This is a text version of the notes presented in class.

Feel free to email the instructor (alisonjs@kent.edu) with any
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1.  Today we will look at two major features of the ocean floor, the mid-ocean ridges and the trenches.  They each have their own set of characteristics:

 mid-ocean ridges are characterized by high heat flow, shallow earthquakes, hydrothermal (hot water) vents and volcanic activity

 trenches, on the other hand, are characterized by low heat flow, and earthquakes ranging from shallow to intermediate to deep.  Near each trench we can always find a volcanic mountain chain-whether it is a volcanic island arc or a continental mountain belt.

 Clues to their significance can be found by looking at
Earthquakes – sudden release of energy that occurs when pieces of earth’s crust suddenly move relative to one another.
Volcanos-vents in the earth’s crust through which hot molten rock (magma), steam, and gases exit.
Heat Flow-the amount of heat reaching the earth’s surface from the earth’s interior.

2.  Are Earthquakes and Volcanos randomly distributed? Is heat flow randomly distributed?  No, there is a distinct pattern in their distribution.

3.  Interior Structure of the Earth
a. compositional description of layers-crust, mantle, core
b. physical description of layers-looking at behavior of materials-lithosphere, asthenosphere, mesosphere, outer core, inner core

4. Chemical Properties of Earth's Layers
Continental crust
- mostly granite (2.7g/cm3)
Oceanic crust
- mostly basalt (2.9 g/cm3)
Mantle
- silicon, oxygen, iron, magnesium (~3.3 g/cm3 for upper and 4.5 g/cm3 for lower mantle)'
Core
- mostly iron (~13 g/cm3)
Graphic: Garrison, Fig. 3.6, pg 65.

5. Physical Properties of Earth's Layers
Lithosphere
- crust and upper mantle
- cool and rigid
Asthenosphere
- hot, partially melted, slowly flowing part of upper mantle
Lower mantle
- hot, but not melted
- flows very, very slowly
Core
- liquid outer core
- solid inner core
Graphic: Garrison, Fig. 3.6, pg 65.

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, Fig. 3.8, pg 67.

7. 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, Fig. 3.7, pg 66.

8. 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

9.  So now let us take a look at what is happening to the denser parts of the lithosphere, the ocean floor.  In the mid-ocean ridges, we see magma from the asthenosphere rising up to the surface.  This is where new ocean lithosphere, which is also new ocean floor, is made.

a. rift valley of the mid-ocean ridge-has hydrothermal vents
b. fracture zones cross the ridge system
c.  example-Atlantic Ocean mid-ocean ridge

10.  Trenches, slabs of ocean lithosphere (ocean floor) are descending down into the asthenosphere, and are melted and recycled.

a. shallow/intermediate/deep earthquakes
b. rising melted material forms volcanic mountains
c. examples-Andes Mountain Chain, South America/Peru-Chile Trench
d. and Japan/Japan Trench

11.  How does this heat and magma move?  Convective Cells in the asthenosphere:

a. convection cells transport heat in the atmosphere, oceans, earth’s interior, lakes-a common way of dispersing heat.  Other ways-radiation, conduction.
b. mid-ocean ridges:  convection cells move heat & magma up
c. trenches:  convection cells move cool material down

12.  If this motion occurs now, then it must have occurred in the past, and if so, then the ocean basins and continent positions must have changed.

a. Old idea-Francis Drake, 16th century, fit of South America/Africa
b. Alfred Wegener, early 20th century, proposed “Continental Drift” Hypothesis
c. Technology  breakthroughs beginning before WWII and continuing through the 20th century led to modern concept of Plate Tectonics as the over-arching model or paradigm in Geology.

13.  Next lecture:  Plate Tectonics & the Dynamic Earth, Part II
Focus Question:  Why do the lithospheric plates move, and how do we know?
Readings:  Chapter 3, p. 70-86