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Lab 1: Geologic Time

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Amanda Whaling

on 2 October 2016

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Transcript of Lab 1: Geologic Time

Lab 1: Geologic Time
Monday, 12:30-2:20pm
What is geology?
the science that deals with the earth's physical structure and substance, its history, and the processes that act on it.

Quiz next time!
Review: Principles of relative ages
Read: Chapter 3
Relative Geologic Time
Like early scientists such as James Hutton and Nicholas Steno, we can use the concept of relative age to understand the complexities of earth's history.
We need to think of the rock record as a sequence of events.
Uniformitarianism
Principle of uniformity: states that the present is the key to the past.
Look at geological features of today’s Earth as being shaped by its past.
States that the processes that are going on today to shape the Earth are the same processes that have always been going on.

...The mudstone must have been soft when the dinosaur stepped here.
This dinosaur footprint is in a mudstone.
It tells us something about the relative age of the mudstone
"The past history of our globe must be explained by what can be seen to be happening now. No powers are to be employed that are not natural to the globe, no action to be admitted except those of which we know the principle."
~James Hutton
Additional Principles in determining relative age
Principle of Original Horizontality
Principle of Superposition
Principle of Inclusions
Principle of Cross-cutting relationships
Principle of Original Horizontality
Based on the premise that sedimentary and volcanic rocks are deposited in flat-lying layers, much like sleet or snow.
This means that if a sedimentary layer is not horizontal, it has been subjected to forces that have moved or changed it.

Horizontal Sedimentary bedding in E. Washington

Principle of Superposition
When first accumulating, younger layers of rock overlie older layers of rock
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Principle of Inclusions
The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation occurs when fragments of rock get caught up in magma or lava so as to become trapped in younger igneous rock.
Principle of Cross-cutting relationships
A rock feature that cuts across a body of rock is younger than the body across which it cuts.
One more principle of relative age...
Unconformity: when part of the rock record is destroyed, the erosional gap that is formed is called an unconformity.
Absolute Age: When we know the dates
EX: Civil War 1861-1865, World War II 1939-1945
How do we assign ages to rock?
Absolute Age
Isotopic (radiometric) decay
A chemical element consists of atoms with a specific number of protons in their nuclei but a different number of neutrons. Atoms of the same element with differing atomic weights are called isotopes.
Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter). The rate of decay is conveniently expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay.
Some isotopes decay slowly, or they have long half lives, and several of these are used as geologic clocks. The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks.
The Geologic Time Scale
The earth is ~4.5 billion years old
Humans have been around for less than .001% of the Earth’s life
How the time chart is organized:
Eons – Largest Time Segments
Era -  span of time marked by character, events, changes on earth, etc.
Period - the basic unit of geological time in which a single type of rock system is formed
Epoch – commonly used in the younger Cenozoic era since we have a more complete geologic record and understanding of events occurring in those times.

Other methods for determining relative age: Faunal Succession
Recognizes that life has changed throughout time.
As one group of organisms disappears from the fossil record it is replaced with new ones.
The same form is never exactly duplicated
This allows fossils to be put in chronological order based on their appearance in the strata and the fossil record.

An index fossil is a fossil that occurred widely, but for a very brief time period. The use of index fossils and the other methods of reading fossils to make it possible to compare rocks that have been exposed in two locations

Methods of Absolute Dating include:
Dendochronology
Varve counting
Coral growth
Radiometric dating
Earth's history in a nutshell
Relative Age, cont'd.
Let's get started:
Relative Age
Relative age: we know the order of events but not the dates, specifically
EX: the civil war occurred before WW1
Why is geology important?
Geologists study some of
society’s most important
problems, such as energy, water, and mineral resources; the environment; climate change; and natural hazards like landslides, volcanoes, earthquakes, and floods. 
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