Transcript Lecture 2 -- Geology 117

GEOLOGY 117 -- THE OCEANS
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Instructor: Prof. Tom Johnson
[email protected]
Web Notes and Readings
•One set of notes for each class; linked to class
schedule on web page
•Short reading assignments to be done before
classes; listed in class schedule on web page
•Get to the web page through Compass
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 geology
Chemical oceanography
Physical oceanography
Biological oceanography
This is the basic order of topics in this course.
Class #2 -- Geology 117
The Earth, Part 1
The Oceans- Part 1
Marine Geology
•How did the Earth and Oceans form?
•What does the ocean floor look like (shape)?
•Why is it shaped that way?
•Why does the earth have oceans and continents
instead of just a single shallow ocean covering
everything?
•How do we “know” all this stuff anyway?
•Do we really “know” or is it more “inference” than
knowledge?
Latitude
Latitude lines are “parallels”:
constant distance between
Longitude (East-West)
Why measure longitude in degrees???
Continents mostly in the N hemisphere:
Gives Earth a slight “pear” shape
When did the earth form and
how did it form? How did it get
to be what it is today?
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
Layering of the Earth
Based on
Chemical Composition
Layer
Depth
(km)
State
Comp.
g/cm3
TÞC
Crust -55
Continental (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
15003000
CoreOuter
29005300
liquid
Fe, Ni
metal
11.5
4000
CoreInner
53006370
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/e
dumod/waves/Pwave.htm
http://www.eas.purdue.edu/~braile/e
dumod/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_depo
t/2002/eq_021103/ak_seismic_w
aves.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
uranium
Represents amount of
lead