Lecture 11: Plate Tectonics and the Earth’s Geography
generate colossal heat, which can melt rock and drive up mountain chains.
The Andes were formed above a subduction margin created as the Paci¿ c
plate dove beneath the South American plate.Where plates slide past each other at “transform margins,” such as the San
Andreas Fault in California, friction builds before being suddenly released in
earthquakes. Satellite images now enable us to measure tectonic movements
with great precision. Driving the entire machinery is heat from the center of
the Earth, created when the Earth was ¿ rst formed 4.5 billion years ago.It is now clear that the Earth’s plates have joined periodically to form
supercontinents such as Pangaea. Here, we summarize changes in the Earth’s
geography over the last two of these cycles. About 540 million years ago,
as the ¿ rst multi-celled organisms formed in the Cambrian period—the
oldest period of the Phanerozoic eon—the ancient supercontinent of Rodinia
was breaking up. About 420 million years ago, in the Silurian period, most
continental plates were gathered in the south; the ¿ rst bony ¿ shes and the
¿ rst trees appeared. About 300 million years ago, in the Carboniferous
period, continental plates were reassembling. This is the period of the ¿ rst
reptiles, winged insects, and amphibians, and the period in which fossilized
trees began to lay down huge beds of coal. About 180 million years ago, in
the Jurassic period, most continental plates joined to form the supercontinent
of Pangaea. The dinosaurs À ourished during this period, as did ferns and
conifers—and early forms of mammals, which (disappointingly) probably
looked a bit like shrews! About 60 million years ago, Pangaea was splitting
into two large continents, Laurasia and Gondwanaland. This is just after an
asteroid impact wiped out most species of dinosaurs, allowing the mammals
to À ourish in the niches they left behind. Today, the Earth is as fragmented
as it has ever been, a geographical reality that has had a profound impact on
human history. ŶChristian, Maps of Time, chap. 3.
Redfern, The Earth.Essential Reading