Page 88 - Earth's Climate Past and Future

64 PART II • Tectonic-Scale Climate Change

Ocean ridge crest
3.4 Myr ago IN SUMMARY, we can reconstruct the positions of
continents on Earth’s surface with good accuracy
Normal polarity
_ Reversed polarity back to 300 Myr ago and less accurately back to 500
_ Myr ago or earlier. Within the last 100 Myr, we can
compile spreading rates over enough of the world’s
2.75 Myr ago
ocean to attempt to estimate the global mean rate of
creation and destruction of ocean crust.
_
+
_ +
0.78 Myr ago Polar Position Hypothesis
An early hypothesis of long-term climate change focused
_ on latitudinal position as a likely cause of glaciation of
_ +
_ continents. The polar position hypothesis made two
_ + Today key predictions that can be tested over the younger part
of Earth’s history: (1) ice sheets should appear on conti-
_ nents that were located at polar or near-polar latitudes,
_ + but (2) no ice should appear if the continents were
_ + + 3.4 5.0 located outside polar regions. This hypothesis calls not
_ + 2.75 on worldwide climate changes to explain the occurrence
0.78
0.78 0 of icehouse intervals but simply on the movements of
3.4 2.75 Age (Myr) continents and tectonic plates across Earth’s surface.
5.0
The fact that modern ice sheets occur on the polar
FIGURE 4-6 Magnetization of ocean crust As molten lava continent of Antarctica and the near-polar landmass of
erupts at the seafloor, cools, and solidifies, successive bands Greenland makes this hypothesis seem plausible. Modern
of ocean crust form and are magnetized in the normal or ice sheets exist at high latitudes for several reasons: cold
reversed polarity prevailing at the time. As the plates move temperatures caused by low angles of incident solar radia-
apart, equal amounts of magnetized crust are carried away tion, high albedos resulting from the prevalent cover of
from the ridge axis in both directions and can be used to date snow and sea ice, and sufficient moisture to maintain ice
the seafloor. (Modified from F. Press and R. Siever, Understanding sheets despite melting that may occur along their lower
Earth, 2nd ed., © 1998 by W. H. Freeman and Company.) margins (companion Web site, pp. 3-11, 27-30).

4-3 Glaciations and Continental Positions since
back the recent motions of the seafloor and restore 500 Myr Ago
the continents and oceans to their positions during
the last 175 Myr. Second, the lineations in ocean crust We can directly test the validity of the polar position
can be used to reconstruct the rate of seafloor hypothesis against evidence in the geologic record. Over
spreading. Changes in the rate of spreading define the last 450 Myr, seafloor spreading has slowly moved
both the rate at which new ocean crust and lithos- continents across Earth’s surface between the warmer
phere are created at ocean ridges and the rate at which low-latitude climates and the colder high-latitude
older ocean crust and lithosphere are subducted at climates (Table 4-1). If latitudinal position alone con-
ocean trenches. trols climate, these movements should have produced

TABLE 4-1 Evaluation of the Polar Position Hypothesis of Glaciation
Time (Myr ago) Ice sheets present? Continents in polar position? Hypothesis supported?

440 Yes Yes Yes
425-325 No Yes No
325-240 Yes Yes Yes
240-125 No No Yes
125-35 No Yes No
35-0 Yes Yes Yes