Rock record Condensed version of complete Rock Record –relative dating portion [See Rock Record to see complete version including more details on Rock layers, Record with fossils, and Absolute dating [radioactivity dating]

INTRODUCTION In the 18th century, James Hutton proposed the idea that the Earth was about 4.6 billion years old. (1726-1797) Hutton’s ideas raised serious questions about Earth’s age, because up until that time, most scientists thought Earth was only 6,000 years old, and that all Earth’s geologic features had formed at the same time.

EARTH’s AGE Hutton theorized that most geologic processes occur very slowly and that the same forces that change Earth’s surface now, such as volcanism and erosion, are the same forces that were at work in the past. In other words: The PRESENT is the KEY to the PAST

PRINCIPLE OF UNIFORMITARIANISM: Geologic processes that occurred in the past can be explained by current geologic processes.

GEOLOGIC TIME RELATIVE DATING ABSOLUTE DATING To estimate the time during which those changes occurred, geologists rely on: RELATIVE DATING ABSOLUTE DATING

Relative Dating Relative dating describes the process by which the age of an object or event is estimated in comparison to the age of another object or event. Rocks and fossils are placed in sequence of appearance. Ex: Who is older Mrs. Daniel or Mr. Riehemann?

Rock Layers One way to learn about Earth’s past is to determine the order in which rock layers and other rock structures formed. Sediments and lava flows form distinct layers as they are deposited Those layers of rock, called strata, show the sequence of events that took place in the past. [stratigraphy: study of strata]

Relative Age Using a few basic law and principles, scientists can determine the order in which rock layers formed. Once they know the order, a relative age can be determined for each layer. Relative age indicates that one layer is older or younger than another layer but does not indicate the rock’s age in years.

LAW and PRINCIPLES of relative dating Law of Superposition an undisturbed rock sequence will have the oldest layer at the BOTTOM “You build a layered cake from the bottom-up.”

Law of Superposition As the sediments accumulate, they harden into layers called beds.

Law of Superposition

Principle of original horizontality Sedimentary rocks and lava flows are generally deposited horizontally, i.e. they form horizontal layers. Left undisturbed, those layers will remain horizontal. Scientists can then assume that sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened AFTER the layers formed.

Principle of original horizontality

Superposition and Horizontality in the Grand Canyon

FOLD

Principle of cross-cutting relationships features, such as tilt, fold, intrusions, fault are YOUNGER than the rocks they affect.

FOLD

Crosscutting Relationships

Layers 1-4 are positioned oldest to youngest (superposition) 4 5 Formation 5 is younger than 1-4 since it cuts through them 3 2 6 1 Fault 6 is youngest because it cuts through 5

Which layer is the youngest? B C D

Which layer is the oldest? B C D

Which statement MUST be true? B cuts thru A C cuts thru B B cuts thru C D cuts thru A

Principle of INCLUSION when a feature contains pieces of other layers, it has to be YOUNGER. Notice that the dike has incorporated pieces of other layers.

An unconformity refers to a gap in the rock record due to EROSION Sedimentation can be disrupted periodically by major environmental changes such as sea level variations and tectonic activities. An unconformity refers to a gap in the rock record due to EROSION

D, J, I, A, G, TILT, B [intrusion], EROSION, E, C, F[intrusion], H

Can you date the features from oldest to youngest?

HIMALAYAS AND APPALACHIANS are FOLDED MOUNTAINS

5 4 6 3 7 2 1

Which of the letter combinations below belong to the same layer of rock before the fault disrupted the layer? C and D C and F G and I G and F

Which structure is older: B or X, and which structure is the youngest of all? B, B X, A B, A X, C

Absolute Dating Used to determine the numeric age of a rock formation Absolute age: the numeric age of an object or event, often stated in years before the present

Absolute Dating Methods Rates of Erosion - less dependable because rates of erosion can vary over millions of years. Rates of Deposition – an estimate, not always accurate Radiometric Dating – most accurate and reliable way of dating

Radiometric Dating Rocks contain radioactive materials that act as natural clocks Radioactive isotopes – nuclei emit particles and energy Scientists measure breakdown of isotopes to measure the absolute age of rocks.

Radiometric Dating Compares the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope. Half-life: the time required for half of a sample of a radioactive isotope to break down by radioactive decay to a daughter isotope The more daughter cells – the older the rock

Parent Daughter Half-Life Rubidium-87 Strontium-87 48.8 billion years Uranium-238 Lead-206 4.5 billion years Carbon-14 Nitrogen-14 5,730 years Carbon-Dating: Used to determine the age of bones, shells and other organic material less than 70,000 years old.