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Figure 2.18 K/T boundary at Raven Ridge, Colorado, 6 December 2008. Photograph ISS018-
E-11127, Gateway to Astronaut Photography of Earth, Lyndon B. Johnson Space Center, NASA.
of Pangaea's breakup in the Middle Jurassic around 175 Ma is shown in
fi gure 2.19 and at present in fi gure 2.20. Toward the end of the Cretaceous
and into the early Tertiary, the North American and South American plates
continued to separate, and the Pacifi c plate and its subplates were subduct-
ing under Mexico and Central America along the Middle American Trench.
The result of these tectonic events was compression, extension, and uplift
that created the Transvolcanic Belt beginning in the early Miocene about
23 Ma. The region includes many highlands, such as the Pico de Orizaba
(5650 m), Volcán Popocatépetl (5452 m), and Ixtacihuatl (5286 m). The
site of Paricutín in the state of Michoacán emerged to great fanfare in the
fi eld of a Mexican farmer in 1943. Ash completely buried the villages of
Paricutín and San Juan Parangaricutiro, leaving only the steeple of a church
pointing symbolically out of the ash toward heaven. Another prominent vol-
cano in Mexico is El Chichón, 650 km southeast of Mexico City. It erupted
in 1982, ejecting ash into the atmosphere and creating the spectacular sun-
sets of that year. It also focused attention on the climatic effects of volcanic
eruptions. Initially temperatures warm because of the greenhouse effect of
the ash layer, then cool from defl ection of incoming solar energy by ash and
droplets of sulfuric acid. The effects are short-term (a few months to a few
years), but with massive eruptions such as Toba in Sumatra around 74,000 BP
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