159
eventually becoming sedimentary rocks. Steno also observed that younger rock
units are deposited on top of older rocks, in what is now known as the law of
superposition.
Hutton reasoned that if the angled strata at Siccar Point were originally deposited
as horizontal sediments, as required by Steno, then after the sediment turned to
rock, the rocks received their upward tilt by a later episode of mountain building or
some other geologic activity. According to the law of superposition, the horizontal
sediments deposited on top of the angled strata had to be younger than the episode
of tilting, before they too were turned to rock.
Hutton’s breakthrough was in assuming that the geological processes he could
observe around him such as sediment deposition and erosion had proceeded at similar rates in the past. Using these rates, Hutton did the calculations for how long it
would take to create the angular unconformity at Siccar Point and came up with an
age for the Earth of hundreds of millions of years. We now know that even this age
was far too young, because data from radiometric age dating of the oldest terrestrial
rocks, corroborated by the ages of slightly younger moon rocks indicate that the
Earth formed about 4.6 billion years ago (4.6 Ga). Hutton’s observations resulted in
other scientists thinking about the geological time scale and expanding it.
The names of geologic time intervals have come from a variety of origins. Some,
like the Devonian period, are named after places where rocks typical of the time
period are found; in this case Devonshire, England. Other place names include the
Mississippian, Pennsylvanian, Permian, and Jurassic periods, the latter two named
after the Russian city of Perm and the Jura Mountains on the border of France and
Switzerland. Several time periods are named after the dominant rock of the age. The
Carboniferous is the name for a period in Earth’s history when the landscape was
dominated by carbon-rich coal swamps. The Cretaceous Period refers to chalk, a
dominant rock type of that age. The name is derived from the Latin word cretus,
which means “grown,” because chalk is a collection of microscopic plant and animal shells. The White Cliffs of Dover in the U.K. are a famous Cretaceous chalk
outcrop, and there are many others.
Geologic time periods are subdivided into epochs, and there tend to be more of
these in recent periods because more details are decipherable. The Tertiary, for
example, contains two geological periods, the Paleogene and Neogene, and these
are subdivided into a number of epochs such as the Paleocene, Eocene, Oligocene,
Miocene and Pliocene. The Quaternary is subdivided into the Pleistocene and
Holocene epochs. Humans or at least hominids have been around since the beginning of the Pleistocene. The Pleistocene is the time of Ice Ages, and the Holocene
began when the last ice sheets melted about 12,000 years ago. Many geoscientists
now advocate that a new epoch be added after the Holocene to describe the influence of humans on planet Earth. The name for this is the Anthropocene.
The Greek word ánthrōpos (ἄνθρωπος), which means “human,” is the root for
terms like anthropology, anthropogenic, and Anthropocene. Human influence on the
geology of the planet is widespread and obvious. People have cleared forests, built
railroads, canals, highways, and cities, sculpted coastlines into harbors and waterfronts, drained wetlands, altered natural streamflow and sediment transport by
9.1 The Sixth Mass Extinction
eventually becoming sedimentary rocks. Steno also observed that younger rock
units are deposited on top of older rocks, in what is now known as the law of
superposition.
Hutton reasoned that if the angled strata at Siccar Point were originally deposited
as horizontal sediments, as required by Steno, then after the sediment turned to
rock, the rocks received their upward tilt by a later episode of mountain building or
some other geologic activity. According to the law of superposition, the horizontal
sediments deposited on top of the angled strata had to be younger than the episode
of tilting, before they too were turned to rock.
Hutton’s breakthrough was in assuming that the geological processes he could
observe around him such as sediment deposition and erosion had proceeded at similar rates in the past. Using these rates, Hutton did the calculations for how long it
would take to create the angular unconformity at Siccar Point and came up with an
age for the Earth of hundreds of millions of years. We now know that even this age
was far too young, because data from radiometric age dating of the oldest terrestrial
rocks, corroborated by the ages of slightly younger moon rocks indicate that the
Earth formed about 4.6 billion years ago (4.6 Ga). Hutton’s observations resulted in
other scientists thinking about the geological time scale and expanding it.
The names of geologic time intervals have come from a variety of origins. Some,
like the Devonian period, are named after places where rocks typical of the time
period are found; in this case Devonshire, England. Other place names include the
Mississippian, Pennsylvanian, Permian, and Jurassic periods, the latter two named
after the Russian city of Perm and the Jura Mountains on the border of France and
Switzerland. Several time periods are named after the dominant rock of the age. The
Carboniferous is the name for a period in Earth’s history when the landscape was
dominated by carbon-rich coal swamps. The Cretaceous Period refers to chalk, a
dominant rock type of that age. The name is derived from the Latin word cretus,
which means “grown,” because chalk is a collection of microscopic plant and animal shells. The White Cliffs of Dover in the U.K. are a famous Cretaceous chalk
outcrop, and there are many others.
Geologic time periods are subdivided into epochs, and there tend to be more of
these in recent periods because more details are decipherable. The Tertiary, for
example, contains two geological periods, the Paleogene and Neogene, and these
are subdivided into a number of epochs such as the Paleocene, Eocene, Oligocene,
Miocene and Pliocene. The Quaternary is subdivided into the Pleistocene and
Holocene epochs. Humans or at least hominids have been around since the beginning of the Pleistocene. The Pleistocene is the time of Ice Ages, and the Holocene
began when the last ice sheets melted about 12,000 years ago. Many geoscientists
now advocate that a new epoch be added after the Holocene to describe the influence of humans on planet Earth. The name for this is the Anthropocene.
The Greek word ánthrōpos (ἄνθρωπος), which means “human,” is the root for
terms like anthropology, anthropogenic, and Anthropocene. Human influence on the
geology of the planet is widespread and obvious. People have cleared forests, built
railroads, canals, highways, and cities, sculpted coastlines into harbors and waterfronts, drained wetlands, altered natural streamflow and sediment transport by
9.1 The Sixth Mass Extinction