Questions: Continental Rifting and Extensional Tectonics

Question 1

Geologists find active volcanoes erupting basalt along a continental rift zone. There is no subducting plate nearby and no evidence of a mantle hot spot. What process most likely explains the volcanism?

• A. Frictional heat generated by movement along the bounding normal faults melts the lower crust
• B. Asthenospheric upwelling into the thinning lithosphere undergoes adiabatic decompression melting as pressure drops, crossing the mantle solidus
• C. Radioactive decay concentrated in the thinned crust generates enough heat to produce partial melting
• D. Seawater infiltrating along rift faults reacts exothermically with hot mantle peridotite

Answer: (B)

Rift volcanism is driven by adiabatic decompression melting, not an external heat source. As the lithosphere thins, hot asthenosphere rises to fill the space. As this material ascends, pressure decreases while its temperature stays roughly constant — it crosses the solidus (the pressure-temperature curve below which rock is solid) and begins to melt. No extra heat is added; the same material that was solid under high pressure becomes molten at lower pressure. This is a fundamentally different mechanism than subduction-zone or hot-spot volcanism, which involve fluid flux and excess heat respectively.

Question 2

In a continental rift zone, the characteristic basin-and-range topography consists of alternating down-dropped blocks and upstanding blocks. What are these structures called?

• A. Horsts (down-dropped) and grabens (upstanding)
• B. Grabens (down-dropped) and horsts (upstanding)
• C. Anticlines (down-dropped) and synclines (upstanding)
• D. Thrust sheets (down-dropped) and footwalls (upstanding)

Answer: (B)

In extensional tectonics, normal faults form where the hanging wall slides downward relative to the footwall. Grabens are the down-dropped fault blocks that form the basin floors of rift valleys; horsts are the upstanding fault blocks between grabens that form the ridges and escarpments. The Basin and Range Province of the western United States and the East African Rift Valley both display this characteristic alternating graben-horst topography. The terms are easy to reverse — grabens are 'graves' (down) and horsts are 'high.'

Question 3

The volcanism associated with continental rifting is primarily driven by a mantle hot spot supplying anomalous heat from depth.

• T. True
• F. False

Answer: False

Rift volcanism does not require a hot spot. It is driven by adiabatic decompression melting: as the lithosphere thins and the asthenosphere rises, the ascending mantle material decompresses, crosses its pressure-dependent solidus, and melts — without any addition of extra heat. Hot spots (like Hawaii or Yellowstone) involve a separate mechanism: a thermal plume of unusually hot mantle material. While some rifts do overlie hot spots (e.g., the East African Rift may have a component), the rift process itself generates volcanism independently of hot spot activity.

Question 4

If extensional rifting continues long enough, the continental crust thins to zero, a new mid-ocean ridge forms, and the previously rifted continental edges become passive continental margins.

• T. True
• F. False

Answer: True

This describes the complete Wilson Cycle of ocean basin opening. Initial rifting (like today's East African Rift) progresses to a narrow sea (like the modern Red Sea), then a widening ocean (like the Atlantic). The stretched, formerly rifted continental edges — now separated by growing ocean floor — subside as the underlying lithosphere cools and become passive margins. The eastern United States coastline is a classic passive margin, preserving in its sedimentary record the entire history from rift-phase tilted fault blocks through passive subsidence and sediment accumulation.

Question 5

Explain why rising asthenosphere in a continental rift produces magma without requiring heat from an external source such as a hot spot or subducting plate.

Model answer: The key is that mantle rock's melting point depends on pressure. Rock that is solid at great depth — where pressure is high — can melt at shallower depths where pressure is lower, even at the same temperature. As the lithosphere thins and asthenosphere rises, this upwelling material decompresses (pressure drops) while remaining at roughly the same temperature (the process is adiabatic — too fast for significant heat exchange). When the rising rock crosses its pressure-dependent solidus, it begins to melt. The melting is caused entirely by pressure reduction, not by heating. This is adiabatic decompression melting.

This is the conceptual core of rift volcanism and also applies to mid-ocean ridge volcanism. Students often assume all magma requires an external heat source (like a hot spot) or a chemical flux (like water from subducting slabs). Decompression melting is a third mechanism that requires neither — just the pressure drop that accompanies mantle upwelling. The East African Rift volcanoes, Icelandic volcanoes, and mid-ocean ridge basalts all arise from this same process.