Laboratory Manual for Introductory Geology 4e

in all five Earth System components to see which might cause problems for human
society in the next 50 years and which we will barely notice. We will also consider
the extent to which humans can modify these changes and, perhaps, avoid or
lessen the potential problems.
18.2.1 The Nature of Change: Cyclical versus Unidirectional
The Earth is essentially a closed system, meaning that its composition is fixed and that,
except for an occasional meteorite, it neither gains nor loses matter. As a result, the
Earth is the original recycler. Geologic processes reuse atoms to create new materials
from old ones, commonly in large-scale cycles (TABLE 18.1). Some Earth System
processes, however, are unidirectional: the materials that are changed do not have the
potential to return to their original state, so the process is never repeated exactly.
Unidirectional changes include the evolution of the solid Earth, the development
of the structure and composition of the atmosphere and oceans, and the origin and
evolution of life. Extinction of a species is a dramatic example: although the carbon
and other elements returned to the environment when a species becomes extinct
are indeed used by other species, that species is gone forever.
We have seen that organisms can play important roles in geologic processes, such
as physical and chemical weathering and the formation of organic and biochemical
sedimentary rocks. Even the evolution of the Earth’s unique oxygen-rich atmosphere
was due largely to the activity of blue-green algae (cyanobacteria) in early
Precambrian time. But in the last 2.5 billion years, humans, by far, have been the
organism that has most drastically affected global processes and cycles.
18.2.2 Changes in the Geosphere
Some properties of the geosphere will definitely not change much in 50 or 100 years,
or even in 100 million years. For example, the Earth’s shape and size will remain
the same, and it will continue to orbit the Sun at the same distance and velocity.
The Earth’s magnetic field may reverse polarity, but it will still exist. Plate-tectonic
TABLE 18.1 Selected Earth System cycles
Cycle What is recycled? Processes
Rock cycle
Tectonic cycle
Hydrologic cycle
Carbon cycle
Atoms in igneous, sedimentary,
and metamorphic rocks are
recycled to make new rocks
and minerals.
Lithosphere is created at midocean
ridges and consumed at
subduction zones.
Water changes between liquid,
solid, and gaseous states. It is
incorporated into and released
from plant and animal tissue.
Carbon passes from one
reservoir to another as carbon,
carbon dioxide, carbonate
minerals, plant and animal
tissue, and hydrocarbons.
Melting, crystallization, physical
and chemical weathering, chemical
and physical phase changes in
response to new conditions
Seafloor spreading, subduction
Evaporation, condensation,
freezing, thawing, chemical
weathering, volcanic eruption,
photosynthesis, decay
Photosynthesis, animal respiration,
decay, solution in water, chemical
weathering, volcanic eruptions,
fire, human use of fossil fuels
Earth System
reservoirs involved
Geosphere, atmosphere,
hydrosphere, biosphere
Geosphere
Geosphere, atmosphere,
hydrosphere, cryosphere,
biosphere
Geosphere, atmosphere,
hydrosphere, cryosphere,
biosphere
18.2 CHANGES IN THE EARTH SYSTEM
473