Heat Flow at Mid-Ocean Ridges: Evidence for Plate Tectonics I 5/08
Integrated Science 4 Name Per.
Introduction
The Earth is a hot planet. In certain areas water is heated to its boiling point by hot rock just below the surface. Pressure builds up and a geyser of hot steam and water erupts. Volcanoes, spewing out molten rock and steam, also give direct evidence of internal heat. The temperature of molten lava is about 1,200º C. Indirect evidence indicates that temperatures in the Earth's core approach 6,500º C.– hot enough to melt iron! The First Law of Thermodynamics states that heat moves spontaneously from areas of high temperature to areas of low temperature until an equilibrium of temperature is achieved. Because the Earth's interior is hotter than the surface, there is a flow of heat energy outward to the surface. The quantity of heat lost is very small. However, with sensitive instruments scientists can measure the heat flow through continental and oceanic crust.
Even in areas far from volcanic action there is a small but measurable flow, and can be measured at the ocean floor by thrusting a device with two thermometers into the bottom sediments. After a period of time the lower thermometer will register a higher temperature than the upper one. The difference between the two readings determines the amount of heat flow. This is a very low value, with the worldwide average being 1.2 millionths of a calorie per square centimeter of surface area per second (0.0000012 cal/cm2/sec).
The theory of plate tectonics predicts that new (and hot) oceanic crust is produced by volcanic activity at divergent plate boundaries, which are marked by mid-ocean ridges. At the same time old (and cold) oceanic crust is melted as these tectonic plates plunge into the mantle at convergent plate boundaries. These areas are marked by ocean trenches and island arc volcanoes and should evidence increased heat flow. Extensive studies have been made in attempting to verify these predictions.
In this exercise you will map heat flow data from the Mid-Atlantic ridge to determine if there are any relationships between major geological features, tectonic activity and heat flow. As you work, consider the following questions:
• How is heat flow data for the Earth's crust collected and analyzed?
• Are there any patterns in world-wide heat flow data?
• What are explanations for the heat-flow characteristics of a mid-ocean ridge system?
• What is the relationship between heat flow data and plate tectonics theory?
Hypothesis
Consider this hypothesis as you gather and analyze your data:
If you move further from an ocean ridge, then heat flow values will decrease. This is because sea floor spreading is occurring at the ocean ridge causing lava to be exposed and land to be formed (also creating plate tectonic movement).
Procedures and Data
1. Heat flow values have been plotted on the attached map of the Atlantic Ocean Basin. Color the squares on the map using this color code to make any relationships more apparent.
3.0 and higher / Red
2.0 to 2.9 / Yellow
1.0 to 1.9 / Green
0.0 to 0.9 / blue
Procedures and Data, continued
2. Here are the names, locations and ages of oldest lavas for six active volcanoes in the Atlantic. Mark the location of each on your heat flow map with an X and note the date of the oldest lava flow in millions of years.
Volcano / Location / Oldest lava flowAscension / 7º S./14º W. / 1 million years ago (mya)
Azores / 38º N./28º W. / 20 mya
Canary Islands / 28º N./18º W. / 32 mya
Cape Verde Islands / 16º N/24º W / 150 mya
St. Paul Rocks / 2º N/29º W / 0.5 mya
Tristan de Cunha / 37º S/12º W / 1 mya
Data Analysis
1. What is the frequency distribution of each color in squares where ocean ridge is present? Show work:
(# of squares of certain color on a ridge ÷ total # of squares on a ridge of any color).
2. What is the frequency distribution of each color in squares adjacent to (next to) ocean ridge? Show work:
(# of squares of certain color next to ridge ÷ total # of squares next to ridge of any color).
3. What is the frequency distribution of each color in squares NOT adjacent to ocean ridge? Show work:
(# of squares of certain color not on or next to ridge ÷ total # of squares not next to or on ridge of any color).
Discussion
1. Consider the trends in your data analysis. Where are heat flow values generally greatest? Why?
2. Consider the trends in your data analysis. Where are heat flow values generally least? Why?
3. Describe trends in the location (relative to ridges) and age of volcanic lava flows (X’s on map).
Conclusion
4. Restate the theory of sea floor spreading.
5. Discuss the relationship between heat flow data patterns at mid-ocean ridges and the theory of sea floor spreading.
6. Describe sea floor spreading as a mechanism for plate tectonics. Include a drawing to illustrate the relationship between these two processes.
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