GEOLOGY 117 -- THE OCEANS

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Instructor: Prof. Tom Johnsontmjohnsn@uiuc.edu

•One set of notes for each class; linked to class One set of notes for each class; linked to class schedule on web pageschedule on web page

•Short reading assignments to be done Short reading assignments to be done beforebefore classes; listed in class schedule on web pageclasses; listed in class schedule on web page

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Web Notes and ReadingsWeb Notes and Readings

What is Science?

•A collection of facts about nature?-It’s much more than that

•A process or method used to decide what is “true” and what is not?

-It is that, too.

•A collection of ideas dreamed up by scientists that keeps changing and is never really reliable?.....

Science is often reliable.

How science progresses, succeeds, and fails:

-We observe the world around us.

-We try to make sense of it all. If we succeed at that, then maybe we can predict what will happen, or find a way to make something useful.

-We make educated guesses- Hypotheses.

-Some don’t hold up – useless.

-Others do withstand the tests of time- useful!

-So we test them- BIG TIME

How science progresses, succeeds, and fails:

-Sometimes an idea becomes more than just an educated guess. After much testing, it really seems to work, and we call it a theory. Some theories are well established and reliable.

How science progresses, succeeds, and fails:

-Sometimes an idea becomes more than just an educated guess. After much testing, it really seems to work, and we call it a theory. Some theories are well established and reliable.

-Some theories are revised or overturned

-This does not mean that science is haphazard- it is part of the normal process

-We will examine the development of the theory of plate tectonics in this class. 

Subdivisions of ocean science:Marine geologyChemical oceanographyPhysical oceanographyBiological oceanography

This is the basic order of topics in this course.

Class #2 -- Geology 117

The Earth, Part 1

•How did the Earth and Oceans form?How did the Earth and Oceans form?•What does the ocean floor look like (shape)?What does the ocean floor look like (shape)?•Why is it shaped that way?Why is it shaped that way?•Why does the earth have oceans and continents Why does the earth have oceans and continents instead of just a single shallow ocean covering instead of just a single shallow ocean covering everything?everything?•How do we “know” all this stuff anyway?How do we “know” all this stuff anyway?•Do we really “know” or is it more “inference” than Do we really “know” or is it more “inference” than knowledge?knowledge?

The Oceans- Part 1The Oceans- Part 1

Marine GeologyMarine Geology

Latitude lines are “parallels”: constant distance between

Latitude

Longitude (East-West)

Why measure longitude in degrees???

Continents mostly in the N hemisphere: Gives Earth a slight “pear” shape

About 5 B.Y. ago: Formation of the solar system Gravitational collapse, rotating disk… dust and gas (mostly H and He)

•Then… accretion -- planets.

•Earth, other planets, meteorites at ~ 4600 m.y.

•Early molten(?) stage of Earth (first 200 m.y.):

- silicate mantle and metallic core formed

When did the earth form and how did it form? How did it get

to be what it is today?

Layering of the Earth Based on

Chemical Composition

Layer Depth

(km)

State Comp. g/cm3 T˚C

Crust --

Continental

55

(25-90)

solid

(rigid)

"granite" 2.7 <1000

Crust --

Oceanic

10

(5-10)

solid

(rigid)

basalt 3.0 <1500

Mantle to 2900 solid

(ductile)

Mg, Fe

silicates

4.5 1500-

3000

Core-

Outer

2900-

5300

liquid Fe, Ni

metal

11.5 4000

Core-

Inner

5300-

6370

soild Fe, Ni

metal

13.0 5000

Fig. 1.14 in Text

What is the evidence for this?

• The way earthquake waves travel through• Each earthquake sends out waves• Thousands of seismographs detect these

– e.g., Earthquake in Turkey, detected here

• Two types of earthquake waves– compressional waves, or “P” waves– shear waves or “S” waves

P and S wave material not in textbook

http://www.eas.purdue.edu/~braile/edumod/waves/Pwave.htm

http://www.eas.purdue.edu/~braile/edumod/waves/Swave.htm

P- waves(refraction, shadow zone)

S- waves(blocked by liquid, large shadow zone

Homogeneous earth would be like this:

Homogeneous earth would be like this:

P and S wave material not in textbook

http://neic.usgs.gov/neis/eq_depot/2002/eq_021103/ak_seismic_waves.html

P- waves(refraction, shadow zone)

S- waves(blocked by liquid, large shadow zone

Homogeneous earth would be like this:

More evidence used to infer what’s inside the earth...

More evidence used to infer what’s inside the earth...

• Meteorites– Some are fragments of planets;

broken apart billions of years ago

– Some are Fe Metal

– Some are Mg+Fe silicates like the earth’s mantle

How do we know when all this happened (“age dates”)?

• Radioactive elements– Decay produces “daughter” products

• Some radioactive elements have very long half-lives (e.g., Uranium)

• Buildup of daughter products can be used to determine ages of rocks and meteorites

Represents amount of lead

Represents amount of uranium