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Learning Objectives Apply the geologic time scale to geochronology
List the processes of a mold and a cast formation
Explain how a living thing becomes fossilized Distinguish the difference between a fossil mold and cast Introduction
The vast majority of changes on the continents and in the oceans take place over millions and
even billions of years. Recording those geological changes are Earth's rocks. Rocks can be
dated, giving us an idea of what geologic events were happening on the Earth when these
rocks were created. Early Rock Records
Although the rock record can tell scientists much about the geologic events in Earth's history, it
cannot tell us when the Earth was created. Earth's oldest rocks continue to be buried, melted,
eroded, and recycled during geologic processes; therefore, there aren't any known rocks in
their original state to determine how old the Earth is. Scientists have found other ways to
estimate the Earth's age, including calculating the probable age of the solar system and
assuming the Earth originated at the same time. To pinpoint the solar system's age, scientists
measure radioactive elements. These are sometimes referred to as 'clocks in rocks'. Radioactive
isotopes from the Earth, moon, and meteorites are measured by decay of atomic nuclei,
termed radioactive decay, the basis for all radiometric dating methods. © 2014 eScience Labs, LLC.
All Rights Reserved Figure 1: The concept of half-life is depicted in the graph above, showing how much of the
element is present after sequential half-lives pass. Carbon-14 has a half life of 5,730 years. Today the Earth is estimated to be at least 4.5 billion years old. Rocks from all of Earth's
continents have been dated, with the oldest rocks found in Greenland and northwestern
Canada. One pink-layered basalt with garnets in Hudson Bay, for example, dates to 4.28
billion years ago. © 2014 eScience Labs, LLC.
All Rights Reserved These early rocks are thought to be a
product of lava flows and sediments
deposited in shallow water, which
indicates the Earth was formed before
the rocks in Hudson Bay. Another
indicator of an early planetary origin,
single zircon crystals found in Australia
have been radiometrically dated to 4.4
billion years. The moon has rocks
between 4.4 and 4.5 billion years old,
roughly similar in age to non-lunar
meteorites, the extraterrestrial
fragments that fell to Earth 4.53 and
4.58 billion years ago. In addition to
radiometric dating methods, other
findings such as isotopic composition
of lead samples have helped date the
Earth. Knowledge Check
What are radiometric dating
methods?
The history of the Earth is divided into
units of time larger than days, months,
and years. The typical geologic
timescale consists
of eons, eras, periods,
and epochs encompassing billions of
years. The longest chapter of Earth's
Figure 2: Geological timescale.
history is its first: the once-termed
Precambrian Eon, also known as the Cryptozoic Eon, lasted more than four billion years
starting with the formation of Earth. Of the three eons now designated in this time period, the
Hadean is the first, a time of liquid rock oceans, boiling sulfur, volcanos, and raining asteroids
from space. Rocks resulting from lava continually melted, while the Earth separated into its
internal layers and began cooling. Few remnants of this time have been found making it a time
of conjecture rather than fact. Knowledge Check
© 2014 eScience Labs, LLC.
All Rights Reserved Which is the largest unit of time: eras, periods, eons, or epochs?
As scientists learn more about the Earth, time estimations become more exact. Time closer to
the present is more exact because more geologic evidence remains, however, time estimations
are subject to change as more geologic evidence becomes studied.
The Earth's cooling by the Archean Era allowed solidifying lava to form the ocean floor and
condensed vapor filled the oceans. Sedimentary and igneous rocks such as tonalite and basalt
appeared as did layered minerals that only could develop in the era's low oxygen levels.
During this time, cratons, the interior portions of continents and tectonic plates made of stable
rock such as gneiss and valuable mineral deposits, were created with deep lithospheric roots
extending into the Earth's mantle. Life, too, began in the
ocean with onecelled bacteria later found in fossils.
The geological Proterozoic Eon encompasses a span
when continental landmasses formed from smaller land
bodies along with extensive building of mountains from
plate tectonic processes. Shallowwater marine
environments supported multi-celled organisms, which
released oxygen through photosynthesis and contributed
to the accumulation of oxygen in the atmosphere.
Stromatolites, for one, flourished in shallow waters with
Figure 3: Modern stromatolites in
their remains forming a type of rock known as
Shark Bay, Australia.
stromatolitic chert. Other rocks identified from the
Proterozoic Eon include metallic ores along with iron, gold, copper, uranium, and nickel. Figure 4: The Rocky Mountains are
considered to be a young mountain range. During the much shorter Phanerozoic Eon
were three distinctive eras. In the Paleozoic
Era, ancestors of plants and animals today
appeared, followed by the Mesozoic Era, and
the current time period, the Cenozoic Era.
Within each era were periods signifying
geologic change. For example, during the
Paleozoic Era, shallow seas covered much of
North America in the Cambrian period; much
of North America was south of the equator in
the Silurian period; North America collided
with northern Europe in the Devonian period;
and great mountains arose in the American
West during the Carboniferous period. Eras can contain further time breakdowns. During the present era, the Cenozoic, two periods
subdivide into epochs. The most recent period, the Quaternary Period, contains the Holocene
© 2014 eScience Labs, LLC.
All Rights Reserved epoch in which we live. In this very tiny slice of Earth's history, gigantic ice sheets repeatedly
advanced and withdrew from the northern polar region. They covered the northern part of the
U.S. continent, much of northern Europe, and parts of Asia with ice thousands of feet thick,
and then retreated. Stratigraphical Archives
Geologic age is either how many years ago an event is
known or estimated to have occurred (absolute
geologic age), or when something happened relative to
other events (relative geologic age). Before the
20thcentury, geologists could only determine the
relative ages of rocks - whether one was younger or
older than another. Early geologists used relative
geologic age when observing sedimentary rock
formations. They formulated hypotheses about time
based on the relationship of rock layers and fossils that
have held up under examination. Figure 5: Cross-cutting relationships.
Rock layers A and B must be older
than the Intrusion C that disturbs
them. One of these observers, James Hutton (1726 – 1797), refuted the current thought that only
violent disasters modified the Earth. Hutton maintained "the present is the key to the past,"
saying geologic processes in the present were the same as those of the past and that of the
future. His uniformitarianism concept was used to explain surface rock as an area that has been
uplifted and eroded. He also observed rock layer aberrations and devised the Law of CrossCutting Relationships, which states that relative age of a rock can be determined by
observing which rock cuts into another. Thus, if a fault or lava cuts through another body of
rock, then the fault or lava must be younger in age than the disturbed rock layers.
Nicolaus Steno (1638 - 1686) while working in the mountains of Italy also observed rock layer
sequence changes. His 17th-century observations have become known as: Principle of Superposition: In a sequence of undisturbed sedimentary rock layers or
lava flows, the layer above another is younger than the layer beneath it.
Principle of Original Horizontality: Because sedimentary rock originally was
deposited in horizontal layers, if sedimentary rock layers aren't horizontal, then forces
tilted them after the layers formed.
Principle of Strata Continuity: Sedimentary rock layers with internally consistent
characteristics distinguishable from other layers can be assumed to have developed as
broad layers that extended laterally till they thin out or grade into another environment. © 2014 eScience Labs, LLC.
All Rights Reserved © 2014 eScience Labs, LLC.
All Rights Reserved Pre-Lab Questions
1. Describe the process of fossilization. 2. What is the Principle of Faunal Succession? 3. Index fossils identify geologic periods because these fossils were restricted to
____________. 4. Often, fossils are thought of as the bones of an organism. Describe three other types
of fossils. © 2014 eScience Labs, LLC.
All Rights Reserved Experiment 1: Indirect Fossilization
A living organism that dies and is covered with sediment, decomposes, and leaves a hollow
space. Sediment may fill the hollow space. Mineral-carrying water may also flow through the
space and leave behind minerals. In terms of fossilization, this hollow space is a mold, and
whatever fills the space, for instance, sediment or minerals, is a cast. Both molds and casts
preserve the physical characteristics of organisms.
Body parts that leave the best fossil impression are the hardest parts of the organism’s body
such as a shell, skeleton, or tooth because those parts often have minerals, for example,
calcium carbonate or silica, that don’t decay quickly and allow the sediment to collect around
the organism. In this experiment, you will simulate fossilization of a shell to examine how
different fossilization environments influence the degree of preservation. Materials Permanent Marker
Plaster of Paris
Play-doh®
Ruler
Sand
Shell
3 Styrofoam® cups Toothbrush
Topsoil
*Spoon
*Water
*You Must Provide © 2014 eScience Labs, LLC.
All Rights Reserved Procedure
1.
2.
3.
4.
5. Sketch the shell. Record its length, width and depth in Table 1.
To make mold, label one cup, sand and one topsoil.
Fill sand cup to the top with sand.
Place soil in topsoil cup. Add four tablespoons water. Mix uniformly with spoon
Place the shell with the ridges facing down in the Play-doh®. Press down until shell is
level with material, creating impressions of the ridges in the Play-doh®. Remove shell.
Wash off any debris and dry shell.
6. Repeat mold-making process in the sand and soil
7. Mix Plaster of Paris with one cup of water to consistency of thick pudding.
8. Pour 1-inch-deep layer in each container.
9. Allow plaster to dry. This should take an hour. Remove cast, and allow it to dry 24
hours before handling.
10. After 24 hours, brush any remaining debris of cast with toothbrush. Compare cast
measurements to initial shell measurements. Record any additional observations,
such as ridges retained or shape lost, in Table 2. Table 1: Measurements
Measurement
(cm) Plaster of Paris Cast Shell
Sand Topsoil Play-doh® Length
Width
Depth Table 2: Observations
Sand Topsoil Observations © 2014 eScience Labs, LLC.
All Rights Reserved Play-doh® Post-Lab Questions
1. How are the mold and original shell similar for each material? Different? 2. What type of material does the Play-doh® simulate? 3. Which fossil gives more information about the once living object? Why? 4. What might be found in today’s sedimentary strata to a geologist millions of years
in the future? 5. If trilobites lived from the Lower Cambrian Period to the end of the Permian
Period, how long did they exist? 6. Which type of rock is most likely to contain fossils? Igneous, sedimentary, or
metamorphic? © 2014 eScience Labs, LLC.
All Rights Reserved
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