Geology Reference
In-Depth Information
ing billion saw the invention of distinctive processes that formed new types of rocks and
valuable ore deposits, as well as the first appearance of many new mineral species. And
mostcritically,geologicalevidencefromaroundtheworldisrevealingthatitwasatimeof
coordinated global plate movement that would establish new patterns still in place today.
The Supercontinent Cycle
Earth's familiar geography of oceans and continents is ephemeral, geologically speaking.
The Americas, Europe, and Africa framing the mighty Atlantic Ocean; the great eastward
sweep of the Asian continent; the expansive Pacific Ocean with its plethora of southern is-
landsandthecontinentofAustralia;andthepolarworldofAntarcticaarebutamomentary
configuration. The stately process of plate tectonics not only forms the continents but also
ceaselessly shuttles them across the globe. Land and water have been subject to extreme
makeovers time and time again.
An elite band of geoscientists has learned to tease out our world's ancient, alien carto-
graphy and has produced remarkable, if approximate, maps of the once and future Earth.
They have many clues to work with. First, we know how the continents are moving
today—howfastandinwhatdirections.YearbyyeartheAtlanticopenswider,Africasplits
intwo,andaswewatchamazed,IndiasmashesintoChina,crumplingtheimpactzoneinto
the jagged Himalayan Mountains. It all happens in slow motion, of course, but steadily, an
inch or two a year; over the span of a hundred million years, even a snail's pace can pro-
duce monumental changes. We play the imaginary videotape of Earth's geography back-
ward and forward, and can guess the features of our planet's capricious face. Even as far
back as a half-billion years ago, the rich fossil record of animals and plants can help sci-
entists sketch a picture, especially when the flora and fauna of widely separated continents
follow divergent evolutionary pathways. The varied marsupials of Australia, for example,
andthelargeflightlessbirdsofNewZealandtellacompellingstoryofzoologicalisolation.
Pushing back more than five hundred million years, the picture begins to fade; we must
seek other kinds of clues. Of special import is the fossil magnetism locked into volcanic
rocks. We tend to think of our planet's magnetic field in terms of a north-south orientation,
familiar from the alignment of a compass needle, but it is more complex than that. Mag-
netic field lines intersect Earth's surface at an angle, called the dip. At Earth's Equator, the
dip is close to zero—nearly horizontal—but at higher latitudes, the dip becomes steeper
and steeper until at the poles it is almost vertical. Exacting measurements of the ancient
magnetic field frozen into a volcanic rock thus can reveal both the north-south orientation
andthelatitudeofthecontinentwhenthoserockssolidified.Remarkably,suchsubtleevid-
ence shows that some rocks now at the Equator were once close to Earth's poles and vice
versa. Fossil evidence of former tropical lagoons in Antarctica and frozen tundra in equat-
