Lecture #4 Geologic Time
1.Geologic Time - since 4.6 billion years ago
2.Geology and Astronomy
3.Rocks, fossils, and geologic structures characterize Earth's history, timeline of events
4.Environmental geology = recent events (< 1 million years old)
Need to understand all of Earth history - understand global change
Absolute and Relative Ages
Two different means of dating rocks and geologic events:
1) Absolute age = age of a rock in years.
requires a constant process (radioactive decay) and a record of the process (parent and daughter isotopes)
Examples - 14C to 14N, 238U to 206Pb
Rate of radioactive decay given by the half-life
Age of a rock determined by knowing the half-life of a reaction and measuring the parent and daughter isotopes
Each decay reaction has a time scale for which it is useful:
238U = slow decay (half-life = 4.5 billion years) - date old events
14C = rapid decay (half-life = 5730 years) - date recent events
Igneous rocks yield the most reliable radiometric ages for the timing of rock formation.
2) Relative age - age of one rock (or event) with respect to another rock (or sequence of events).
This method uses several principles
A) The Principle of Uniformitarianism
"The present is the key to the past"
Current physical processes have also operated in the geologic past (James Hutton, 1785)
We can predict future events using this principle
B) The Principle of Superposition
A sequence of sedimentary rocks is youngest on the top and oldest on the bottom
Processes of deposition (or lava flows) lay down a new layer on top of an older layer
C) The Principle of Fossil Succession
Groups of fossil plants and animals change form (evolve) with time in a definite and recognizable order.
Certain fossils are limited to a specific time interval
Example - Dinosaur bones are limited to a specific time 245-65 million years ago.
Using radiometric age determinations and fossils, geologists have developed the Geologic Time Scale - a table of ages (and divisions)that represents Earth's history. Below is a summary of the various divisions (e.g., eon, era, period, epoch) within the time scale. (the time scale is under constant revision):
Age of the earth = 4.5-4.6 billion years (4,600,000,000 years!) - meteorites, moon dust
the oldest rocks preserved on Earth are ~ 4.1 b.y. old
Precambrian = the largest block of time (88% of Earth's history) from 4.6 b.y. to 543 m.y.
Not well documented, lack of fossils (no hard parts)
The oldest evidence of life is 3.5 b.y. old bacteria
Precambrian/Phanerozoic boundary (~543 m.y.) = appearance of animals with shells ("evident life")
Paleozoic ("Old life") = Trilobites, crinoids, coral reefs, snails, appearance of fish, plants, amphibians, and insects.
abundant forests and swamps = massive coal deposits (Carboniferous Period, Miss. and Penn.)
Paleozoic/Mesozoic boundary (245 m.y.) = Mass extinction (Permian)
Over one-half of the families of organisms disappeared
Mesozoic ("Middle life") = Dominance of reptiles (dinosaurs), appearance of birds.
Includes the Jurassic Period (the real Jurassic Park!)
Mesozoic/Cenozoic boundary ("K-T boundary" ~65 m.y.) = Mass extinction (Chicxulub crater?)
one fourth of all families of organisms disappeared
extinction of dinosaurs opened the way for mammals.
Cenozoic ("Recent life) = Dominance of mammals, appearance of "humans"
Homo Sapiens sapiens, ~100,000 years ago
Ways to describe the depth of geologic time:
1) Think of geologic time as compressed into one calendar year.
2) Think of the age of the earth as the length of your arm.
People often think of the dinosaurs as an unsuccessful life form since they went extinct after 180 million years.
Can humans hope to survive even one percent (1.8 m.y.) as long?
Geologic Time Links:
Geologic Time and a glass of Beer
Do you believe geologic time could be that long?
Dating using Radiocarbon--how it works
Lecture 5: Plate Tectonics
Plate tectonics is a "grand unifying theory" in geology today.
Proposed by Alfred Wegener in 1910 as the theory of continental drift (Rejected!)
Plate tectonics - tektonikos = Greek for builder
Not adopted by a majority of scientists until the mid-1960's.
Explains many geologic processes and events, and the fit of continents (S.America & Africa)
Basic components
The outer layer of the Earth is broken into a number of pieces, or plates, that move with respect to each other.
Plates can consist entirely of oceanic crust (e.g. the Pacific plate) or of both oceanic and continental crust (e.g. the North American plate)
A tectonic plate is defined as the lithosphere
~cold, outer rigid layer where rocks are brittle
includes the entire crust and the upper part of the mantle, up to 100 km thick
Below the lithosphere is the asthenosphere, a layer of hot, weak rock that flows plastically instead of breaking,
from 100 to 300 km depth, contains only mantle rock (peridotite)
plastic layer allows the plates to "drift"
Heat flow and movement in the mantle drive plate movements
Plate Boundaries
Plate boundaries are identified by volcanism and earthquakes, and there are three different types of plate boundaries:
1.Divergent boundary - where plates move apart and new magma fills the void - CREATES CRUST
This boundary is defined by an oceanic ridge or spreading center
At the ridge, solid material moves away laterally from the central ridge
Produces matching stripes on both sides of the ridge.
Divergent boundaries on land and these areas are called rifts (example - East Africa = volcanoes and lakes).
2.Convergent boundary - where plates come together - DESTROYS CRUST
Often characterized by the presence of a subduction zone and oceanic trench (deep trough).
Old oceanic lithosphere is subducted, descends back into the mantle, and often partly melts
Magma is produced that rises to the surface and often becomes part of explosive volcanic eruptions
Forms volcanic mountains (Andes) or islands (called an island arc, e.g. Japan).
As subduction occurs, the plates grind past each other, and often the result is the generation of earthquakes
Varieties of convergent boundaries include:
ocean/continent (e.g., Andes, Cascades - Mt. St. Helens)
ocean/ocean (e.g., Japan, Indonesia)
continent/continent - continental crust will NOT subduct, huge mountain range develops (Himalayas)
The margin of the Pacific ocean is called The Ring of Fire
many earthquakes and volcanoes all around the edges
presence of convergent boundaries at all locations
3.Transform boundary - where two plates slide past each other on a vertical plane.
Boundary is defined by the presence of a major transform fault (e.g. San Andreas)
No crust is destroyed or created.
Plates slide past each other, build up friction, stress
Infrequent releases of tension, leading to numerous shallow, damaging earthquakes
Plate Movements
Plates move at speeds of 2 - 10 cm/year.
This means that Earth's surface is constantly changing, rearranging continents (or pieces) and recycling (oceanic) crust
Thus, in the past, continents were in different orientations (Pangea and Gondwanaland), at different latitudes (Antarctica has tropical fossils), and will continue to change position in the future.
How is plate tectonics relevant to Environmental Geology?
1.Geologic hazards - earthquakes, volcanoes, and landslides (this lecture)
2.Global change - use past changes to better predict future changes.
Plate tectonics rearrange the continents
Affects ocean and atmospheric circulation,
Played an important role in ice ages and climate variations throughout geologic time
Probably important to the evolution of life on Earth
3.Helps drive the formation and recycling of rocks (the Rock Cycle) and mineral and energy resources.
Plate Tectonics Links:
Plate Tectonics Page--UC Berkeley (animations of plate motions)
Global Earth History Page--N Ariz U
Lecture 6: Geologic Hazards
A geologic hazard is a natural Earth process that can cause death and/or destruction
Examples: earthquakes, volcanic eruptions, landslides, sinkholes, and floods
We will not discuss meteorological hazards such as hurricanes, tornadoes, or drought
Obviously negative impacts on society and the environment
But there are some benefits:
Floods bring rich soils to the floodplain for better crops
smaller earthquakes & volcanic eruptions may help prevent larger, more devastating events, etc.
The magnitude and frequency of hazards are two key components:
Magnitude - the size of the hazard (energy or matter relaeased)
Frequency - the number of times a hazard occurs over a given period of time (days, months, years)
Typically, there is an inverse relationship between the magnitude and frequency of hazards
small, harmless events are very common
large, destructive events are fairly rare
good examples are earthquakes and floods
Prediction and Risk Assessment
Predication of geohazards is of great importance but not very accurate at this time
Aiding in prediction are:
A) knowledge of the location
B) estimates of an event's probability
C) understanding of precursor events
D) ability to forecast events
E) Warning system/network
Risk assessment - combination of probability and consequences of an event if it does occur.
little risk to people and property if a hazard occurs in a remote and unpopulated area.
high risk if a hazard occurs in a densely populated urban area (Los Angeles or San Francisco).
Calculation of risk is complex and involves use of geologic, geographic, biologic, and meterological data
Human response to geologic hazards - three main classes
Reactive response - what happens after a hazard occurs?
Anticipatory response - pre-hazard response.
Prevention - what can humans do to stop a hazard?
Hazards Links: