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ISL #6
Heat Flow at Mid-Ocean Ridges: Evidence for Plate Tectonics
Integrated Science 4
Name
5/15
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.
Heat Flow Values
3.0 and higher
2.0 to 2.9
1.0 to 1.9
0.0 to 0.9
Color
Red
Yellow
Green
blue
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 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 flow
Ascension
Azores
Canary Islands
Cape Verde Islands
St. Paul Rocks
Tristan de Cunha
7º S./14º W.
38º N./28º W.
28º N./18º W.
16º N/24º W
2º N/29º W
37º S/12º W
1 million years ago (mya)
20 mya
32 mya
150 mya
0.5 mya
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).
Red
Yellow
Green
Blue
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).
Red
Yellow
Green
Blue
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).
Red
Yellow
Green
Blue
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 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
1. Restate the theory of sea floor spreading.
2. Discuss the relationship between heat flow data patterns at mid-ocean ridges and the theory of sea floor
spreading.
3. 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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