Geology Reference
In-Depth Information
Most of the rock-forming processes of the rock cycle depend on forces of movement,
heat, or pressure. For example, building mountains requires pressure exerted in two dir-
ections, forcing rocks upward or folding them together. This type of movement and the
associated forces of pressure are a result of continental plate movements. The idea that
the surface of the earth is separated into different puzzle-like pieces that move around is
a relatively new concept in earth sciences, called
plate tectonics theory
(the subject of
Part III).
Unifying geology with plate tectonics theory
For many decades, earth scientists studied different parts of the earth without knowing
how all the features and processes they examined were tied together. The idea of plate
movements came up early in the study of geology, but it took a while for all the persuas-
ive evidence to be collected, as I describe in Chapter 8.
By the middle of the twentieth century, information about the age of sea floor rocks
gathered during World War II had provided scientists with the evidence they needed to
build a theory of plate tectonics. After they recognized and accepted that new ocean
floor was formed along
mid-ocean ridge plate boundaries
(a place on the earth's surface
where two crustal plates are moving apart from one another), scientists could explain
how continental plates moved around on the earth's surface.
One of the things they realized was that the earth's crustal plates interact in different
ways. These interactions are described as plate boundary types and include:
Convergent boundaries:
At convergent boundaries, two crustal plates are moving
toward one another and come together. Depending on the density of the crustal
plates, this collision builds mountains, creates volcanoes, or causes plate
subduc-
tion
(meaning one plate goes beneath another).
Divergent boundaries:
At divergent plate boundaries, two crustal plates are separ-
ating or moving apart from one another. These boundaries are most commonly ob-
served along the sea floor, where the cooling of magma along the boundary cre-
ates a mid-ocean ridge, but they may also occur on continents, such as in the
African rift valley.
Transform boundaries:
At transform boundaries, the two plates are neither collid-
ing nor separating; they are simply sliding alongside one another.
In Chapter 9, I provide the details on the different characteristics of continental plates
and how they interact as they move around earth's surface, including the particular geo-
logic features associated with each plate boundary type.