Geology Reference
In-Depth Information
hundred thousand years was visibly different from the last. Part of the reason is that we
have a more detailed record, but it's also the nature of life. Animals and plants, especially
creatures that colonize the lands, respond to Earth's cycles quickly—they evolve fast or
they die. As old species die out, new species take their places.
All the World's a Stage
Shifting continents of the past 550 million years continued to provide a changeable stage
fortheevolutionofEarthanditsincreasinglydiversebiota.Thestoryiswellunderstoodin
its basic outlines—a rather simple play in three acts.
Act One: The beginning of the Cambrian Period, 542 million years ago, found the
Proterozoic supercontinent of Rodinia broken into several large and scattered pieces. The
largest expanse, stretching from the south pole to beyond the Equator, was the sprawling
continent of Gondwana, named for a geologically revealing region of India. All of today's
southern continents plus a big swath of Asia were jumbled together in this one giant land-
mass measuring more than eight thousand miles from north to south. Other post-Rodin-
ia continents, all of which were located in the southern hemisphere, included the core of
Laurentia (what is today North America and Greenland) and several other big islands (in-
cluding much of Europe). A global ocean, all but devoid of land, dominated the northern
hemisphere. Over the next 250 million years, plates moved all the continents northward.
Laurentia more than doubled its size, by merging first with what would become Europe,
then with a significant part of Siberia.
Act Two: About three hundred million years ago, northward-tracking Gondwana col-
lided with Laurentia to form the most recent supercontinent, Pangaea. One of the most
spectacular geological consequences of this merging of Gondwana and Laurentia was the
closing of the ancient sea between North America and Africa, a collision that birthed the
Appalachian Mountains. Today the Appalachians, stretching from Maine to Georgia, seem
a relatively benign, well-rounded sort of mountain range. Such gently rolling topography
speaks to the power of erosion, for three hundred million years ago their jagged still-rising
peaks soared six or seven miles high, rivaling today's Himalayas as some of the mightiest
mountains in the history of the world. Lopsided Pangaea concentrated almost all of Earth's
dry land on one side of the planet, three-quarters of it located in the southern hemisphere.
For one hundred million years the appropriately named superocean Panthalassa (Greek for
“all sea”) surrounded Pangaea.
Act Three: The opening of the Atlantic Ocean commenced 175 million years ago, when
the great Pangaean landmass began to fragment into seven major pieces. First Laurentia
and Gondwana fractured, forming the incipient North Atlantic, while the growing contin-
ental split stretched ever farther to the northwest and southeast. Antarctica and Australia
