Case #1: Expedition Rift Zone
"Satellite lasers reveal that two landmasses are pulling apart by 2.5 cm annually. A long, sunken valley has opened with volcanic fissures venting hot basaltic lava."
Boundary:
Supporting Evidence:
Case #2: The Oceanic Abyss
"Sonar scans show an oceanic trench 10,000 meters deep. Directly parallel on the continental coast is an explosive volcanic mountain chain with frequent deep earthquakes."
Boundary:
Supporting Evidence:
Case #3: The Offset Orchard
"An orange orchard planted in 1960 in a straight grid line is now severed. Half the trees shifted 4 meters northward horizontally relative to the other half after a seismic event."
Boundary:
Supporting Evidence:
Case #4: Sky-High Marine Fossils
"Climbers at 20,000 feet in an inland mountain range discovered fossilized sea shells embedded in folded limestone rock layers. No active volcanic eruptions are recorded."
Boundary:
Supporting Evidence:
PART 4
Prompt: If new crust is constantly being created at mid-ocean divergent ridges all around the globe, why isn't planet Earth continuously swelling and expanding like a giant inflating balloon?
Write your scientific explanation using the concepts of subduction and rock recycling:
Plate Boundaries Investigation • 6th Grade Earth Science Page 2 of 2
| (Sliding Past) | Past each other: ⇅ or ⇄ (Lateral slide) | Neither (crust is conserved, not created or melted) | Fault lines, offset rivers/roads, frequent shallow earthquakes (no volcanoes). | San Andreas Fault (California), Alpine Fault (NZ) |
Plate Boundaries Assessment • Teacher Reference Page 1 of 2
Answer Key: Case Studies & Scientific Reasoning Teacher Reference Guide
PART 3 KEY
Case #1: Expedition Rift Zone [2 pts]
Boundary: Divergent (Continental Rift)
Key Evidence: Plates are pulling apart (2.5 cm/yr) and forming a sunken valley with volcanic fissures erupting new basaltic rock.
Case #2: The Oceanic Abyss [2 pts]
Boundary: Convergent (Oceanic-Continental Subduction)
Key Evidence: Presence of a deep 10,000 m oceanic trench paired with a parallel continental volcanic mountain chain and deep earthquakes.
Case #3: The Offset Orchard [2 pts]
Boundary: Transform (Strike-Slip Fault)
Key Evidence: Horizontal ground offset of 4 meters shifting trees sideways along a fault line during an earthquake; no volcanism.
Case #4: Sky-High Marine Fossils [2 pts]
Boundary: Convergent (Continental-Continental)
Key Evidence: Ancient seabed pushed up to 20,000 ft by massive crustal folding; towering mountain peaks with no active volcanoes.
PART 4 KEY
Exemplar Student Response (Full Credit):
"Earth does not expand because the amount of new crust being created at divergent boundaries is balanced by an equal amount of old crust being destroyed at convergent boundaries. In subduction zones, heavy oceanic plates dive back down into the mantle, where they melt and are recycled. Earth's total surface area and volume stay constant through this planetary conveyor belt cycle."
• 1 pt: States Earth remains constant size.
• 2 pts: Identifies subduction/recycling of crust.
• 1 pt: Mentions balance between creation & destruction.
Scaffolding & Differentiation Strategies
For Struggling Learners: Provide hand gestures: fist-to-fist collision (convergent), hands pulling apart (divergent), and hands rubbing past (transform).
For Advanced Learners: Challenge them to explain why oceanic crust always subducts under continental crust (higher density of basalt vs. granite).
Plate Boundaries Assessment • Teacher Reference Page 2 of 2
