Geology Reference
In-Depth Information
intense mountain building, extensive glaciations, widespread shallow seas, and the con-
tinued buildup of material onto the continental cratons, building the continents into
shapes resembling what you see today.
Constructing continents
The history of each continent is told in its rocks. Beginning with the ancient cratons that
formed during the Precambrian (see Chapter 18), geologists interpret the geologic his-
tory of the continents from the rock sequences and the stories they tell.
When the Paleozoic era began, there were six major continents on earth, none of them
as large as the modern continents. These continents moved around under the influence
of plate tectonics (see Part III). Rock layers on the modern continents indicate intense
periods of mountain building that occurred during the Paleozoic era as the continents
crashed into each other. Each continent grew larger through the
accretion of terranes,
and mountains were built along
mobile belts.
Here's what these processes entail:
Enlarging continents through accretion of terranes:
Rocky material from one con-
tinent can be added to another continent in a process called
accretion.
The foreign
materials (the new rocks) have a different history than the continent they are ad-
ded to and are called
terranes.
As the accretion of terranes occurs repeatedly
through time, the continents grow larger and have different shapes.
Building mountains along mobile belts:
When two continents collide, crustal ma-
terial along the edges is forced upward (I describe the details of this process in
Chapter 9). The result is elevated areas of topography — mountains — in a linear
pattern parallel to the edge of the colliding plates.
Reading the rocks: Transgressions and regres-
sions
Extensive sedimentary rocks formed in the Paleozoic era indicate times when vast, shal-
low seas covered the continents, depositing sandstone, shale, and limestone. These
rocks are informative because sedimentary rocks, as I describe in Chapter 7, record im-
portant information about the environments in which they are deposited — particularly
when the sedimentary rocks are formed by the settling of particles through water. In
such cases, the sediment particles are subject to the laws of physics and gravity that
still apply today. For example, a larger, heavier particle will settle out of water more
quickly than a smaller, lighter particle.
