Biology Reference
In-Depth Information
Figure 2.19
The New World
and adjacent plates near
the time of breakup in the
Middle Jurassic circa 175 Ma:
1
=
North American plate;
2
=
South American plate;
3
=
African plate; 4
=
Ant-
arctic plate; 5
=
Indian plate;
6
=
Australian plate; a
=
Greenland; b
=
Madagascar;
c
proto-Rocky Mountains
and the western Mexican
cordillera; d
=
eastern
Mexican cordillera; e
=
=
Guiana highlands; f
Brazil-
ian highlands; hatched areas
represent low mountains.
=
(Oppenheimer 2002), Tambora in Indonesia in 1815, resulting in “the year
without a summer,” Krakatoa in Indonesia in 1883, Mount St. Helens in
the Cascade Mountains of Washington in 1980, and Mt. Pinatubo in the
Philippines in 1991, climates may cool by more than 1°C for a somewhat
longer period of time. During periods of active plate movement, 12-15 cm
a year when volcanism is greatest, as it was in the Jurassic and Early to
Middle Cretaceous, this may cause some cooling, but it is counterbalanced
by the warming effect of increased CO
2
.
Plate movement has decreased since the Middle Cretaceous and cur-
rently averages about 2-3 cm a year. Volcanic activity in the Transvolca-
nic Belt continues to the present; in 1985 Mexico City was damaged by a
severe earthquake of 7.8 magnitude in which an estimated 10,000 people
were killed and more than two hundred buildings were destroyed. Volcán
Popocatépetl began it latest eruption on 30 June 1997, videos of which can
be seen on the Web site of the Centro Nacional de Prevención de Desastres
(www.cenapred.unam.mx/mvolcan.html). In the eastern Transvolcanic
Belt, there is an assemblage of plant macro- and microfossils in the late
Oligocene Pié de Vaca Formation being studied by Sergio Cevallos-Ferriz,
Susana Magallon-Puebla, Enrique Martinez-Hernández, and colleagues at
the Universidad Nacional Autónoma de México (UNAM). It is an impor-
