The rates of these processes over time vary Before Hutton Earth was only 6,000 years old Observations that changed his land happened very slowly Earth must be older than we originally thought Studying the order rock layers and other structures formed we can learn about Earth’s past

Relative Age Relative Age: the age of an object or rock layer in relation to the ages of other objects. Determining age from surrounding objects. Indicates that one layer is older than another layer But does not indicate the age in years. Sedimentary rocks are studied to determine relative ages Sedimentary rock layers are called strata

Exit Ticket  How old is Earth?

3/14 Warm up Explain relative age.

Yesterday Recap James Hutton Uniformitarianism Relative Age Age of Earth

What is the Rock Record? The “record” of rocks that currently exist. Help tell us how rock features formed and their relative ages

Which rock layer is the oldest?

Law of Superposition The principle that a sedimentary rock layer is older than the layers above and younger than the ones below it. Sedimentary rocks form when new sediments are deposited on top of old layers Sedimentary rock layers are called beds The boundary between two beds is called the bedding plane

Principle of Original Horizontality Sedimentary rocks normally form in horizontal layers Principle of original horizontality states that sedimentary rocks will stay in horizontal layers if left undisturbed. No geologic forces acting on it. Tectonic forces can overturn the sedimentary layers If sed layers are not horizontal the law of superposition cannot be easily applied

Sedimentary Rock Features Graded bedding Cross-beds Ripple marks

Superposition handout Look at column A What can you tell just by looking at this? What two principles would apply to A? Label the oldest rock layer and the youngest rock layer Color in the layers based on rock type Make a key of rock types between A and B

Exit Ticket  Draw an example of graded bedding, ripple marks or Cross bedding

3/15 Warm-Up Explain the principle of original horizontality

Yesterday Recap Law of Superposition Principle of original horizontality Sedimentary rock features

Unconformities Unconformity: a break in the rock record created when rock layers are eroded away or there is a gap in deposition Shows that deposition stopped for a brief period of time Rock has been eroded away by natural processes Uplift and erosion Creates a break in geologic time

3 Types of unconformities Nonconformity Stratified rock is deposited on top of unstratified layers Sediments may be eroded away New sediment is deposited on the eroded material

2. Angular conformity Horizontal layers are deposited on tilted layers Layers are folded or tilted Then new layers are deposited on top

3. Disconformity Layers are NOT folded, tilted or eroded Layers sink Deposition of new layers continues on top May look continuous but there is a large time gap

An unconformity is a gap in the rock record. According to the law of superposition all rocks beneath an unconformity are older than the rocks above it. An unconformity is a gap in the rock record. Missing information

Law of Cross-cutting relationships The principle that a fault or intrusion is younger than the layer it cuts through. Fault: is a break or crack in Earth’s crust which shifts its position Intrusion: igneous rock is formed when magma is injected into a rock which cools and solidifies

Superposition Handout Look at B: What is happening? Order of oldest to youngest layers? New rock type?  add to key Look at C: What type of unconformity is pictured? Look at D: What is pictured?

Section 1 Quiz #1 what is the age of Earth? a. 2017 years b. 6,000 years c. 13.6 Billion years d. 4.6 Billion years

#2 Sedimentary layers will remain horizontal if left undisturbed, this is based off of what idea? a. Law of Superposition b. Principle of original horizontality c. Stratification d. Uniformitarianism

#3 Determining the age of a rock layer based on surrounding objects is called? a. Absolute Age b. Assumed Age c. Relative Age d. uniformitarianism

#4 What is a break in the rock record caused by erosion and geologic activity known as? a. Unconformity b. A break c. A fault d. An intrusion

#5 A fault or intrusion is always older than the layers it cuts through, true or false? a. True b. False

Section 2: Determining Absolute Age

3/16 Warm-up Explain the principle of cross-cutting relationships

Absolute Age Relative age only indicates that one rock formation is older or younger than another layer Absolute age: the numeric age of a rock formation. Scientist use different methods to measure absolute age

Rate of erosion Studying the rate of erosion Ex: measuring the rate a stream erodes Only practical for features that formed in the past 10,000 to 20,000 years Niagara Falls 1.3 m eroded per year For about 9,900 years

Rates of Deposition Calculating the rate of sediment deposition Measure the amount of sediment deposited in an area over a long period of time = estimate age of rock Average of 30 cm of sedimentary rock are deposited every 1,000 years Not very reliable, provides just an estimate

Varve count Varve: sedimentary layers that are deposited in an annual cycle. Light colored bands of coarse sediment Dark bands of fine sediment Scientists can count the layers of varves and estimate the age Generally form in glacial lakes Summer large amounts of deposition occurs coarse grained Winter smaller amounts of deposition occurs  finer grained

Radiometric Dating A method of determining absolute age by comparing radioactive isotopes and stable isotopes Radioactive isotopes have nuclei that emit particles and energy at a constant rate. As large amounts of energy are released scientists use the natural breakdown of isotopes to date rocks

Half-Life As atoms emit particles and energy the atom changes into a different isotope of the same element or a different element. Original isotopes are called Parent isotopes Newly formed isotopes are called Daughter isotopes comparing the proportions of parent and daughter isotopes to determine absolute age Half-life: the time required for half of the parent isotope to breakdown to for daughter isotope. Ex: 10 parent isotope ----- 5 parent isotope left after 1 half life Second half life 2.5 and so on

Radioactive Isotopes Larger the percentage of daughter isotopes the older the rock is Scientist use different radioactive elements when dating rocks Uranium- used for dating rocks more than 10 million years old As long as they contain Uranium Argon-argon dating- used for dating rocks between 10,000 to 4.6 billion years old

Carbon Dating Used to date younger rocks that are less than 70,000 years old Wood, bones, shells and other material with in the rock layer may cause inaccurate dating Carbon-14 dating or radiocarbon dating Plants and animals contain carbon12 and carbon 14 Scientist compare the ratio of C12 and C14 in the sample

Exit Ticket  Describe what a varve is and where they form