Notes - Rock Formation and Age File
Transcript Notes - Rock Formation and Age File
FOSS: Earth History
• The earth is estimated to be about 4.5 billion years old.
Our knowledge of its history comes from a number of
sources. The geologic time scale is constructed through
scientific methods and calculations as well as from the
interrelationships of geological features as observed in
the field. The principle of uniformitarianism (“the
present is the key to the past”) is helpful in that we can
accurately measure the rates of geologic processes we
see today and apply them to the geologic past.
• For example, we know layers of sediment build up on
the ocean floor at the rate of about 1 millimeter per
year. Thus, it would take over one million years of
sedimentation to form a unit of shale 1,000 meters
Evidence of the dynamic changes of Earth’s
surface through time is found in the geologic
Earth is approximately 4.6 billion years old. Earth history is
based on observations of the geologic record and the
understanding that processes observed at present day are
similar to those that occurred in the past
(uniformitarianism). There are different methods to
determine relative and absolute age of some rock layers in
the geologic record. Within a sequence of undisturbed
sedimentary rocks, the oldest rocks are at the bottom
(superposition). The geologic record can help identify past
environmental and climate conditions.
“the present is the key to the past”
• the concept that the earth's surface was
shaped in the past by gradual processes, such
as erosion, and by small sudden changes, such
as earthquakes, of the type acting today.
Law of Superposition
• the principle that in any sequence of
sedimentary rocks which has not been
disturbed, the oldest strata lie at the bottom
and the youngest at the top
Law of Crosscutting
• if a fault or other body of rock cuts through
another body of rock then it must be younger
in age than the rock through which it cuts and
These are thin syenite dikes that cut a fine-grained gabbro, clearly after the
gabbro was solid.
The Law of Original Horizontality
• layers of sediment were originally deposited
horizontally under the action of gravity.
• The forces of nature beat up on the earth and
break it into small pieces (weathering), which
then wash down rivers (erosion) to the ocean
and settle out on the seafloor, in horizontal
layers. Lots of exceptions here! i.e. sand may
be deposited at angles up to 15 degrees.
Law of Lateral Continuity
• states that layers of sediment initially extend
laterally in all directions; in other words, they
are laterally continuous. As a result, rocks that
are otherwise similar, but are now separated
by a a valley or other erosional feature, can be
assumed to be originally continuous.
Geologists recognize two different
kinds of time: relative time and
absolute age. Relative time concerns
the sequence of geologic events, and
absolute age measurements concern
the actual age of a rock or mineral.
• Principles Of Relative Age
Relative dating doesn't really give us an actual 'age,' but it
does put things in sequential order. This allows geologists
to determine the age of a rock or strata relative to
another rock or strata. So instead of saying 'when'
something happened, it puts events in the 'order' they
happened. It's basically the difference between saying
'I'm 25 years old and my sibling is 20 years old' and 'I'm
older than my sibling.' Geologists use a variety of
techniques, or principles, to determine the relative age of
a rock unit.
• Absolute dating is the process of determining
an approximate computed age in archaeology
• Absolute dating provides a computed
numerical age in contrast with relative dating
which provides only an order of events.
• Geologists use radiometric dating to estimate
how long ago rocks formed, and to infer the
ages of fossils contained within those rocks.
Radioactive elements decay.
is full of naturally occurring radioactive elements. Radioactive
atoms are inherently unstable; over time, radioactive “parent
atoms” decay into stable “daughter atoms.”
• When molten rock cools, forming what are
called igneous rocks, radioactive atoms are
trapped inside. Afterwards, they decay at a
predictable rate. By measuring the quantity of
unstable atoms left in a rock and comparing it
to the quantity of stable daughter atoms in
the rock, scientists can estimate the amount
of time that has passed since that rock
Radiometric Dating Carbon
• thermoluminescence, emission of light from some minerals and
certain other crystalline materials. The light energy released is
derived from electron displacements within the crystal lattice of
such a substance caused by previous exposure to high-energy
radiation. Heating the substance at temperatures of about 450° C
(842° F) and higher enables the trapped electrons to return to their
normal positions, resulting in the release of energy. The intensity of
the emission can be correlated to the length of time that a given
substance was exposed to radiation; the longer the time allowed for
the radiation to build up an inventory of trapped electrons, the
greater the energy released. Because of this feature,
thermoluminescence has been exploited as a means of dating
various minerals and archaeological artifacts.
Relative Age of Sedimentary Rocks
• Bracketing the fossils Fossils are
generally found in sedimentary
rock—not igneous rock.
Sedimentary rocks can be dated
using radioactive carbon, but
because carbon decays relatively
quickly, this only works for rocks
younger than about 50 thousand
• So in order to date most older
fossils, scientists look for layers of
igneous rock or volcanic ash
above and below the fossil.
Scientists date igneous rock using
elements that are slow to decay,
such as uranium and potassium.
By dating these surrounding
layers, they can figure out the
youngest and oldest that the
fossil might be; this is known as
“bracketing” the age of the
sedimentary layer in which the
BEDROCK GEOLOGIC MAP OF OHIO
Fun Facts About Ohio'S Geological History
• Fun Facts About Ohio'S Geological History
1. The oldest exposed rocks found in Ohio were deposited here 470 million years ago.
2. Flint is the official gemstone of Ohio. It originated in sea sponges that fell to the
sea floor in pre-historic times. The sponges' openings were filled with mud or sand
and then compressed over time. Flint was used by Native Americans to shape
spear points, knives and other tools.
3. Want to collect a million seashells? Pick up a piece of limestone rock. Limestone
was formed from the remains of seashells. Many prestigious and architecturally
significant buildings and monuments in Ohio are made from limestone.
4. The trilobite Isotelus is Ohio's state fossil. Many states have a state fossil.
5. Sandstone found in Ohio's Berea layer of the earth is known for three amazing
things. First, it holds oil between its course grains. (Oil is a substance formed by
sea creatures that decomposed in the absence of oxygen.) By 1886 more that
2,000 wells were drilled in eastern Ohio. Second, sandstone is a perfect material to
make grindstones. In the 1870s, the Berea layer produced more than 75 percent of
the world's entire supply. Grindstones were shipped to Australia, Japan and South
America. Third, buildings throughout North American are made from this
Fun Facts About Ohio'S Geological History
More than 80 percent of the electricity Ohioans use is made by burning coal, a
7. About 290 million years ago a huge rock from space cashed into what is now
Adams County in southern Ohio, shattering rocks in a five mile diameter. Rock
layers at the center were pushed into the earth and then bounced back up.
Those formations are now a 1,000 feet above their original position.
8. Much to the disappointment of grade-school dinosaur lovers everywhere in
Ohio, the state can't boast of any dinosaur bones or fossils found here. During
the age of dinosaurs, massive erosion occurred here, preventing bones from
being buried and turning into fossils. But we have great trilobite fossils.
9. Mammoths and mastodons, two elephant-like creatures, roamed Ohio during
the Ice Age.
10. The rare and endangered Lakeside Daisy grows in only a few locations near the
Great Lakes shores of the United States and Canada. The plant can be seen at the
Lakeside Daisy State Nature Preserve on the Marblehead Peninsula on Lake Erie.
The flower grows on an alvar, a shallow soil system over limestone.
(SOURCE: Ohio Governor's Residence and Heritage Gardens; Geologic Walk Guide
and Companion Educational Materials)
Ohio's Geologic Periods