Page 264 - Earth's Climate Past and Future

240 PART IV • Deglacial Climate Changes

BOX 13-2 CLIMATE INTERACTIONS AND FEEDBACKS
Giant Deglacial Floods

n an unusual landscape called the channeled scab- shaped and molded all of the features we see on Earth.
Ilands in Idaho and east-central Washington, the Overenthusiastic application of this otherwise useful con-
bedrock consists of thick sequences of basalt deposited by cept left little room for infrequent catastrophic phenom-
lava flows during a time of heightened volcanic activity ena; these events were rejected because they had not
some 15 Myr ago. As the North American ice sheets were occurred during historical time. The problem with this view
melting during the most recent deglaciation, the surface is that the human life span, and indeed all of recorded
of these ancient lava flows was eroded into shapes sug- human history, is extremely short in relation to the age of
gesting the violent action of water on an immense scale. In the Earth, and our perspective on “normal” processes is
the scabland region, deep canyons with nearly vertical narrow. With much longer life spans, we would naturally
walls were gouged into bedrock. At some locations, these take a much broader view of what is normal.
now-dry channels abruptly plunge over steep cliffs into In the 1950s, aerial photography revealed that the sca-
larger channels with depressions like those found at the blands were covered by giant ripple marks and gravel bars
base of modern waterfalls (but much larger). Huge boul- more than 20 ft high and spaced at intervals of 400 ft.
ders and displaced gravel and sand lie in the channels, but Working on foot, Bretz had not recognized these enor-
upland areas nearby have a thin cover of windblown loess mous features because they were masked by scrubby veg-
typical of much of the rest of the Pacific Northwest out- etation. This new evidence convinced most geologists that
side the scabland region. the scablands had indeed been flooded, and further stud-
The geologist Harlen J. Bretz, working in the 1920s and ies suggested that a discharge of some 25,000 to 30,000 m 3
3
1930s, came to the conclusion that these erosional fea- (750,000 ft ) per second flowing at 80 km/h (50 mi/h) was
tures must have resulted from a flood of immense propor- required to carve such a landscape. All the features Bretz
tions, one that within a few days carried a volume of water had noted were indeed the result of rushing water on an
equivalent to all of Earth’s rivers today. He suggested that unimaginably large scale.
the water in this flood ran wildly across the landscape, A likely source of the water was glacial Lake Missoula, a
gouging and eroding the lower terrain but leaving the proglacial lake ponded against the side of the ice sheet in
higher areas untouched before eventually flowing down Idaho. Although Bretz proposed only a single flood, the
the Columbia River into the Pacific Ocean. Bretz inferred multiple layers of sediment left by the waters implied
that the source of all this water was a rapid melting event dozens of floods. One possibility is that each time a lobe
on the southern margin of the Cordilleran ice sheet that of Cordilleran ice advanced far enough south to act as a
covered western Canada and extended southward into the dam, Lake Missoula filled up and released water in cata-
northwest United States. He speculated that a volcano strophic bursts when the blocking ice lobe melted back.
erupting beneath the ice margin had caused rapid melting Geologists have also found evidence that large lakes hun-
and the sudden release of an enormous torrent of water. dreds of kilometers wide and tens of meters deep existed
For decades Bretz’s ideas were rejected by geologists. underneath the thin western lobes of the ice sheet in the
At that time, most geologists took too literally the principle Canadian provinces of Alberta and Saskatchewan, and
of uniformitarianism, the concept that slow geologic that these lakes could have periodically released large vol-
processes working today have, over immense spans of time, umes of water to the north.

Other Climate Changes During and rose toward maximum values 10,000 years ago and as
the effects of the ice sheets and the reduced green-
After Deglaciation
house-gas levels diminished. By 6000 years ago, with
Scientists have investigated two important changes the ice sheets melted and the greenhouse gases close to
during the late-deglacial and postglacial interval—the full interglacial levels, the major factor left to influence
strength of north tropical monsoons and the warmth of northern hemisphere climate was the drop in summer
summers in north polar latitudes. Monsoons grew insolation and the rise in winter insolation toward
stronger and summers warmer as summer insolation modern values.