1. DO NOW
Turn in Review #8.
Pick up notes sheet.

2. Looking Back….
Which is:
Sedimentary?
Metamorphic?
Igneous?

3. Earth’s Crust and ISOSTASY
SES2. Students will understand how plate tectonics creates certain geologic features, materials, and hazards.
e. Explain how plate tectonics creates and destroys sedimentary basins through time.

4. Earth’s Lithosphere: Two types of Crust
Tectonic plates "float” on the asthenosphere. Level depends on thickness and density: Recall that density = M/V Changes in Mass OR Volume affect Density!
Ocean Crust
More dense
Continental Crust
Less dense

5. Continental crust Found under land masses. Older than oceanic crust.
Made of less dense rocks such as granite, schist and gneiss.
Thickness varies between 10 and 75 km (6 to 47 miles).
Felsic rock
silica
lighter color
Density avg. 2.7

6. Oceanic Crust Found under the ocean floor.
Made of more dense rocks such as basalt and gabbro.
About 5-10 km (4 miles) thick.
Mafic rock
less silica
heavier elements – magnesium & iron
darker
Density avg. 3.0

7. Crust responds to forces
Gravity:
Force pulling down on crust.
Buoyancy:
Upward force exerted by fluid asthenosphere.
To balance:
Less dense plates “float” higher.
More dense plates “float” lower.

8. Isostasy Balancing forces of gravity and buoyancy.
Equilibrium between the Earth's lithosphere and asthenosphere.
Isostatic Equilibrium = forces balanced.
Tectonic plates of different densities float at different “levels” depending on balance of forces.

9. Changes in Crust
Changes in crust density (mass or volume) affect how tectonic plates “float” on asthenosphere.
Mountain building increases mass, crust “sinks” into asthenosphere.
Weathering and erosion reduce mass, crust “rises” on asthenosphere.
Deformations can occur when the weight of part of the Earth’s crust changes.

10. Effects of Erosion & deposition
Erosion decreases mass and tectonic plate rises on asthenosphere.
Deposition increases mass and tectonic plate sinks into asthenosphere.

11. Isostatic Subsidence Occurs where sediment is being deposited:
Volcanic areas
Glaciers
Under water
Increases in mass on the crust causes it to sink until a new point of equilibrium is reached.
Often rock layers that sink become metamorphic rock.

12. Measuring Isostasy Hoover Dam: Constructed in 1935.
Added the weight of 24 billion metric tons of water.
Caused the circular area around the lake to settle.
Area sank approximately 5.5 feet.

13. isostatic rebound Occurs in areas of erosion where:
Mountains and landforms are eroding away.
Glacial ice is melting.
Lake or sea water is decreasing.
Decreases in mass on crust causes it to rise until a new point of equilibrium is reached.

14. Animation….
Another animation

15. Something to Think about…
Gravity is one of the forces that affects isostasy. What is the other force? A. weight B. momentum C. buoyancy D. electromagnetism

16. Isostasy Lab How does density effect isostatic equilibrium? Materials:
2 blocks of wood of different densities
Weights
Container/water
Metric ruler

17. ISOSTASY LAB Procedures:
1. Measure the mass (one decimal place) in grams and measure the length, width, and height in centimeters (two decimal places). Place measurements in the table below.

18. ISOSTASY LAB
2. Calculate the volume of each block of wood in cubic centimeters, cm3. (to two decimal places) and record below.
3. Calculate the density of each block of wood in g/cm3. (to one decimal place) and record below:

19. ISOSTASY LAB
4. Float Block A in the container of water. Using a metric ruler measure the amount of wood above and the amount of wood below the water's surface in centimeters. Record your measurement in the data table on the backside of this sheet.
5. Carefully place ten weights on the surface of block A evenly. Measure the amount of wood above and below the water's surface in centimeters. Record this information in your data table on the backside of this sheet.
6. Continue adding washers 10 at a time. Carefully measure and record the amount of wood above and below the water's surface after each addition. Stop adding washers when the wood sinks or the washers spill into the basin of water.

20. ISOSTASY LAB
7. Float Block B in the container of water. Using a metric ruler measure the amount of wood above and below the water's surface in centimeters. Record your measurement in the data table. 8. Carefully place ten weights on the surface of block B. Measure the amount of wood above and below the water's surface in centimeters. Record this information in your data table. 9. Continue adding washers 10 at a time. Carefully measure and record the amount of wood above and below the water's surface after each addition. Stop adding washers when the wood sinks or the washers spill into the basin of water.

21. ISOSTASY LAB

22. ISOSTASY LAB Analysis – Write in complete sentences:
1. Are there any differences in the way the two blocks of wood float before the washers are added? WHY?
2. What happens after the washers are added?
3. Which block of wood is able to hold more washers before it sinks? Explain.
4. How do the two blocks of wood resemble continental and oceanic crust? Which one (A or B) represents continental crust? Which represents oceanic crust? WHY?
5. How does the water represent the Earth's asthenosphere?
6. Which type of crust is able to support the most weight (continental or ocean)?
7. How does this investigation illustrate isostasy?

23. TO DO
Work on lab.