Heat Flow at Ocean Trenches: Evidence for Plate Tectonics II
Integrated Science
Name
11/04
Per.
Introduction
In Heat Flow at Ocean Ridges: Evidence for Plate Tectonics I, we found evidence to support the idea plate tectonics
produces new (and hot) oceanic crust by volcanic activity at divergent plate boundaries. These areas are marked by
mid-ocean ridges. Plate tectonics also predicts that at the same time, old (and cold) oceanic crust is destroyed as these
tectonic plates plunge into the mantle at convergent plate boundaries (subduction zones). These boundaries are
marked by ocean trenches and island arc volcanoes. Extensive heat-flow studies have been made in attempting to
verify these predictions.
This lab exercise is designed for students to analyze heat-flow data from the Japanese Island arc/trench to
determine the relationships between major geological features, tectonic activity and heat flow. As you work, consider
these questions:
• How can heat flow data for the Earth's crust be collected and analyzed?
• What are possible explanations for the heat-flow characteristics of an island arc/trench 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 heat flow values are measured on both sides of an ocean trench, then heat flow values will be greater beyond
the subduction zone (ocean trench) as compared to measurements taken prior to the subduction zone.
Procedures and Data
1. Go to the website http://www.nature.nps.gov/geology/usgsnps/pltec/converge.html and read the notes on that
page. Be sure to study the three diagrams on the page.
2.
Use a tectonic plate map to write the names of the two tectonic plates shown below.
3. The Japanese Island Arc Map details heat-flow data collected at thirty different sites along the length of the arc.
Average world heat flow is 1.2 units (1 heat flow unit = 0.000001 cal/cm2/sec). Circle all sites that are greater than
this amount (and shade them RED, if possible). Box all points that are less than this average value (and shade them
BLUE, if possible). Do not mark areas that are equal to the world average value.
Data Analysis
Find the mean heat flow and standard deviation for each of the following areas.
Area
Mean
Standard deviation
Japan Trench Axis
(4 closest points)
Sea of Japan
Pacific Ocean Side of
Japan Trench
Japanese Islands
Discussion
1. On which side of the trench are heat flow values generally greater than the world average?
2.
On which side of the trench are heat flow values generally smaller than the world average?
3.
For each area studied, explain why the mean heat flow value is greater/less than the average world heat flow.
Area
Greater/less
Explanation
than world avg.?
(In terms of plate tectonics)
Japan Trench
Axis
Sea of Japan
Pacific Ocean
Side of Japan
Trench
Japanese
Islands
4.
What type of geological activity would you expect to occur in the highest heat-flow region? (Draw arrows  on
your Japanese Island Arc Map to show the plate movement that would cause the geological activity you describe.)
5.
Based on your analysis of the data (see questions 1 and 2), respond to the Hypothesis statement. Cite specific data
(mean values and standard deviation) to support your conclusion.
Conclusion
Discuss the relationship between your heat flow data patterns and the theory of plate tectonics. In particular, how does
your data support and/or contradict the hypothesis of sea floor spreading as the mechanism behind plate tectonics?