EPSC201 2004 Final Exam Review Section 7&8
1. Explain why ridge mountains form in continental rifts.
As the ridges get stretched out, they change their gradient from subtle to steep. There are steep ridges
on the edge of the stretched out basin. The basin also gets ﬁlled with sediment from erosion of the rifts.
There is also block rotation, which occurs when the normal faults are have the same slope, forming a
domino wedge. This is shown below. The professor called this block rotation, and simply means that as
the zone is extended (it occurs in extensial zones) the dominos will tilt further and further. Hence block
2. Explain three different criteria for establishing relative ages of rocks.
Based on cross cutting relationships, we know the vertical crosscut is the youngest.
Using fossil records can help with the determination of age. This can help us form a global age. Any
rocks with trilobites are about 400 million years old. Due to the progress of evolution, we see tiny
changes in fossil records, that can accurately determine the age of the rock. Looking at an outcrop with
fossils, and other rock layers, can help us determine rock layers a few thousand kilometers away, with the
exact same fossil records.
Fossils are good for relative ages, while radioactive dating is good for absolute aging. Combining these
two techniques makes it easy to date rocks.
3. What is an unconformity? Discuss the origin of two types of unconformity.
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of
different ages, indicating that sediment deposition was not continuous.
Angular unconformity - An angular unconformity is an unconformity where horizontally parallel strata of
sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the
overlying horizontal layers.
The ﬂat top layers have an angular relationship with the rocks layers below.
A nonconformity between sedimentary rocks and metamorphic or igneous rocks occurs when
sedimentary rock has been deposited above pre-existing (eroded) metamorphic or igneous rock,
indicating an environmental alteration in mode of formation of strata.
4. How are fossil assemblages used to determine the relative ages of rocks? Why is
this helpful in identifying disconformities?
The sea level has changed levels. When the sea level was low, walking animals were able to walk on it,
and form fossils. Then the sea level rose back up, and there are more marine fossils, in the more recent
layer of sediment. So we get layers of marine to land to marine fossils.
You can identify this gap in deposition by noting gabs in fossil records. If the land got covered by water,
no land fossils would be able to form. Then it got re-exposed, and more fossils will be able to deposit.
However, there will be a gap in the fossil record, showing that no deposition occurred when the land was
covered by the sea. There will be a gap in rock layers.
Disconformities mark the difference in age of layers next to each other. So there must have been a time
when no deposition occurred. This can be represented by a gap in the fossil record. Disconformities can
be the result of erosion, where the top layer has been transported away, or an era of non-deposition.
5. Using an equation, explain the principles of the radiometric method of age dating.
Radiometric dating is used to determine age of rocks and construct a map. Up to this point, all the ages
have been relative, not absolute. If we can calibrate how old rocks are, then we can tell a lot about other rocks using relative dating. From the pre-cambrian era, and earlier, we don’t have fossils. So we must
use radioactive dating.
What is an isotope? Well elements are deﬁned by the number of protons they have. They can have
variable amounts of neutrons. For example, all hydrogen isotopes have one proton, but they have
different amounts of neutrons. Normal hydrogen has one neutron, deuterium has two, and tritium has
three. The number of protons present in the atom deﬁnes the chemical properties.
As we go to heavier elements, we ﬁnd more and more unstable isotopes. For example, uranium has a
very big nucleus, and is unstable. It undergoes alpha decay, and decays to a more stable nucleus. The
alpha particle contains two neutrons and two protons, which equates to a helium nucleus. So alpha
decay is just the breakdown of a large nucleus into a slightly smaller nucleus plus a helium nucleus.
There are other types of decay. Beta particles are electrons and positrons. Positrons are just the positive
equivalent of an electron.
Rutherfords big discovery was that alpha decay proceeds at a constant rate. If you know the mass of the
uranium initially, and you know the mass of the particle now (some time later after its decayed) you know
how old the rock is.
To reiterate, if a rock 500 million years ago has a certain amount of uranium. This uranium decays at a
constant rate, and forms a bunch of different elements; lets say it produces lead. If we measure the lead
and uranium content now, we can tell how old the rock is, by seeing how much it has decayed since
Zirconium usually has uranium trace amounts in it. If we measure the concentration of lead and uranium
in the zirconium, we know how much uranium was present initially. If we know the rate that uranium
decays, we can determine how old the Zr crystal is.
6. Discuss a chemical theory for how life may have started on Earth.
Where did life originate? In the 1950s, Millar and Uri put methane, ammonia, and hydrogen gases into a
mixture. These represent the simple components of an early atmostphere on Earth. He envisioned an
atmosphere that was very reducing, meaning there was no oxygen present. This is similar to the
atmosphere on Venus. He put these gases in a closes system, and passed electricity through it
(representing lightning strikes).
They analyzed the contents after a bunch of electricity had been passed through. They observed a huge
number of organic molecules, including amino acids, which represent the building blocks of life.
There is a lot of p