Estimated Time: 45 minutes Materials: Computer with internet access, “Puerto Rico Trench Gravity Anomaly” simulation
Engage
- Observe the Phenomenon: Start the simulation. Observe the initial gravity profile cross-section of the Puerto Rico Trench.
- Identify the Anomaly: Look at the “Target Anomaly” metric on the right. What is the value of the free-air gravity anomaly over the Puerto Rico Trench? (Answer: -380 mGal). Why is this negative anomaly considered “extreme” or a “mystery mass deficit”?
Explore
- Investigate Tectonic Parameters: In the left sidebar, locate the “Tectonic Parameters”.
- Adjust the Subduction Angle ($\theta$), Oceanic Crust Density ($\rho_c$), and Asthenosphere Density ($\rho_m$).
- How does increasing the subduction angle change the gravity anomaly profile?
- What combination of subduction angle and density contrast between the oceanic crust and asthenosphere produces the lowest (most negative) gravity anomaly?
- Collect Data: Use the following table to test different parameters. Record the minimum anomaly produced.
| Trial | Subduction Angle ($\theta$) | Oceanic Crust Density (g/cm³) | Asthenosphere Density (g/cm³) | Trench Depth (km) | Minimum Anomaly (mGal) |
|---|---|---|---|---|---|
| 1 | 45° | 2.9 | 3.3 | 8.4 | _____ |
| 2 | 75° | 2.5 | 3.6 | 10.0 | _____ |
| 3 | 15° | 2.9 | 3.3 | 4.0 | _____ |
| 4 | _____ | _____ | _____ | _____ | _____ |
Explain
- Analyze the Relationships: Based on your exploration, describe the relationship between trench depth and the gravity anomaly. Why does displacing dense rock with seawater create a negative anomaly?
- Investigative Phenomena: Turn on “Enable Crustal Thinning”. How does extreme tectonic extension affect the mass deficit? Now turn on “Enable Carbonate Platform”. How does adding this low-density sedimentary mass affect the overall anomaly profile?
Elaborate
- Model the System: Using the data you collected, adjust the simulation to recreate the target anomaly of -380 mGal.
- Deliverable: Write a short paragraph explaining the specific combination of processes (trench depth, slab subduction angle, density contrast, and any additional factors like crustal thinning or a carbonate platform) that are required to produce a gravity anomaly of -380 mGal in the Puerto Rico Trench.
Evaluate
- Extension: How might the changes you made to the parameters in the model relate to actual geological processes occurring over millions of years (temporal scale)? Explain how the relative ages of the subducting North American Plate and the overriding Caribbean Plate influence these density values and the resulting trench features.
Teacher Notes
- HS-ESS2-1 Alignment: Students use the simulation to develop a model (Deliverable) to illustrate the relationships between specific ocean-floor features (trench) and internal/surface processes (subduction, crustal thinning, carbonate platform deposition).
- Evidence Statements:
- 1a(i) Descriptions and locations of specific ocean-floor features: Students identify the Puerto Rico Trench and analyze its extreme gravity anomaly.
- 1a(iii) Internal processes and surface processes: Students model internal processes (subduction angle, crustal density, crustal thinning) and surface processes (carbonate platform deposition, ocean trench depth) to reproduce the gravity anomaly.
- 2a(iii) The flow of matter by convection… sinking of cold, dense crust back into the mantle exert forces on crustal plates…: Students observe how subduction parameters (angle, density contrast) affect the gravity profile as the crust sinks into the mantle.
- 2a(v) Matter is cycled between the crust and the mantle at plate boundaries…: Students model the boundary where the North American plate sinks beneath the Caribbean plate.