Astrobiology, Fall 2005
Name(s):
Exercise 5: A crash course in the development of geology
Geology is the scientific study of the Earth, its materials and processes, and is as
old as the hills. Even older.
Plate tectonics is the unifying theory of geology, much in the way that evolution
is the unifying theory of biology. Plate tectonics can be summed up in four key
points:
• The Earth’s lithosphere comprises continental and oceanic plates. They number
in the dozens, with only a dozen or so large plates.
• The plates move over a semi-fluid asthenosphere. The motion of the plates is
directed by convection cell currents in the asthenosphere, which are in turn
directed by still deeper convection cells, all the way down to the outer core.
• Plate boundaries, that is, the meeting line between two plates, is where
geologic phenomena, such as earthquakes and volcanoes, occur.
• The interior areas of plates are relatively geologically quiet, except for hotspots.
With this theory, geologists were able to explain how core processes could be
connected to surface processes and vice versa, to explain how the diversity of
rocks and minerals occurs, and to explain in greater detail than ever, the history
of this planet.
But it was not always thus.
Classifying rocks (pre-historic)
The first humans probably studied geology by trying to classify different kinds
of rocks, usually by purposes: some rocks yielded valuable metals; other rocks
were easy to carve.
The modern system of classification is derived from the origin of rocks; rocks
have two principle methods of “forming”: rocks can lithify, or they can
crystallize.
“Lithifying” means that the rock is made by breaking down other rocks,
depositing the sediment somewhere and then compressing it enough to give it
cohesion — in other words, making it a rock. Rocks made in this fashion are
called sedimentary rocks.
“Crystallizing” means that a rock freezes from a molten (in which case it is an
igneous rock) or a semi-molten (metamorphic rock) state. Of course, to make
both rocks are made at much higher temperature and pressures (and therefore
depth) than sedimentary rocks, but igneous rocks are the only ones that are
frozen from a completely liquid state.
The rock cycle below shows how one rock can give rise to another in an endless
cycle.
Igneous Rock
Sedimentary
Rock
Metamorphic
Rock
1. Obtain rock specimens R1, R2 and R3. Identify each rock as igneous,
metamorphic or sedimentary and complete the table on the next page.
Rock sample
R1
R2
R3
Igneous, metamorphic or
sedimentary?
Reason for classification
choice
The texture and the mineralogical composition of a rock frequently reflect its
geologic history and help us determine whether it has an igneous, metamorphic
or sedimentary origin. Texture refers to the size, shape and relationship of
minerals within a rock. In general, igneous rocks have a crystalline texture, in
which different mineral crystals have grown together and are interlocking.
Metamorphic rocks often have a foliated texture, where rippled layers of single
minerals align roughly parallel to each other (some single mineral metamorphic
rocks have a crystalline texture). Sedimentary rocks usually have a clastic texture
because they are made up of fragments (clasts) of other rocks, and often are
layered. Sedimentary rocks may also contain fossils, which are the preserved
remains or traces of organisms.
A mineral is a naturally-occurring, solid, usually inorganic element or
compound with a definite crystal structure and chemical composition which
varies only within specific limits. Rocks are merely aggregates of minerals.
2. a. Assuming that minerals can be distinguished by color (not always a good
test), how many minerals can you see in specimen R2? Describe each by color
and by any other distinctive characteristic.
b. Does it appear that all of the minerals in this rock had been melted at the time
this rock was forming? What observations led you to your conclusion?
Steno’s Principles (1669)
Nicolaus Steno (born Niels Stensen) was a court physician in Florence who
published a book called Prodomus (the title is actually much longer, so it’s known
by its first word). In it, he writes about three principles that would eventually
bear his name.
• The principle of original horizontality — all layers of sediment are laid down
parallel to the Earth’s surface.
• The principle of lateral continuity — all layers originally extend unbroken until
they end.
• The principle of superposition — layers of sediment stack in order of age, with
the oldest layer found at the bottom of a stack of sediment layers.
To this, more principles were added by later researchers:
• The principle of cross-cutting relationships — a rock that has been cut by a
fault is older than the fault. A rock that has been intruded by another rock is
older than that second rock.
• The principle of faunal succession — if a fossil organism is found in a layer
above another fossil organism’s layer, then, regardless of what type of sediment
or where in the world they are found, the first fossil organism will always be
found in a layer above the second fossil organism’s layer.
Using these principles, consider the diagram below, which is a cross-section of the
crust in central Oregon. The letters refer to the events which placed those rocks
there; for instance, "A" represents the deposition of the shale layers near the
bottom and "C" represents the intrusion of the steeper dike.
3. Order the lettered units from oldest (least recent) to youngest (most recent).
4. a. The surfaces between formations G and I and between (A, D, E) and F are
called unconformities (a time gap in the rock record). Which layers violate the
principle of original horizontality? Dikes like B and C are not layers, so they
don’t count.
b. Suggest a theory why the layers you chose violate that rule. Hint: they don’t
really violate the principle of original horizontality.
We don’t have to stay on Earth to apply these principles; here is a photograph,
taken by the Viking Orbiter in 1976, of a small area on Mars.
5. Notice what appear to be river channels to the left of the photo. Notice that this
area is cratered by impacts. Which is older, the craters or the river channels? Use
cross-cutting relationships to explain your answer.
The geological timescale (1750s to present)
From the principle of faunal succession, geologists and paleontologists
worldwide began to realize that layers of sedimentary rock could be correlated
across oceans. For instance, a formation called the Old Red Sandstone in
Scotland, England and Wales had the same color, composition and especially
fossils as the Catskill formation in New York. The inference was made that at one
point, the Old Red Sandstone had been laterally continuous (as Steno said) from
Great Britain to North America and that later erosion or some other force had
removed an ocean’s worth of rock.
Thus, the Old Red Sandstone was of a particular time in the past, and by
correlating other rocks with key fossils (see back pages) worldwide, geologists
and paleontologists began developing the geological timescale (see back pages).
6. Go back to the cross-section on the previous page. As you wander in this area,
you find (oh, happy day) fossil F-1 near the top of layer E. What is it? What is the
youngest geologic period that layer E could have been deposited? What is the
oldest geologic period it could be?
7. Later in your exploration, you come upon fossil F-2 near the top of layer G.
What is it? What is the youngest geologic period that layer G could have been
deposited? What is the oldest geologic period it could be?
8. a. Which strata (write the letter) must be Mesozoic in age?
b. Which strata may be Mesozoic (in part or wholly) in age?
c. Given the answers to a and b above, what limits relative dating methods, in
terms of figuring out when in the geological timescale you are?
Radiometric dating (1900s to present)
Dating Earth events using fossils can give you a rough order of events, but
whether these events took place over thousands of years, or over billions of years
was a hard question to answer until numerical dating methods were developed.
With the discovery of radioactivity in naturally-occurring rocks by Henri
Becquerel in 1896, geologists quickly realized that there might be a tool in
determining the numerical ages of rocks.
Radiometric dating relies on the fact that a radioactive isotope will decay into a
stable isotope at a set rate. After a time of one half-life of a radioactive isotope,
only one half of the amount of a radioactive isotope will remain in the rock; the
text has an excellent explanation of the mathematics of half-lives and dating on
pages 87 through 89.
What’s important is how the geoclock is set; in other words, what event is being
dated when a rock is said to be 4.6 billion years old? It is the crystallization of the
rock that is the key event; prior to crystallization, radioactive isotopes are free to
wander in and out (they’re in a liquid after all) of an area that might become a
rock, but after crystallization, the isotopes are trapped and are part of the rock.
Thus, we assume that none of the isotopes in the rock can enter or leave the rock,
which means that we can tell how much of the original radioactive isotope has
decayed. This assumption is pretty good for certain rocks.
9. The formula used to determine a numerical age for a particular isotope is:
1 ÊÁ N0 ˆ
˜ , where t is the age of the sample in years, l is a constant related
t =
ln
l ËN ¯
to the half-life of the radioactive isotope used (l = 0.693/t1/2), N is the number of
atoms of that radioactive isotope in the sample now, and N0 is the number of
atoms of that radioactive isotope when the sample crystallized (this is generally
calculated by measuring the amount of daughter product). For uranium-lead
dating, l = 1.54 ¥ 10-10 yr-1. Calculate the numerical age for dikes B and C and fill
in the table below. (Hint: you really do need a scientific calculator for this one)
Rock sample
Number of
uranium atoms
originally (N0)
Number of
uranium atoms in
sample (N)
Dike B
10000
9667
Dike C
10000
9940
Calculated
numerical age in
years (t)
10. What event do the ages in question 8 represent?
11. Are the numerical ages consistent or inconsistent with the principle of crosscutting relationships? Explain how the numbers support your answer.
12. Suppose another research team finds and radiometrically dates orthoclase
crystals (a mineral that usually can be radiometrically dated) in layer J at 315 my
(million years). Is it reasonable to conclude that layer J is of late Paleozoic age?
Short answer: NO! What is the reason for the apparently errant date (assume
their lab technique was superb)?
13. You may have heard that Moon rocks and meteorites have been
radiometrically dated to 4.6 billion years old. What assumption are we making
about these rocks? Fortunately for this class, the assumption has been validated
by other means.
Plate tectonics (1960s to present)
In his 1915 book The Origin of Continents and Oceans, Alfred Wegener, a German
climatologist, came up with a radical notion that at one point, all of the
continents on the Earth had been one large supercontinent called Pangaea (“allEarth”) and that continental drift had torn Pangaea apart to spread the
continents the way we see them today. His theory, though having a lot of rock
and fossil evidence that the continents were at one point continuous, was not
accepted due to the lack of a plausible mechanism for moving continents.
Two American researchers, Harry Hess and Robert Dietz, in the early 1960s,
proposed seafloor spreading as a mechanism to generate seafloor at mid-ocean
ridges (spreading centers). Ocean floor would then be pushed away from midocean ridges, move across the ocean to finally be melted and recycled at the
trenches bordering continents and other parts of oceans. They proposed thermal
convection cells in the mantle as the mechanism that drove this engine.
Nearly coincidentally, a team of American and British geophysicists, Fred Vine,
Drummond Matthews and Lawrence Mosely, found that new seafloor at midocean ridge crystallized and became magnetized to the direction and strength of
the magnetic field of the Earth at that time. When the Earth’s magnetic field
suddenly (geologically speaking) reverses, the reversal is recorded as a different
orientation “stripe” on the seafloor rocks. By following the pattern of “normal”
polarity and “reversed” polarity rocks across the Pacific Ocean and correlating
those reversals with well-dated rocks showing the same magnetic reversals on
land (at Jaramillo, New Mexico), they concluded that seafloor rocks were
youngest near the mid-ocean ridge and oldest near the trench, finally getting
evidence for seafloor spreading.
In the mid-1960s to 1970, Canadian geologist J. Tuzo Wilson and American
geologist Tanya Atwater published a series of papers outlining plate tectonic
theory.
The Plate Tectonic Map of the Circum-Pacific Basin Region is a classic map that
summarizes a recent (1985) state of knowledge about plate tectonics in the Pacific
Rim (it’s not called the “Ring of Fire” for nothing, you know). Unfortunately, the
American Association of Petroleum Geologists (AAPG) no longer prints this, so
please be careful (especially folding it and not writing on it)! Other sources that
might help conceptually are the Physical Earth map and the Shaping of a Continent
map, both published by the National Geographic Society. Please use the
numbers off of the AAPG map, though.
14. This map shows bathymetry; darker shading indicates deeper water depths.
Find the East Pacific Rift. Is this a ridge or a valley in the ocean? Does it ever
exceed sea level? Read "Explanations" and determine what kind of plate
boundary this rift is.
15. What compass direction is the Pacific Plate moving near the East Pacific Rift?
So would older ocean floor be found to the west or to the east?
16. Why does the ocean floor get deeper the further you are away from the rift?
Hint: Think where the rocks of the ocean floor might be hotter.
17. a. In fact, the map of the world on the next page (courtesty of the US
Geological Survey), which shows the heat flow out of the Earth at various spots,
should confirm your idea in question 16. The units of the numbers for the
shading given on the map are milliwatts per square meter. What kind of
spectroscopy (in other words, what form of electromagnetic radiation) allows
this type of information to be obtained?
b. Back on the plate tectonic map, what do you notice about the depth and
number of earthquakes which occur near or at rifts? Why might this be so?
18. Another way to view where hot material is rising in the asthenosphere is by
using seismic wave speeds. The speed of a seismic wave is proportional to the
density of the material it is traveling in. In other words, the denser the material, the
faster the seismic wave spreads. In other other words, seismic waves traveling
through hotter (and therefore less dense) material tend to show up at a seismometer
later than expected.
a. The diagrams on the lower half of the previous page (courtesy of Yuancheng
Gung and Barbara Romanowicz, 2002, at UC Berkeley) show seismic wave velocities
along two lines of cross-section shown (M to N and S to T) through the southern
Atlantic Ocean, Africa and the Indian Ocean. The cooler colors represent denser
(cooler) rocks and the hotter colors represent less dense (hotter) rocks. Is there
evidence for the mid-Atlantic ridge being an area with hot rocks (and would
therefore confirm the heat flow map)? Circle these confirming areas on the crosssections.
b. The big red tongue of flame in the lower mantle on the M-N cross-section is called
the “African Superplume”. Using your textbook or any plate tectonic map that
shows this area of the world, find out what (if any) hotspot this superplume directly
feeds, or fed. (I) What does this suggest about the likelihood of continental hotspots
being directly fueled by core-mantle boundary instabilities that result in
superplumes?
19. One of the classic ways in which researchers found evidence for plate tectonics
was by looking at the paleomagnetism of ocean floor rocks. When the rocks
crystallized at the rift, the iron minerals aligned with the prevailing magnetic field at
that time, and as the plate spread away from the rift, this magnetic orientation was
preserved. Viewed from above by the right equipment, the preserved magnetic
grain orientations look like stripes, as shown below.
a. Back on the plate tectonics map, find the magnetic stripe labeled "3" near the
East Pacific Rift south of the Wilkes Fracture Zone. What is the distance from the
middle of the stripe to the middle of the rift (in kilometers)?
b. How old is this magnetic stripe (in millions of years (My))? Hint: look for the
magnetic timescale in the "Explanations" section.
c. Divide the answer to part a) by the answer to part b) and you obtain the speed
of the Pacific Plate in km/my. (Remember that distance/time = speed) Divide this
result by 10 to convert the speed into units of centimeters per year (cm/yr).
d. What is the speed of the plate, according to the map (first of all, are the colored
or clear arrows the ones you want to look at?)? What are the units of speed of the
numbers on the map? How does it compare to your number (give a rough
percent estimate of how your values differ)?
20. Find the west coast of South America.
a. What is the long linear feature in the ocean near here?
b. What is the long linear feature on the coastal land?
c. What type of plate boundary is this?
21. Determine the approximate distance from the trench to the volcanic arc for
the Cascades (the trench is where the thrust is mapped). Do the same for the
Andes in South America. Why are these distances different?