Chapter 21 Section 4Geologic Time

Chapter 21 Section 4 – What You’ll Learn - page Before you read - write the reading’s objectives in this space: 1 – _____________________________________________ ________________________________________________ 2 -______________________________________________ ________________________________________________ 3 -______________________________________________ ________________________________________________ 4 - ______________________________________________ ________________________________________________ 5 - ______________________________________________

Chapter 21 Section 4 – What You’ll Learn - page Before you read - write the reading’s objectives in this space: 1 – Distinguish among geologic eras, periods, and epochs. 2 – Explain how scientists decide when a geologic time period ends and another begins. 3 – Explain the methods and logic used in relative dating. 4 – Interpret a geologic cross section. 5 – Describe the methods and logic used in absolute dating.

What is absolute dating? _________________________________________ _________________________________________ _________________________________________ What is relative dating? _________________________________________ _________________________________________ _________________________________________ NOTE - relative dating can only determine the sequential order in which a series of events occurred, not when they occur. 1

What is absolute dating? The process of assigning a precise numerical age to an organism, object, or event. What is relative dating? The process of placing objects or events in their proper sequence in time. NOTE - relative dating can only determine the sequential order in which a series of events occurred, not when they occur. 1

What does Uniformitarianism state? __________________________________________ __________________________________________ __________________________________________ NOTE - The only changes might be in their rate or scale. State the Principle of superposition: __________________________________________ __________________________________________ __________________________________________ __________________________________________ __________________________________________ 2

What does Uniformitarianism state? The laws of nature operate today as they have in the past. “The present is the key to the past.” NOTE - The only changes might be in their rate or scale. State the Principle of superposition: The oldest rocks in an undisturbed sequence of rock layers will be at the bottom of the sequence. 2

If I have sedimentary layers that are not horizontal what information can I say has happened? ____________________________________________ ____________________________________________ ______________________________ Sideling Hill, Md. Reveals a 350 million year old section of rock which was folded by a massive compression of the earth's crust some 245 million years ago (during formation of Pangaea), leading to the cut's distinctive "U" shaped fold which is called a syncline. 3

If I have sedimentary layers that are not horizontal what information can I say has happened? Some type of disturbance. Sideling Hill, Md. Reveals a 350 million year old section of rock which was folded by a massive compression of the earth's crust some 245 million years ago (during formation of Pangaea), leading to the cut's distinctive "U" shaped fold which is called a syncline. 3

The principle of cross cutting relationships states what? _________ _________________________________________ _________________________________________ _________________________________________ _ 4

The principle of cross cutting relationships states what? Any rock formation or fault is younger than the rock or feature that it cuts through. You can't cut across something that isn't already there! 4

What do Unconformities represent? ________________________________________________________ Angular unconformity 5

What do Unconformities represent? Gaps in the rock record during which erosion occurred or deposition was absent. Angular unconformity 5

Tim and Moby What are fossils? _____________________________ _________________________________________ The process of matching distinctive rock units from different regions is called correlation. What makes correlation useful? ____________________________ _________________________________________ What is an index fossil? _________________________________________ _________________________________________ _________________________________________ _____________________________ Ammonites lived about 240 million years ago. 6

Tim and Moby What are fossils? The remains or traces of organisms found in the geologic rock record The process of matching distinctive rock units from different regions is called correlation. What makes correlation useful? Scientists can use fossils to match the ages of rock units around the world. What is an index fossil? Fossils that lived for only a short time period but were widely spread throughout the world. Ammonites lived about 240 million years ago. 6

Paleontologists observe changes in the fossil record and associate them with boundaries of time units. At these boundaries, fossils of certain life forms are no longer present and new life forms begin to appear. 7

Radioactive dating is based on the fact that a radioactive isotope (form) of an element changes (breaks down) into an isotope of another element at a fixed rate. Each radioactive isotope, or radioisotope, has its own rate, expressed in terms of its half-life. Half-life is the time taken for a substance or collection of particles to decay by half of its original amount. 8

What is a geologic map? ______________________ ______________________ What does a geologic map show? ______________________ ______________________ 9

What is a geologic map? Geologic cross-sections represent a vertical slice taken through Earth. What does a geologic map show? Geologic maps show the horizontal surface distribution of various rock formations. 9

Explain how basins and domes are formed: _________________________ _________________________ _________________________ **In basins, the youngest rocks are exposed in the center and tilt or dip inward. **Domes are essentially the opposite of basins. 10

Explain how basins and domes are formed: First - The principle of original horizontality states that sediments are laid down in horizontal layers. Next- Compressional forces cause the layers to fold or to dip downward. **In basins, the youngest rocks are exposed in the center and tilt or dip inward. **Domes are essentially the opposite of basins. 10

Rocks subjected to the compression might wrinkle or fold, what is an upward wrinkle of rock formation called? __________________________________________ What is a downward wrinkle of a rock formation called? __________________________________________ Anticline and syncline in layered Precambrian gneiss along NJ Route 23 near the rest area exit ramp (west of Butler, NJ). 11

Rocks subjected to the compression might wrinkle or fold, what is an upward wrinkle of rock formation called? An anticline What is a downward wrinkle of a rock formation called? A syncline. Anticline and syncline in layered Precambrian gneiss along NJ Route 23 near the rest area exit ramp (west of Butler, NJ). 11

What are Plunging Folds? _________________________ _________________________ _________________________ _________________________ _________________________ ____________________ 12

What are Plunging Folds? If the forces are applied unevenly the axis of the fold might plunge back into the Earth, producing a surface pattern of rock like the one shown in the figure. 12

Section Check Question 1 _______ is the process of placing objects or events in their proper sequence in time. A. Relative dating B. Absolute dating C. Carbon dating D. Fossil dating

Section Check Answer The answer is A. When you date events relatively, you know which came first, second, and so on

Section Check Question 2 Why is Fossil C considered to be between 2.1 and 2.4 million years old?

Section Check Answer You could conclude from the rock layers at Location A that Fossil C is between 2.1 and 2.4 million years old, Fossil A is older than 2.4 million years, and Fossil E is younger than 2.1 million years

Section Check Question 3 Which time unit of the geological time scale is the shortest? A. eon B. epoch C. period D. era

Section Check Answer The answer is B. An epoch is the shortest time unit of the geological time scale