Geoscience Reference
In-Depth Information
If the sedimentary rocks remain near the surface, they too will
be weathered, causing sediments to be loosened from the host
rock mass. These sediments will then accumulate someplace
else to form rock of a potentially different character. Remember
that these are possible directions that the rock cycle might take.
It is possible that sedimentary rocks may be re-melted directly
to magma or that metamorphic rocks may be weathered to form
new sedimentary rock.
A good example of how the rock cycle works is the carbonate
environment of the Bahama Islands (see Figure 12.13). Recall
that in this area recrystallized calcium carbonate accumulates
on the shallow ocean floor. In the context of the rock cycle, this
carbonate likely originated from continental limestones that
were eroded, and the resulting calcium carbonate was carried
in solution by rivers to the ocean. Some of this carbonate is cur-
rently recrystallizing in the Bahama Islands to form new rock.
Some of that rock has been eroded and deposited on the shore
to create the beautiful white, powdery beaches of the islands.
Naturally, it has taken a long time for these combined processes
to occur.
represents 88% of Earth history, during which very little life
of any kind existed. Notable events during early Precambri-
an time include the initial stages of atmospheric develop-
ment, the emergence of the first cyanobacteria, and the onset
of photosynthesis. Toward the end of Precambrian time the
modern atmosphere evolved.
Beyond Precambrian time is the
Phanerozoic
Eon, which
is subdivided into three major
eras
, the
Paleozoic
,
Mesozoic
,
and
Cenozoic
. These eras are further subdivided into
periods
.
The beginning of the Paleozoic Era (the
Cambrian
Period) is
marked by the emergence of the first fish and shellfish about
540 million years ago, whereas the end of the Paleozoic Era is
related to a major extinction during the
Permian
Period around
250 million years ago. Other significant events in the Paleozoic
Era include the development of the first trees and amphibians at
the beginning of the
Devonian
Period (~417 million years ago)
and the first winged insects at the onset of the
Mississippian
Period (~354 million years ago).
The Mississippian Period is particularly noteworthy be-
cause organic deposits began to accumulate in sufficient quan-
tity to ultimately form the extensive coal and other fossil fuel
resources we use today. This period of massive organic depo-
sition began because the average global temperature during
the early Mississippian Period was quite warm, about 22°C
(72°F). Thus, many parts of Earth were covered by steamy
swamps that contained copious amounts of organic carbon. As
plants died, they slowly sank to the bottom of the swamps and
began to decay. These deposits were later buried by thousands
of meters of sediment; the resulting compaction and heating
caused thick beds of coal to form and many petroleum and
natural gas reserves to be created. This period of massive or-
ganic deposition terminated at the end of the Pennsylvanian
Period about 290 million years ago.
Following the Paleozoic Era is the Mesozoic Era. This
era is a definable unit of deep time because it is the inter-
val in which dinosaurs dominated life on Earth, beginning
around 250 million years ago and ending about 65 million
years ago.
Isn't it amazing that dinosaurs existed for over
180 million years?
In addition, the first birds and mammals
evolved during this era of time, specifically at the onset of
the
Jurassic
Period 206 million years ago. The Cretaceous
Period is significant because dinosaurs reached their maxi-
mum dominance during this time, most notably in the form
of the
Tyrannosaurus Rex
, and the first flowering plants ap-
peared. In addition, two major episodes of mountain build-
ing occurred in North America.
The end of the Cretaceous Period at 65 million years ago
is marked by the extinction of the dinosaurs. Most geologists
believe that this extinction was caused by the catastrophic im-
pact of a large asteroid with Earth. This impact is thought to
have caused widespread fires and high levels of atmospheric
dust that may have blocked sunlight for months. As a result,
the vast majority of animals that depended directly on plants,
either by eating them or by eating the animals that ate plants,
became extinct. In contrast, it is thought that animals such as
small rodents and insects survived because they fed on the
Geologic Time
In the context of the rock cycle, what does a long time mean?
To answer this question requires considering the concept of
geologic time
(or
deep time
), which is radically different from
the human timescale. Given our relatively short lifetimes, hu-
man beings tend to think that anything 100 years old, or older, is
ancient. For example, the American Civil War seems like a very
long time ago, does it not? After all, it occurred in the 1860s,
long before your grandparents, or even great, great grandpar-
ents, lived. As far as the history of Earth is concerned, however,
100 years is a blink of an eye. Scientists accept that Earth is
about 4.6 billion years old and that it lies within a much larger
and older universe that is approximately 14 billion years old.
This kind of timescale is naturally difficult to comprehend. It
is important to understand the concept of deep time, however,
because it provides a context within which we can study Earth.
It also helps us realize the incredibly rapid nature of environ-
mental change associated with ongoing human impacts on the
planet.
In an effort to make the concept of geologic time more
understandable, geologists have devised a geologic times-
cale that is subdivided on the basis of major events in Earth
history. Figure 12.19 shows this timescale and its various
subdivisions. The timescale is categorized into (from largest
time expanse to shortest) eons, eras, periods, and epochs.
All time earlier than 540 million years (my) ago is lumped
into the division of
Precambrian
time, which includes both
the
Archean
and
Proterozoic
Eons. This interval of time
Geologic time
The period of time that encompasses all of
Earth history, from its formation to the present.
