Lecture 23 - Causes of Long-Term Climate Change (p.3)

Q: What are the average global temperature ranges during the Phanerozoic?

Average annual temp ~+17°C, lowest ~+12°C, highest ~+22°C — a variation of about 10°C between icehouse and greenhouse conditions.

Q: What is the Polar Position hypothesis?

It proposes that ice sheets form when continents are located at or near polar latitudes, because ice sheets grow more easily on land than ocean and more easily at the poles than the equator.

Q: What evidence supports the Polar Position hypothesis?

Current ice sheets (Antarctica, Greenland) sit on polar landmasses, extensive Gondwana glaciations occurred 325–240 mya with continents at polar latitudes, and a brief glaciation occurred ~430 mya.

Q: What evidence contradicts the Polar Position hypothesis?

At 360 mya and 100 mya, continents occupied polar positions but no ice sheets were present, showing that polar position alone does not guarantee glaciation.

Q: What is the main weakness of the Polar Position hypothesis?

It considers only plate movement (Earth-internal processes) and ignores external climatic drivers, so it cannot fully explain icehouse–greenhouse transitions.

Q: What is the Sea Floor Spreading hypothesis?

It proposes that faster mid-oceanic ridge spreading increases CO₂ emissions from divergent and convergent plate boundaries (and hot spots), raising atmospheric CO₂ and causing greenhouse warming.

Q: What are the three CO₂ sources associated with plate tectonics?

Divergent plate boundaries (mid-oceanic ridges), convergent plate boundaries (subduction zones), and hot spots.

Q: How are sea floor spreading rates determined?

By age-dating oceanic crust and measuring distances between paleomagnetic reversals; velocity = distance ÷ time.

Q: What is the range of sea floor spreading rates?

Less than 1 cm/yr to over 10 cm/yr, with an average of ~5 cm/yr (equivalent to 5,000 km per 100 million years).

Q: Why can we only estimate sea floor spreading rates for the last 200 million years?

Because oceanic crust older than ~200 million years has been subducted and no longer exists on the ocean floor.

Q: How does the Sea Floor Spreading hypothesis explain the absence of ice sheets at 100 mya?

Despite extensive landmasses near the South Pole, fast sea floor spreading produced high CO₂ levels, keeping global temperatures too warm for ice sheet formation.

Q: What is the moderating role of silicate weathering in the Sea Floor Spreading hypothesis?

Silicate weathering acts as a negative feedback — increased CO₂ and warming from faster spreading accelerate weathering, which removes CO₂ and partially offsets the warming.

Q: What happens when sea floor spreading decreases, according to this hypothesis?

Less CO₂ is emitted, temperatures drop, silicate weathering slows (negative feedback), and icehouse conditions can develop.

Q: What is the Uplift Weathering hypothesis?

It proposes that chemical (silicate) weathering is an active driver of atmospheric CO₂ rather than just a negative feedback — increased mountain building exposes more rock, accelerates CO₂ removal, and causes cooling.

Q: How does uplift increase chemical weathering rates?

Mountain building increases physical weathering (wind, freeze-thaw cycles, steep slopes, mass wasting), creating fresh surface area of silicate rock available for chemical weathering.

Q: What happens during times of increased uplift, according to the Uplift Weathering hypothesis?

Chemical weathering increases, removing more CO₂ from the atmosphere and causing global cooling (icehouse conditions).

Q: What happens during times of little uplift?

Chemical weathering decreases, less CO₂ is removed from the atmosphere, and greenhouse warming results.

Q: What two types of convergent boundaries cause mountain building?

Oceanic–continental collisions (e.g., Andes), which are more or less continuous, and continental–continental collisions (e.g., Himalayas), which are sporadic events.

Q: How does the Uplift Weathering hypothesis explain the Permo-Carboniferous glaciation?

The formation of the Appalachian Mountains (320–240 mya) from continent–continent collision increased chemical weathering, removed atmospheric CO₂, and contributed to major glaciation.

Q: Are the Sea Floor Spreading and Uplift Weathering hypotheses independent of each other?

No — more mid-oceanic ridge activity leads to more subduction and continent collisions, meaning more uplift AND more CO₂ emissions simultaneously.

Q: Which hypotheses are consistent with major icehouse–greenhouse conditions in the last 320 million years?

Both the Sea Floor Spreading hypothesis and the Uplift Weathering hypothesis are consistent with the observed pattern.

Q: Why is the Polar Position hypothesis insufficient on its own to explain long-term climate change?

Because there are periods when continents sat at polar latitudes without ice sheets forming, indicating that other factors (atmospheric CO₂ levels driven by volcanism and weathering) are also required.