Geologic History
Geologic History is the study and interpretation of Earth’s past
Describing age:
Relative age
Age of something compared to the age of other objects or events
Cannot tell the exact age
Absolute age
Age of something in years, months, days, etc.
How is relative age determined?
Major principles:
Uniformitarianism:
Geologic processes that took place in the past are generally similar to those that take place now
Principle of Original Horizontality
Sediments are deposited (usually in water) in horizontal layers (strata) that are parallel to the surface they were deposited on
Principle of Superposition
(in undisturbed layers) oldest layer is on bottom and each overlying layer is younger
Does not apply if layers have been overturned or faults have forced older rock over younger rock
Law of Cross-cutting Relationships
A disruption that cuts through rock or another geologic feature must be younger than the rock or the other geologic feature
Law of Inclusions
If a rock body contained fragments of another rock body, the rock with the inclusions must be younger than the fragments of rock it combined
What events can disrupts horizontal derock?
Igneous intrusions
Magma squeezes into rocks (at cracks and zones of weakness)
causes contact metamorphism (“baking”) of surrounding rock
An intrusion is younger than the rock it intrudes upon
Igneous extrusions
Lava reaches surfaces
Causes contact metamorphhism (“baking”) of rock below it (and surround it underground)
Erosion
Carrying away of weather rock materials often after uplift and emergence from below water’s surface
Unconformity: “gap” in rock record/a boundary between 2 non-sequential layers (most often an erosional surface) buried by younger sediments
Shown on a rock outcrop by a wavy line
Folding
Layers of sedimentary rock that have been bent/warped by crustal forces
Folds are younger than the rock from which they are formed
Tilting
Formally horizontal rock layers that have been tilted at an angle by crustal activity
Faulting
A crack of weakness in Earth’s crust along which movement occurs during an eq
Faults are younger than the rock in which they form
Why do scientists use patterns in rock strata (layers)?
To try to determine if the rock layers (and evidence of geologic events) in different locations are related
How can scientists use data from rock strata to see patterns in geologic events?
Correlation: matching rocks and geologic events in one location with rocks and geologic events in other locations
Method of correlation
Walking the outcrop:
Matching nearby rock layers that appear to be the same types and thickness
Outcrop: surface of exposed bedrock
Comparing properties
Matching the color, texture, and composition of the rock strata
How to Correlate Rock Outcrops In Diagrams
Draw arrows to match the layers in different outcrops
Helpful to look at the layers above and beloved the one you’re working with (although they could be missing for various reasons)
It may also be easier to use 2 arrows, 1 at the top of the layers and 1 at the bottom
To determine which layers are missing (due to an unconformity), compare sequences and look for the missing layer(s)
You can determine the oldest layer of all of the columns. It will be the bottom layer that does not correlate with another rock outcrop.
You can determine the youngest layer of all of the columns. It will be the top layer that does not correlate with another rock outcrop
Volcanic markers and meteorite deposits
Volcanic eruptions leave a layer of ash and meteorites leave rock particles and debris
Effective in correlation bc took place rapidly and covered a large area
Can be used to pinpoint the age of the ash layer or meteorite particles/debris
Index Fossils
Lived for a short time, so it appears in ONLY ONE layer per outcrop
Covers a wide geographic area
Found over large areas so that we can used them to match layers separated by huge distances
Layers with same index fossils are the same age
What are major events that have occurred on Earth?
Big Bang was ~13.7 billion years ago
Earth formed ~4.6 billion years ago
Geologic time scale
Geologists have subdivided geologic time into units based on fossil evidence and events in Earth’s history
ESRT pages 8-9
For major decisions of time:
Precambrian Eon: first 88% of Eath’s history
No fossils (organisms were small and soft and most rocks were not sedimentary)
Precambrian Eon lasted ~4.05 billions years / Earth’s age ~4.6 billion = ~88%
Paleozoic Era (of Phanerozoic Eon): ~6.5% of Earth’s history
Invertebrates, fishes, amphibians, vertebrates, land plants, and land animals all appear
Paleozoic Era lasted ~300 million years / Earth’s age ~4.6 billion = ~6.5%
Mesozoic Era (Phanerozoic Eon): ~4.3% of Earth’s history
Evidence of dinosaurs, early birds, and early mammals
Mesozoic Era lasted ~200 million years / Earth’s age ~4.6 billion = ~4.3%
Cenozoic Era (Phaneorozoic Eon): Most recent ~1.1% of Earth’s history
Cenozoic Era lasted ~50 million years / Earth’s age ~4.6 billion = ~1.1%
Fozzils of modern plants and mammals
First modern humans emerged late in the Cenozoic era (~1.8 mya) and have existed for about ~0.04% of Earth’s history
1st modern humans ~1.8 million years / Earth’s age ~4.6 billion = ~0.04%
How has Earth changed over time?
Evolution of Earth’s atmosphere
Outgassing: Volcanoes began releasing water vapor, CO2, and N2 on early Earth
Photosynthesis begins: With CO2 in the atmosphere, photosynthetic organisms were able to evolve
Atmospheric composition evolved: B/c photosynthesis removed CO2 from the atmosphere and released O2
Oxygen revolution: increase led to evolution of most life forms that exist today
Evolution of Organisms:
Fossil record supports the change from simple to complex life forms (generally).
Extinction: end of a species
Most life forms have been extinct
Mass extinction of the dinosaurs 65.5 million years ago:
Large craters from asteroids found on Earth supports the hypothesis that impact events have caused mass extinction of life forms and global climate changes
How do scientists give events or fossils and absolute (exact) age?
Radiometric dating using radioactive decay: an unstable radioactive (“parent”) isotope will break down into another more stable element called a decay (“daughter”) product
Isotope: atoms of same element that have different masses (numbers of neutrons)
Ex.: C12 and C14 are both carbon atoms, but they are isotopes because C14 has 2 extra neutrons
Each radioactive isotope has a known rate od decay (can be used to determine age)
Half-life
Amount of time it takes for HALF of the atoms in a radioactive sample to decay
Decay of any individual atomic nucleus is random, but a certain fraction will decay in a given time
Will never change (even if you apply heat, pressure, involve it in chemical reactions, break it, etc.)
Each time one half-life passes, divide the amt of radioactive substance remaining in half (and amount of daughter increases so total of both = 100%)
Different radioactive substances have different half0lives. See ESRT pg1
**NOTE: A radioactive sample will never get to zero bc you can always divide the amount remaining in half even if it gets really, really small!)
Half-lives of each dating element:
Cabron 14: 5,700 years
Used to determine the age of “recent” (past 50,000 years) fossils/anything organic
Potassium 40: 1.3 × 109 (1.3 bil) years
Used to determine age of “old” fossils and rocks (~50,000 →~4.6 byo)
Uranium 238: 4.5 × 109 (4.5 bil) years
Used to determine age of Earth and solar system
Rubidium 87: 4.9 × 1010 (49 bil) years
Used to determine age of very old rocks
Rubidium-87 commonly occurs in minerals that contain potassium-40, so it can be used to verify the age of rocks previously dated by using potassium-40