1. What Processes Shape our Earth?
Record in the Rock
What Processes Shape our Earth?

2. Geology
Geology: the scientific study of the origin, history, structure, and composition of the Earth
Importance:
Understanding of the forces that shape our Earth, so we can better forecast potential disasters

3. Earth’s internal structure
Characterized by a gradual increase in temperature, pressure, and density with depth
Crust is the outer most layer of Earth, consists of two types:
Continental: makes up continents, older than oceanic crust, thick
Oceanic: makes up the ocean floor youngest rocks, thin, mostly made of basalt (igneous rock)

4. Age of The Earth & Geologic Time
Principle of Uniformitarianism:
The order in which layers are deposited is how they relate to time.
The same processes that are at work today were at work in the past
*The present is the key to the past!

5. Age of The Earth & Geologic Time
Relative Dating: Finding the age of something, compared to something else. We use a number of principles and laws to do this:
Law of Original Horizontality – soil is deposited horizontally, then forms rock layers
Law of Superposition - the layer below is older than the layer above.
Scientist use law of superposition by using relative age (something compared to the age of something else)
Problem: uncomformity – buried surface that represents a break in the rock record.

6. Age of The Earth & Geologic Time
Relative Dating
3. Lateral Continuity- layers of sediment extend in all directions when they form.
4. Law of Cross-cutting Relationships - A rock is older than any rock across which it cuts. Folds & faults are younger than the layers they cut across.

7. Age of The Earth & Geologic Time
Inclusions: the inclusions (rock pieces) are older than the surrounding rock
Faunal Succession: fossils can be used to identify relative layers of rock
Index fossil:
Lived in a certain time span in many places
Lived in great numbers
Distinct features to identify

8. Age of The Earth & Geologic Time
Correlation: matching rocks by index fossil in different places
1st person to use correlation was William Smith
Smith & others developed the Geologic Column – a diagram of the sequence of rock layers in an specific area, in order of age

9. Early Estimates of Earth’s Age
Methods
Sedimentation – estimated the total thickness of the earth’s sedimentary rocks (rate of sedimentation)
Range from 3 million to 1.5 billion years
Problems
Rate can vary at different times & places
No accurate way to measure thickness
Started with different measurements

10. Early Estimates of Earth’s Age
2. “Salt” Method – estimated the salt content of the oceans, then compared it with the rate at which salt is being added to the oceans.
Range from 9 million to 2.5 billion years
Problems
1. assumed at first the oceans contained fresh water
2. did not account for all way salt is added to or removed from the oceans
3. Each scientist estimated amounts of salt

11. Early Estimates of Earth’s Age
3. Kelvin Method – assumed Earth was hot molten rock, he measured rate of Earth’s cooling to present. Also, took into account heat coming from within the Earth and from the Sun.
Range from 20 million years to 100 million years
Problems
No one knew about radioactivity

12. Absolute Age Dating
Absolute Dating: Enables scientists to determine the exact numerical age of rocks and other objects
Radioactive Decay :
Radioactive isotopes break down into other elements by emiting radiation
An element is defined by the number of protons it contains
As the number of protons changes with each emission, the original radioactive isotope (parent) gradually converted to a different element (daughter)
Ex: Uranium – 238, parent, changes into Lead -206, daughter

13. Radiometric Dating : Dating an object using radioactive isotopes
As the number of parent atoms decrease during radioactive decay the number of daughter atoms increase
Rate of decay never changes, but is based on elements half life
Half life: time it takes for half of the isotope to break down into another element
Uranium 238 use to determine age of non- living things
Carbon used to determine age of once living things

14. Dendrochronology :
Dendrochronology: use tree rings to determine absolute age
Uses tree rings called annual rings (early & late season)
The width of the rings depends on certain conditions in the environment
Trees from the same geographic region tend to have the same patterns of ring width for a give span of time
By matching the rings in these trees scientists have established tree ring chronologies up to 10,000 years

15. Ice Cores :
Ice Cores: they contain a record of past environmental conditions in annual layers of snow deposition
Summer ice tends to have more bubbles and larger crystals than winter ice
Ice-core chronologies study glacial cycles and climate

16. Varves :
Varves: bands of alternating light and dark colored sediments of sand, clay and silt
Occur with seasonal deposition of sediments around lakes usually
Sand-sized particles in summer (more) and thinner, fine-grained particles in winter (few)
Scientists can date cycles of glacial sedimentation over periods as long as 120,000 years

17. Radioactive Half-Life
Half Life: The amount of time required for half of a substance to decay
After one half life there is 1/2 of original sample left.
After two half-lives, there will be
1/2 of the 1/2 = 1/4 the original sample.

18. Example 1
You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years.
How many grams are left after one half-life?
Answer:50 g
How many grams
are left after two
half-lives?
Answer:25 g

19. Half-Life Example The half-life of iodine-131 is 8 days.
If you start with 36 grams of I-131, how much will be left after 24 days?
24 days/8days = 3 half lives
36 g  g  g  g
1 half life
2 half lives
3 half lives