Record in the Rock

1. 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  18 g  9 g  4.5 g 1 half life 2 half lives 3 half lives