Biology Reference
In-Depth Information
shortening (crumpling)—hence the devastating instability of the landscape.
Charles Darwin witnessed an earthquake at Valdiva, Chile, on 20 February
1835 that elevated the coast 3 m in a matter of seconds; and on 19 January
1835, he saw a “great glare of red light” that was the eruption of Corcovado
Volcano near the island of Chiloé in southern Chile. On 22 May 1960, the
largest earthquake ever recorded in the New World occurred near Concep-
ción with a magnitude of 9.5 on the Richter scale. Such events result from
three forces driving the uplift of the Central Andes. First, subduction of the
Nazca plate into the Peru-Chile Trench at 50-85 mm each year accounts for
the volcanic activity in the region and about one-fi fth the present height of
the mountains. Second, compression generated from the Atlantic spreading
center to the east forces South America against the Pacifi c/Nazca plate at an
annual rate of about 25-35 mm, causing shortening of the crust along the
western coast. The Nazca and Cocos plates originated from breakup of the
older Farallon plate in the late Oligocene around 28 Ma. Third, lithosphere
thinning (Garzione et al. 2008), for example, beneath the Ojos del Salado
Mountains in the southern part of the Central Andes, has contributed to
regional instability. The heat coming through the thin lithosphere weakens
the crust and allows it to be buckled to especially high elevations, as at
Cerro Aconcagua (6960 m).
Once great heights are attained, other processes are required to sustain
them to avoid widespread collapse. High elevations usually result when two
continental plates collide. Thus, the collision of the Indian plate with the
Asian plate caused the uplift of the Himalaya Mountains and the Tibetan
Plateau. Where one plate subducts under another, elevations are usually not
so high. If some sustaining mechanism were not operating in the region of
the Central Andes, it is estimated they would average about 2 km in eleva-
tion rather than the present 4 km.
The dryness of the coasts of Peru and Chili due to the cold offshore
Humboldt Current and the rain shadow of the Andes is one such mecha-
nism. It reduces outwash and river fl ow that would normally bring lubricat-
ing sediments into the subduction zone. As a result, the downward slide of
the Nazca plate is impeded, friction builds up, compression forces increase,
and high elevations are sustained (Lamb and Davis 2003).
Several other features of the Central Andes are explained by ongoing
geologic processes. The corrugated aspect of the Yungas is due to the Cor-
dillera Oriental being compressed against the subterranean extension of the
Brazilian Shield. The mountains are being held in place along their western
margin by movement of the Pacifi c/Nazca plate into the Peru-Chile Trench,
