Unit 2: Historical Geology

• Topic 1: Historical Developments and Geologic Time

Principle of Uniformitarianism

• Proposed by James Hutton. (1726-1797).• Important in understanding current and past

geologic events.• After studying landforms and rocks, he came to

the conclusions that (1) “the present is the key to the past” and that (2) “the physical, chemical, and biological laws that operate today to shape Earth also operated in the past”.

• What does this mean? It means that the same geological processes at work now were also at work (i.e. shaping Earth’s surface) in the geologic past.

Principle of Uniformitarianism

• Example 1: If we wish to understand how the Grand Canyon formed, we need to study present day rivers with a focus on weathering and erosion.

• Example 2: Will a species go into extinction in the future? YES, since it has already happened in the geologic past. E.g. dinosaurs and

meteorite impact.

Principle of Uniformitarianism

• Points about the principle!

• Change is gradual - (effects can be seen slowly over time)• Uniform processes – (same natural processes occur over

time)• Long time period – (geologic time)• Processes have explainable causes

Principle of Catastrophism

• Catastrophists thought that Earth’s physical features (i.e. mountains, canyons, and almost all landforms) formed by sudden spectacular events called catastrophes. They also thought that catastrophes were produced by unknowable causes that no longer operate and cannot be explained by nature. Since these events would have had to be extremely large, the only way they could be explained is by the supernatural.

Principle of Catastrophism

• This principle has been rejected by many! SUPERNATURAL?

• But be careful of this! There are many good reasons to suggest that catastrophism may have some ability to account for certain events. Mass extinctions such as the one that removed the dinosaurs, for example, can be accounted for with catastrophic models.

Good Bye Dinosaurs!

Principle of Catastrophism

• Points about the principle!

• Sudden Events• Unknowable Causes• Unexplainable• Short Time Period – (rate of change is fast in relation to

processes)

• Would this not suggest that Earth is much younger? Maybe millions and not billions of years old.

• Example: In the Christian world, Noah’s Flood (catastrophic event) has been used to explain such phenomena as fossils, extinction of species, oceans, and rock/soil layers.

Absolute Versus Relative Time

• Relative Time: Places events in a before and after sequence without reference to when they occurred. Relative dating is often done by comparing events.

• SEQUENCE OF EVENTS!• NO NUMBERS!

Absolute Versus Relative Time

• Absolute Time: Tells how long or how long ago. INVOLVES NUMBERS!

• Absolute Dating: Refers to an exact date or time known for an event. For example, a birth date.

Relative Time Absolute Time- Superposition - Glacial Varves- Cross-cutting Relations - Growth Rings- Horizontality - Radioactive

Dating- Inclusions- Fossil Succession

(Index Fossils)- Unconformities

Techniques for Relative Time

ALL USED FOR INTERPRETING THE GEOLOGY OF AN AREA!

Techniques for Absolute Time

Relative Time• Stack of Books - Superposition

Which book was placed first? (I.e. the oldest)

Which book was placed last?

(I.e. the youngest)

What is up with the meter stick? Cross-cutting relations! The meter stick represents a latter event in the relative sequencing of events.

Law of Horizontality

• All sedimentary layers (or beds or strata) are originally deposited horizontally (or flat).

• If you observe sedimentary layers (or beds or strata) that are not horizontal (or flat), then you can conclude that they were moved following their formation.

Law of Horizontality

• Sedimentary rocks are laid down in a horizontal (i.e. flat)

manner.

Law of Superposition

• Developed by Nicholas Steno.

• States that in an undisturbed or undeformed sequence of sedimentary rocks (layers or beds or strata), the oldest layer is on the bottom and they get progressively

younger upward.

• It is more difficult to apply the Law of Superposition if the sedimentary layers have been disturbed by volcanic activity, folding, faulting, etc.

• Example:

Principle of Cross-cutting Relations

• An igneous rock is younger than the rock strata (or beds or layers) that it cuts across.

• A geological feature such as a fault is younger than the rock strata (or beds or layers) that it cuts across.

Igneous rock (dyke) cuts across rock units A, B, and C.

Fault cuts across rock units A, B, C, and D as well as the igneous rock (dyke).

Principle of Inclusions (Rock Fragments)

• Pieces of one rock found in another rock must be older than the rock in which they are found.

• Inclusions (i.e. rock fragments) from unit D is found in unit E, thereby proving that unit D is older than unit E.

William Strata Smith• He observed that each rock strata (or beds or

layers) in a sedimentary sequence contained fossils unlike those in the beds above or below. Essentially, he observed that the same fossils were in the same rock types.

• Fossils represent certain times and/or time frames. Therefore, some fossils that are found in rocks can be used to calculate the ages of rocks.

• Example: Olenellus Trilobites are Index Fossils (or Guide Fossils) for the Cambrian Period of the Paleozoic Era. INDEX FOSSILS CAN BE USED TO CORRELATE (OR MATCH) ROCKS (OR LAYERS) OF EQUAL AGE.

Principle of Fossil Succession (Index Fossils)• Fossil organisms succeed one another in

a definite and determinable order and therefore, any time period can be recognized by its fossil content. Founded by William Strata Smith!

• Note that fossils show progressive change, thereby documenting the evolution of life through time.

Fossil Correlation

• Also founded by William Strata Smith!

• How can rock layers be correlated (or matched)?

- Colour (NOT Dependable)- Texture Of Rocks- Type of Rocks- Fossil Content (using Index Fossils)

Example – Correlation – Rock Types

Example – Correlation – Fossil Content

Unconformities

• An unconformity is a gap in the rock record.

• Definition - A surface between successive strata (layers) representing a missing interval in the geologic record of time, and produced either by (1) an interruption in deposition or by (2) the erosion of depositionally continuous strata followed by renewed deposition.

• Three types include: angular unconformity; disconformity; and nonconformity.

Disconformity

http://www.pbase.com/dougsherman/stratigraphy

Angular Unconformity

Nonconformity

Contact Metamorphism

• When molten rock comes into contact with older rock, the heat causes a kind of baking that changes the original rock.

Contact Metamorphism

Show Geology Symbols

• Draw Them On The Whiteboard!

• Be sure to include symbols for rock types, contact metamorphism, and intrusions.

• Students NEED to be able to recognize events/features such as faults, folds, unconformities, inclusions, buried lava flows, magma intrusions, and surface erosion.

Some Symbols

Demonstrate Examples Of Interpreting Cross-sectional Diagrams

- Use Drawn Diagrams On Transparencies!- Worksheet/Assignment (Answer Key Included)

NOTE THAT A CROSS-SECTION IS AVERTICAL PROFILE OF EARTH’S SUBSURFACE ALONG A PARTICULAR LINE IN MAP VIEW.

Activity

• Construct a fictitious geologic cross-section on a blank sheet of paper. On the opposite side explain the historical geologic events.

• Exchange cards with classmates to practice interpretation of cross-sections.

Core Laboratory 1

“Interpreting Historical Geologic Events”

Techniques For Absolute Time

• 1) Growth (Tree) Rings

• 2) Glacial Varves

• 3) Radioactive Dating

• ALL PRODUCES NUMBERS!

Growth (Tree) Rings1 Growth Ring = 1 Year

In places where seasonal changes occur (e.g. Newfoundland), plants add on a new growth ring each year.

By simply counting the growth rings in a tree, one can calculate its age.

How many years old?

Glacial Varves• Annual (i.e. yearly) deposit layers.

• A varve is a pair of sedimentary layers that is deposited over the course of a single year.

• They are most common in high latitude areas or high altitude lakes (i.e. glacial locations) where there is a strong contrast in seasonal conditions.

• During the spring and summer, coarse sediments are deposited into the lakes by the melt waters from nearby glaciers. During the winter, the lakes and rivers freeze. Only the fine sediments that settle from the water column is deposited on the lake bottom. These fine sediments are often rich in organic materials from decaying algae. Thus, they create a much darker layer.

• By counting the pairs of coarse-light layers and fine-dark layers, geologists can determine how long it took for a particular sedimentary sequence to develop.

Glacial Varves

Note:Spring and Summer:

Coarse (light) sediments are deposited into the lake by the melt waters from nearby glaciers.

Winter:Lakes and rivers freeze. Fine (dark) sediments from the water column settle out and get deposited on the lake bottom.

Glacial Varves

• How many years?

• For example, 308 sedimentary layers means that there are 154 glacial varves, which represents 154 years of sediment deposition.

• Remember: 1 year = 1 varve = 2 sedimentary layers

Radioactive Dating

• See Notes!