Page 57 - Environment: The Science Behind the Stories

THE SCIENCE BEHIND THE STORY

Have We Brought on changed through time.) Where scien- The idea began to catch on after
a New Geologic Epoch? tists find fossil evidence for major and being proposed by Nobel Prize– winning
sudden changes in the physical, chemi-
chemist Paul Crutzen (p. 488) in 2000.
cal, or biological conditions present on In 2008, a team of 21 scientists from the
Earth between one set of layers and the Stratigraphy Commission of the Geologi-
next, they create a boundary between cal Society of London, led by geologist
geologic time periods. For instance, Jan Zalasiewicz of the University of Leices-
fossil evidence for mass extinctions (pp. ter (U.K.), formally advocated the proposal
77–79, 299–301) determines several in a paper in GSA Today, a journal of the
boundaries, such as that between the Geological Society of America (GSA).
Permian and Triassic periods. In their GSA paper, the British
Traditionally, time periods are defined geologists reviewed a broad sweep
geologist Jan Zalasiewicz, university of and named based on changes actually of published scientific evidence and
leicester seen in the geologic record. So when a advanced several reasons to rename
group of geologists suggested naming the past 200 or so years the Anthropo-
Have the impacts of human beings on the current portion of our planet’s history cene (Figure 1).
Earth been so profound as to warrant the “Anthropocene” (from the Greek word First, humans have caused a
creating a new time period in the geo- anthropos, meaning “human”), scientists sharp increase in erosion worldwide.
logic record? sat up and took notice. By clearing forests and raising crops,
The geologic timescale (appendix e) We live in the Holocene epoch, we have sent immense amounts of soil
shows the full span of Earth’s history—all the most recent slice of the Quaternary downwind and downstream from con-
4.5 billion years of it—and focuses in period. The Holocene epoch began tinents into oceans. This rapid deposi-
on the most recent 543 million years. about 11,500 years ago with a warming tion of sediment in the oceans will be
Geologists have subdivided Earth’s his- trend that melted glaciers and brought noticeable in the stratigraphic record
tory into 3 eras and 11 periods. The most Earth out of its most recent ice age. far into the future as today’s sediments
recent period, the Quaternary period, Since then, Earth’s climate has been become compacted into tomorrow’s
occupies a thin slice of time at the top remarkably constant, and this constancy sedimentary rock layers.
of the scale because this period began provided our species with the long-term Second, our species has rapidly
“only” 1.8 million years ago. stability we needed to develop agricul- altered the composition of the atmos-
Geologists divide this immensely ture and civilization. However, since the phere by emitting greenhouse gases
long timescale using evidence from industrial revolution (p. 22), human activ- as a result of deforestation, agriculture,
stratigraphy, the study of strata, or lay- ity has had major impacts on Earth’s and especially our combustion of coal,
ers, of sedimentary rock. (As we have basic processes. The question for oil, and natural gas. These releases
seen, sedimentary rock is laid down in geologists is: Have those effects been have already brought carbon dioxide
chronological sequence, so by studying strong enough to warrant naming a new and methane to their highest levels in
it researchers can infer how conditions geologic epoch after ourselves? at least 800,000 years (pp. 504–505).

minerals seep through sediments and act as a kind of glue, inferences about Earth’s history (see The Science behind The
binding sediment particles together (cementation). The for- STory above).
mation of rock through these processes of compaction and
cementation is termed lithification. Examples of sedimen- Metamorphic rock Geologic forces may bend, uplift,
tary rock include sandstone, made of cemented sand par- compress, or stretch rock. When any type of rock is sub-
ticles; shale, comprised of still smaller mud particles; and jected to great heat or pressure, it may alter its form, becom-
limestone, formed as dissolved calcite precipitates from ing metamorphic rock (from the Greek for “changed form”)
water or as calcite from marine organisms settles to the (Figure 2.18c). The forces that metamorphose rock generally
bottom. occur deep underground, at temperatures lower than the rock’s
These processes also create the fossils of organisms melting point, but high enough to change its appearance and
(p. 73) and the fossil fuels we use for energy. Because sedi- physical properties. Metamorphic rock (Figure 2.19d) includes
mentary layers pile up in chronological order (Figure 2.19c), rock such as slate, formed when shale is subjected to heat and
geologists and paleontologists can assign relative dates pressure, and marble, formed when limestone is heated and
56 to fossils they find in sedimentary rock and thereby make pressurized.

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