Understanding Earth: A Dynamic and Evolving Planet - The Changing Earth: Exploring Geology and Evolution - page 19

Rocks exposed at Earth's surface are broken into par-
ticles and dissolved by various weathering processes. The
particles and dissolved materials may be transported by
wind, water, or ice and eventually deposited as sediment. This
sediment may then be compacted or cemented (lithified)
into sedimentary rock.
Sedimentary rocks form in one of three ways: consolida-
tion of mineral or rock fragments, precipitation of mineral mat-
ter from solution, or compaction of plant or animal remains
(Figure 1.15c, d). Because sedimentary rocks form at or near
Earth's surface, geologists can make inferences about the envi-
ronment in which they were deposited, the transporting agent,
and perhaps even something about the source from which the
sediments were derived (see Chapter 6). Accordingly, sedimen-
tary rocks are especially useful for interpreting Earth history.
Metamorphic rocks result from the alteration of other
rocks, usually beneath the surface, by heat, pressure, and the
chemical activity of fluids. For example, marble, a rock pre-
ferred by many sculptors and builders, is a metamorphic rock
produced when the agents of metamorphism are applied to the
sedimentary rocks limestone or dolostone. Metamorphic rocks
are either foliated (Figure 1.15e) or nonfoliated (Figure 1.15f).
Foliation, the parallel alignment of minerals due to pressure,
gives the rock a layered or banded appearance.

TABLE 1.3 Plate Tectonics and Earth Systems

Solid Earth
Plate tectonics is driven by convection in the mantle and in
turn drives mountain building and associated igneous and
metamorphic activity.

Atmosphere
Arrangement of continents affects solar heating and cooling,
and thus winds and weather systems. Rapid plate spreading
and hot-spot activity may release volcanic carbon dioxide and
affect global climate.

Hydrosphere
Continental arrangement affects ocean currents. Rate of spreading
affects volume of mid-oceanic ridges and hence sea level.
Placement of continents may contribute to onset of ice ages.

Biosphere
Movement of continents creates corridors or barriers to
migration, the creation of ecological niches, and the transport of
habitats into more or less favorable climates.

Extraterrestrial
Arrangement of continents affects free circulation of ocean
tides and infl uences tidal slowing of Earth's rotation.

Source: Adapted by permission from Stephen Dutch, James S. Monroe,
and Joseph Moran, Earth Science (Minneapolis/St. Paul: West Publishing
Co., 1997).

How Are the Rock Cycle and Plate
Tectonics Related?
Interactions between plates determine, to some extent,
which of the three rock groups will form ( of matter that retain the characteristics of an element. More

than 3500 minerals have been identifi ed and described, but
only about a dozen make up the bulk of the rocks in Earth's
crust (see Table 3.3).
Geologists recognize three major groups of rocks— igneous ,
sedimentary , and metamorphic —each of which is character-
ized by its mode of formation. Each group contains a vari-
ety of individual rock types that differ from one another on
the basis of their composition or texture (the size, shape, and
arrangement of mineral grains).
The rock cycle is a pictorial representation of events lead-
ing to the origin, destruction and/or changes, and reforma-
tion of rocks as a consequence of Earth's internal and surface
processes (

Figure 1.16). For
example, when plates converge, heat and pressure generated
along the plate boundary may lead to igneous activity and
metamorphism within the descending oceanic plate, thus
producing various igneous and metamorphic rocks.
Some of the sediments and sedimentary rocks on the
descending plate are melted, whereas other sediments and
sedimentary rocks along the boundary of the nondescending
plate are metamorphosed by the heat and pressure generated
along the converging plate boundary. Later, the mountain
range or chain of volcanic islands formed along the conver-
gent plate boundary will be weathered and eroded, and the
new sediments will be transported to the ocean to begin yet
another cycle.
The interrelationship between the rock cycle and plate
tectonics is just one example of how Earth's various subsys-
tems and cycles are all interrelated. Heating within Earth's
interior results in convection cells that power the movement of
plates, and also magma, which forms intrusive and extrusive
igneous rocks. Movement along plate boundaries may result
in volcanic activity, earthquakes, and, in some cases, moun-
tain building. The interaction between the atmosphere, hy-
drosphere, and biosphere contributes to the weathering of
rocks exposed on Earth's surface. Plates descending back into
Earth's interior are subjected to increasing heat and pressure,
which may lead to metamorphism, as well as the generation
of magma and yet another recycling of materials.

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Figure 1.14). Furthermore, it shows that the three
major rock groups—igneous, sedimentary, and metamorphic—
are interrelated; that is, any rock type can be derived from the
others. Notice in Figure 1.14 that the ideal cycle involves those
events depicted on the circle leading from magma to igneous
rocks and so on. Notice also that the circle has several internal
arrows indicating interruptions in the cycle.
Igneous rocks result when magma crystallizes or vol-
canic ejecta, such as ash, accumulate and consolidate. As
magma cools, minerals crystallize and the resulting rock is
characterized by interlocking mineral grains. Magma that
cools slowly beneath the surface produces intrusive igne-
ous rocks (

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Figure 1.15a); magma that cools at the surface
produces extrusive igneous rocks (Figure 1.15b).

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