1. Plate Tectonics and Climate

2. Glaciation on Continents – The Polar Position Hypothesis
Two Key Testable Predictions
When continents are near the poles they should have ice sheets
If no continents are near the poles no ice sheets should appear on Earth
Does not consider world-wide climate changes
Only considers positions of the continents

3. Seafloor Spreading Has Moved Continents
During the past 500 Myr continents have changed position between
Warm low latitudes
Colder higher latitudes
If latitude alone is the controlling factor, these movements should have produced predictable glaciations

4. Three “Icehouse Eras” During the Last 500 Myrs

5. South Pole Positions Correlate to Periods of Glaciation
Changes in the position of the pole
Slow movement of Gondwana across a stationary pole
430 Myr ago
S. Pole position consistent with glaciation in the Sahara

6. South Pole Positions Correlate to Periods of Glaciation
From 325 to 240 Myr ago
Gondwana continues to move across the South Pole
A huge region on the southern continent was glaciated

7. South Pole Positions Correlate to Periods of Glaciation
Gondwana’s glaciation ended about 240 Myr ago
It moved away from the pole and merged with northern continents forming Pangaea

8. The Polar Positions Hypothesis: Some inconsistencies
The first southern glaciation (430 Myr ago)
Brief in terms of geologic time
1 to 10 Myr in duration
The slow motion of Gondwana across the South Pole doesn’t easily explain a brief period of glaciation

9. Lack of Ice Sheets on Land over the South Pole
Land existed at the South Pole for almost 100 Myr without glaciation
This argues against the hypothesis being the only requirement for large-scale glaciations.

10. Lack of Ice Sheets on Land over the South Pole
After the breakup of Pangaea Antarctica, India, and Australia moved back over the South Pole.
No ice developed
Antarctica remained directly over the pole from 125 Myr ago to almost 35 Myr ago, but free from ice.
Again, this argues against the hypothesis being the only requirement for large-scale glaciations.

11. Pangaea’s Climate
Extended from high northern latitudes to high southern latitudes
Almost symmetrical about the equator
Wedge-shaped tropical seaway indented the continent from the east
Represented almost 1/3 of Earth’s surface. It spanned:
180o of longitude at it’s northern and southern limits, both near 70o latitude
¼ of Earth’s circumference at the equator

12. Climate Models Input . . . Sea Level Topography
Rock evidence indicates S.L. comparable to today’s
Topography
To minimize errors caused by incorrect guess as to the distribution of mountains
Interior land represented as a low-elevation plateau with a uniform height of 1000 m and gradually sloping towards the sea along the continental margins

13. Climate Models Input . . . Higher CO2 level than today
Compensates for a weaker Sun (about 1%)
This is because geologic evidence indicates a warmer Earth
Absence of polar ice
Fossil vegetation
Palm-like trees at latitudes as high as 40o were not killed by hard freezes on Pangaea
Indicates that the hard freeze limit was at a higher latitude than today’s limit of 30o to 40o

14. Precipitation on Pangaea
Arid low latitudes, especially in the continental interior
Large land area under the dry, descending portion of the Hadley Cell
Large expanse of land in the tropics
Trade winds lose moisture by the time they reach the continental interior

15. Supported by Evaporite Deposits
Mesozoic Rifting
Opens the Atlantic
Evaporites in
shallow basins
Salts precipitated in lakes or in coastal
margin basins
Limited exchanges of water with the ocean
Requires an arid climate
More evaporates precipitated during the later phases of Pangaea than during any time in the last several hundred million years

16. Temperatures on Pangaea
Patterns switch back and forth
between hemispheres with
changes in the seasons.
Continental interior
Season extremes of heating in summer and cooling in winter
May explain lack of ice sheets in high latitudes because summers were so warm that rapid summer melting prevented the build-up of snow.
Freezing average daily winter temperatures extended to 40o latitude

17. Monsoons on Pangaea
Strong reversal between summer and winter monsoon circulations
Winter Hemisphere has high
pressure over the interior of
the continent
- Weak insolation and high
radiative cooling
- Air sinks building high
pressure
- Air flows out towards the
ocean
Summer Hemisphere has
strong solar heating
- Air rises and a strong low
pressure cell develops.
- Causes a net inflow of humid
air

18. Monson Circulation and Seasonal Precipitation
Eastern margins from 0o to 45o latitude
Winds reverse directions between seasons
Extremely wet summers
Dry winters

19. Geologic Evidence – Red Beds
Permian – U.K.
L. Permian, Triassic
Palo Duro Canyon,TX
Triassic - CA
Sedimentary rocks stained red by oxidation
Wet season provides the moisture
Rust forms in the dry season or interval
Red beds are widespread on Pangaea and is consistent with the model of high seasonal changes in moisture

20. Effect of Pangaea’s Breakup on Climate
Northern Hemisphere continents moved farther northward
High latitude ocean water displaced
Steeper global temperature gradient resulted

21. Change in Oceanic Circulation
A single ocean (Panthalassa) with a single continent
Simple pattern
Separate continents
More complex circulation
Affects atmospheric circulation
Warm an cold currents
Conveyer

22. The BLAG Spreading Rate Hypothesis
Also known as the Spreading Rate Hypothesis
Proposes that climate changes in the last several hundred million years:
Caused mostly by changes in the rate of CO2 input to the atmosphere
CO2 input driven by plate tectonic processes
Named using initials of its authors
Robert Berna
Antonio Lasaga
And . . .
Robert Garrels

23. CO2 Released into the Atmosphere by Plate Tectonics
Most CO2 is released
At Mid Ocean ridges
By Subduction Volcanoes

24. CO2 Released into the Atmosphere by Plate Tectonics
A smaller input of CO2 is released at hot spots
Most are not associated with plate boundaries

25. Distribution of Hot Spots
Identified by volcanic activity and structural uplift within the last few million years

26. Rate of Seafloor Movement Controls Delivery of CO2 from Rocks into the Air
Rates of plate motion presently varies from plate to plate
South Pacific spreads up to 10X faster than the Mid-Atlantic Ridge

27. Age of the Seafloor
Magnetic data shows widely varying rates over millions of years
Continue to change

28. Fast Spreading Larger releases of CO2 to the ocean
Results in faster subduction
Larger volumes of carbon-bearing sediment and rock melt

29. Increased CO2 Causes an Initial Shift Towards a Greenhouse Climate
Activates increased chemical weathering
combined effect of temperature, precipitation, and vegetation
CO2 drawn out of atmosphere at a faster rate
Negative Feedback

30. Slow Plate Movement
Slow CO2 input results in cooling

31. A Colder Icehouse Climate
Decreased chemical weathering
Decreased removal of CO2 (greater amount remains in the atmosphere
Reduces the rate of cooling
Negative Feedback

32. Carbon Cycling in the BLAG Hypothesis
Carbon cycles continuously between rock reservoir and the atmosphere

33. Removal of Carbon from the Atmosphere
Carbon from chemical weathering
Ends up in shells of marine life
Forms sediments when marine organisms die

34. Return of Carbon to the Atmosphere
Suduction
Some sediment is scraped off, eroded and redeposited
Most is taken into Earth’s interior
Doesn’t reach the mantle
Returned to the atmosphere by volcanism

35. Does Data Support BLAG?
Data does seem to support the BLAG Hypothesis

36. The Uplift Weathering Hypothesis
Asserts that chemical weathering is:
The active driver of climate change
Not just a negative feedback to BLAG

37. Available Surfaces Affect the Rate of Chemical Weathering
BLAG views chemical weathering as responding to three climate factors:
Temperature
Precipitation
Vegetation
The Uplift Weathering Hypothesis considers availability of fresh rock and mineral surfaces to be weathered
This exposure can override the combined effects of BLAG’s three factors

38. Rock Exposure and the Rate of Weathering
As rocks an minerals physically disintegrate, the total surface area of the particles increases

39. Increased Surface Area Results in a Faster Weathering Rate
The proportional increase of weathering far exceeds the estimated result from changes in temperature, precipitation, and vegetation.

40. Uplift and Weathering
Tectonics results in the uplifting of Earth’s crust and the formation of mountains at many plate boundaries.
In regions of uplift exposure of freshly fragmented rock is enhanced.

41. Factors Increasing Weathering Rates in Uplifting Areas
Steep Slopes
Erosional processes are unusually active
Higher frequency of earthquakes in young mountain regions along plate boundaries
Dislodge debris and further weaken bedrock

42. Factors Increasing Weathering Rates in Uplifting Areas
Steep Slopes
Erosional pocesses called Mass Wasting are unusually active
Rock slides and falls
Landslides
Flows of water saturated debris
Removal of overlying debris exposes fresh bedrock

43. Mass Wasting or Mass Movement is . . .
the movement in which
bedrock,
rock debris,
or soil
moves downslope in bulk, or as a mss, because of the pull of gravity.
Examples

44. Rockfalls

45. Talus
An apron of fallen rock fragments that accumulates at the base of a cliff.

46. Yosemite Valley Rockfall, 1999
Two 80,000 ton slabs of an overhang broke off
Slid a short distance over steep rock and then flew 500 meters, launched as if from a ski jump
They shattered upon impact and created a huge dust cloud.

47. Debris Slide A coherent mass of debris moving along a surface
Rotational debris slide (slump) if the movement is along a curved surface.
La Concita, CA
(1995)
Debris in the upper part
remained mostly intact as
it moved in blocks.
Debris in the lower portion
flowed with rotational
sliding.
Earthflow and
Slumping

48. Earthflow
Earthflow in Santa Tecia, El Salvador, January 13, 2001

49. Factors Increasing Weathering Rates in Uplifting Areas
Steep Slopes
Mountain Glaciation
Pulverizes underlying bedrock
Carries sediment to lower elevations
Increases regional rates of chemical weathering

50. Factors Increasing Weathering Rates in Uplifting Areas
Steep Slopes
Heavy precipitation generated on
High but narrow mountain belts
Intercept moisture carried by tropical easterlies and mid-latitude westerlies
Large plateaus create their only monsoonal circulation (e.g., Tibetan Plateau) by pulling moisture from adjacent oceans

51. Tectonic Uplift Ocean-continent convergence
Subduction occurs relatively steadily over time
Total amount of high mountain terrain on Earth remains constant through time
- Locations and heights of individual ranges may vary

52. Tectonic Uplift
Continent-continent collision – the Himalayas and Tibetan Plateau

53. Active Tectonic Uplift Cools Climate
Uplift accelerates chemical weathering
Draws CO2 out of the atmosphere
Cools climate
Greenhouse Conditions
Slower uplift
Less chemical weathering
More CO2 in atmosphere

54. Does Data Support the Uplift Weathering Hypothesis?
Data does seem to support the hypothesis.