1. Seafloor Magnetism, Seafloor Spreading:
Developing the Theory of Plate Tectonics
Reading:
4th Ed. Ch 3 Sec 5, 12-17, 24-25, 29-30, Ch 4 Sec 5, 12-13, 16
5th Ed. Ch 3 Sec 11, 12, 14-18, 25-26, 29-30, Ch 4 Sec 6, 12-13, 15
Graphics: (top) Earth topography, (bottom) Garrison, 4th Ed. Fig. 3.11, pg 69,
5th Ed. Fig. 3.11, pg 64.
2. Earthquakes and the Dynamic Earth
The Earth is not entirely solid
Earthquakes are not randomly distributed
Observations like these paved the way for a revolution in geology… the theory of
plate tectonics
Graphic: Garrison, 4th Ed. Fig. 3.11, pg 69, 5th Ed. Fig. 3.11, pg 64.
3. What is a Scientific “Theory”?
Popular usage – a hunch or guess which may be unproven or lack credibility
Scientific usage - an explanation of related observations which have been
verified multiple times by many groups of scientists and is generally considered
“true” by scientific experts
A theory: - makes testable predictions - can be enhanced to incorporate new
findings
Examples: atomic theory, gravitational theory, cell theory, evolutionary theory,
plate tectonic theory
Graphic: A theory in the making? Scientific research aboard an oceanographic
vessel. Courtesy Dr. J.Ortiz, KSU Dept. Geology.
4. Distribution of Land and Ocean Today
Northern hemisphere: 61% of surface is ocean, 39% is land
Southern hemisphere: 81% of surface is ocean, 19% is land
North polar area - covered by ocean (Arctic Ocean)
South polar area - covered by land (Antarctica)
Graphic: Garrison, 4th Ed. Fig. 1.2a, pg 4, 5th Ed. Fig. 1.2 pg 3.
5. Buoyancy and Isostatic Equilibrium
Floating objects displace a volume of fluid which is equal to their mass due to
buoyancy
Example:
Empty ships sit higher in the water than loaded ships
In geology, balancing lighter material in heavier fluid matrix is known as
isostatic equilibrium
Graphic: Garrison, 4th Ed. Fig. 3.3, pg 61, 5th Ed. Fig. 3.6 pg 60.
6. The Roots of Mountain Ranges
Large mountain ranges have deep roots due to the physical properties of Earth's
layers
As mountains erode their roots become shallower
This process is known as isostatic adjustment and is similar to buoyancy in
fluids
Graphic: Garrison, 4th Ed. Fig. 3.4, pg 61, 5th Ed. Fig. 3.7, pg 61.
7. Isostacy and the Shape of the Seas
Continental crust sits higher on Earth's surface than ocean crust because it is
lighter (less dense)
The change in elevation from continental to oceanic crust is due to isostacy
Graphic: (top) Garrison, 4th Ed. Fig. 3.2, pg 60, 5th Ed. Fig. 3.5, pg 59,
(bottom) Continental shelf and slope off Los Angeles, California, courtesy of
the USGS Pacific Sea-Floor Mapping Project.
8. Latitude and Longitude
Latitude indicates North-South position (Range: 90°N to 90°S)
Longitude indicates East-West position (Range: 0°E-360°E or 180°W-180°E)
Latitude and longitude uniquely specify the location of each place on Earth
North Pole=90°N 0°=Greenwich,
England Kent,
Ohio=41.15°N, 278.64°E
South Pole=90°S
Graphic: see Garrison, 4th Ed. App. III, Fig. 4, 5th Ed. Box 1.1, pg 9
9. Evidence for a Past "Supercontinent"
- Geometric “fit” of continents
- Fossilized animal and plant remains
- Types of rocks and their locations
Graphic: Configuration of rejoined continents based on geological evidence.
Courtesy of USGS.
10. Continental Drift
Hypothesis: Continents were once joined to form a single land mass which drifted
apart
Proposed Mechanism: continents plowed through ocean crust…
But…
- no “wakes” were found in the seafloor behind continents
- geologists didn’t yet believe that parts of the mantle could flow
Graphic: Garrison, 4th Ed. Fig. 3.9, pg 67, 5th Ed. Fig. 3.9, pg 63.
11. A Piece of the Puzzle - Measuring Ocean Depths
Bathymetry (ocean depth) can be measured using:
- weighted lines
- sound (acoustics)
- Earth-orbiting satellites (new!)
Acoustic bathymetry:
- a sound pulse is sent to the seafloor
- measure time needed for the echo to be received
- this time is related to the distance to the seafloor
Graphic: After Garrison, 4th Ed., Fig. 4.2, pg 94, 5th Ed., pg 89.
12. A Piece of the Puzzle – Seafloor Bathymetry
- Seafloor bathymetry was more complex than imagined
- Discovery of mid-ocean ridges was particularly unexpected
Blue/Purple=deep Green/yellow/red=shallower
Graphic: Garrison 4th Ed., Fig. 4.30, pg 116-117, 5th Ed., Fig. 4.31, pg
108-109.
13. Mid-Ocean Ridges and Abyssal Plains – There are Huge Mountains Under the
Sea!
Graphic: (top) Garrison, 4th Ed. Fig. 4.22a, pg 108, 5th Ed. Fig. 4.22a, pg 101,
(bottom) Garrison, 4th Ed., Fig. 4.21, pg 107, 5th Ed., see Fig. 4.21, pg 100.
14. Another Piece of the Puzzle – Paleomagnetism
Rotation of the Earth and its metal core creates a global magnetic field
The polarity of this field switches through time (“magnetic reversals”)
Graphic: Garrison, 4th Ed., Fig. 3.25b, pg 80, 5th Ed. Fig. 3.26b, pg 76.
15. Magnets and Rocks
When rocks solidify from molten magma, magnetic particles in the rocks align
with the orientation of the Earth’s magnetic field
Scientists can “tell time” by comparing patterns of magnetic reversals preserved
in rocks from different places
Graphic: Garrison, 4th Ed., 5th Ed. Fig. 3.27 pg 76.
16. There are Magnetic Stripes on the Bottom of the Sea!
Observations of the seafloor showed:
- There is a symmetrical “striped” magnetic pattern on either side of the
mid-ocean ridges
- Rocks are youngest at the tops of mid-ocean ridges and are older on either
side
Graphic: Garrison, 4th Ed., Fig. 3.26, pg 80, 5th Ed., Fig. 3.28 pg 77.
17. A Piece of the Puzzle - The Age of the Seafloor
- The seafloor is not all the same age
- The seafloor is much younger than the continents
Red=young Blue=old
Graphic: Age of the seafloor, courtesy of NOAA. See Garrison 4th Ed. Fig. 3.27
pg 81, 5th Ed. Fig. 3.29 pg 78.
18. Sediment Thickness
Sediment = material that accumulates on the bottom of the ocean
It takes a long time to build up a thick layer of sediments
Sediment is thin at the tops of mid-ocean ridges and thick on the abyssal plains
Graphic: Seafloor sediment thickness. Courtesy of NOAA. Light blue=thin, Darker
blue to red= thick.
19. From Observations to the Seafloor Spreading Hypothesis - Putting Together
the Puzzle
The Seafloor Spreading Hypothesis
The formation of new seafloor occurs at “spreading centers” along mid-ocean
ridges
Over time, these newly formed rocks are carried away from the spreading center
Hypothesis explains: age of seafloor, magnetic “stripes”, the existence of
midocean ridges, etc
Seafloor spreading animation
Paleomagnetism animation
20. But If New Seafloor Forms at Mid-Ocean Ridges…
Some of the Earth’s crust must be destroyed elsewhere,
But where???
Earthquakes provided the clue
Graphic: Garrison 4th Ed. Fig. 3.11, pg 69, 5th Ed. Fig. 3.11, pg 64.
21. Plate Tectonics: A New View of the Earth
Earth's surface is divided into rigid plates
Earth dynamics depends on plate motion – as plates move, new lithosphere forms
and old lithosphere is destroyed
Graphic: Garrison, 4th Ed. Figs. 3.14, pg 72 and 3.11, pg 69, 5th Ed. Figs.
3.15, pg 68 and 3.11, pg 64.
22. Preview of Next Lecture
Plate Tectonics: Story of a Dynamic Earth
Reading:
4th Ed., Ch 3 Sec 6, 16, 17, 19-23
5th Ed., Ch 3 Sec 13, 17, 18, 20-24
Graphic: The convergence of the Nazca and South American Plates has deformed and
pushed up marine limestone strata to form towering peaks of the Andes, as seen
here in the Pachapaqui mining area in Peru. George Ericksen, photographer.
Courtesy of USGS.