TIME AND EARTH PROCESSES:
When thinking about geological time, it is useful to think of two kinds:
(1) RELATIVE time (which refers simply to the order of events)
(2) ABSOLUTE time (which includes actual dates in millions of years for those events)
> historically, it wasn't until this century (and, really, only in the last 50 or 60 years) that
we had the techniques for determining absolute time
>> so, we will look at the development of our understanding of time in a historical
perspective
(1) RELATIVE TIME
> refers to the order of events
>> in order to figure out the relative order of events in geology, we need to look at the
ROCK RECORD (i.e., the evidence left by the rocks)
>> this evidence is often fragmentary, and the working out of the geological history of an
area (or of the whole earth!) is like trying to solve a mystery
> we will consider first some of the techniques used to figure out relative age
relationships at a single outcrop, and then look at the problem of relating rocks from two
or more outcrops that may be far apart
Consider Individual Outcrops:
> some of the techniques or principles are:
(a) with sedimentary rocks, we can use the Principle (or "law") of Superposition
>> for sedimentary rocks that have not been turned up-side down, a layer that is on top of
another layer is younger (i.e., layers get deposited sequentially)
(Note: there are ways to tell if sedimentary rocks have been turned upside down)
(b) cross-cutting relationships
>> igneous rocks that cut across other rocks must be younger than the other rocks
>> faults that cut through rocks must be younger than the rocks they cut
(c) unconformities
>> an unconformity is a gap in the rock record; it represents a time period of erosion at
that locality
>> the rocks below the unconformity are older (e.g., flat-lying sedimentary rocks sitting
on top of sedimentary rocks standing at an angle > called an angular unconformity; in
that case, the older sedimentary rocks must have been deposited, pushed up on edge, and
eroded down, all before the seas came in to deposit the youngest flat-lying sedimentary
rocks on top)
>>> it was the recognition of the large time gap that an unconformity can represent
(consider the one in the Kingston area between the metamorphic rocks below and the flatlying sedimentary rocks on top) that made geologists in the 19th Century realize that the
earth must be quite old, even though they had no way of determining absolute ages
Consider outcrops that are widely separated (e.g., on different continents)
> can we say anything about relative age relationships about such widely separated
locations?
YES!! By using
(d) fossil sequences
> sedimentary can preserve the remains (usually the hard parts: shells) of dead life forms
>> we call such remains FOSSILS
> geologists have looked at places where there are thick sequences of sedimentary rocks
(e.g., the Grand Canyon), and, using the "Law" of Superposition, studied how life on
earth has changed over time
>> a pattern emerged of how life changed over time; this became known as the theory of
BIOTIC SUCCESSION
>>> for example, in the oldest sedimentary rocks found, there are only simple, single-cell
organisms; in progressively younger rocks, we find the first appearances of invertebrates,
followed by the first appearance of vertebrates, followed by the first appearance of land
plants, followed by the first appearance of land vertebrates, followed by the first
appearance of mammals, followed by the first appearance of primates, followed by the
first appearance of humans
>>>> furthermore, once a particular biotic group (e.g., vertebrates) appeared in the rock
record, geologists found that, in progressively younger sedimentary rocks the vertebrates
showed a progressive change
>>>>> this showed that there was a change in the nature of life forms over earth history,
and that we could CORRELATE sedimentary rocks in different locations using the
fossils in them (for example, if a limestone in Kingston has fossils distinctive of the time
when the first invertebrates formed, but no younger fossils, and a limestone in Germany
has fossil fish, then the German limestone must be younger
> the fossil record also allowed geologists to produce a RELATIVE TIME-SCALE
>> the idea is that you give a specific name to the time period during which a particular
life-form (or group of life-forms) lived. For example, the time period representing the
first appearance of fossil fish is called the Devonian.
>>> the names chosen are usually based on the geographic location where those fossils
were first found (e.g., the Devonian is named after part of England called Devon where
the fossils of the first fish were found)
>> the boundaries between the different time periods mark a point in earth history when
there was some kind of marked change in its life
>>> for example, the appearance of a life form for the first time (e.g., the first fish at the
Devonian, or the major proliferation of life forms at the end of the Precambrian)
>>> for example, a time of major extinction of a lot of species (e.g., the end of the
Cretaceous, when the dinosaurs (among others) went extinct; or the end of the Permian,
when some 80% of species went extinct)
So, when a geologist finds a sedimentary rock, the nature of the fossils in it allows
her/him to determine where the rock fits on the Relative Time Scale
> the geologist can then 'phone someone up in Africa and say "my rock is Devonian in
age; what is the relative age of your sedimentary rocks, based on the fossils in it?"
Well, that is all fine, but what if i have found a granite here in kingston, and you have
found a granite in southern India
>> can we figure out the relative ages of these two rocks? It seems difficult, doesn't it,
since the rocks contain no fossils
>>> BUT! If we can find the granite cutting older sedimentary rocks with fossils, and
being covered by other, younger, sedimentary rocks with fossils, we can study the fossils
to get the relative ages of the sedimentary rocks, and from this BRACKET THE AGE OF
THE GRANITE
>>>> so we CAN figure out the relative age, on the Relative Time-Scale, of our two
granites, and figure out from that which is older
(2) ABSOLUTE TIME:
> in order to measure the ABSOLUTE ages of rocks, we need to find a natural process
with a constant rate
>> in 1898, Henri Bequerel discovered RADIOACTIVITY, and it soon became apparent
that this was the natural clock that geologists had been looking for
>> it was realized that radioactive isotopes of elements could be used to "date" igneous
and metamorphic rocks to determine when the igneous rock crystallized and when the
metamorphic rock was metamorphosed
This technique is known as GEOCHRONOLOGY:
> it is based on the spontaneous decay of radioactive isotopes of elements at a fixed rate
>> for example , one of the isotopes of K (40K) decays to an isotope of argon (40 Ar)
>>> (we call the radioactive isotope (e.g., K) the parent, and the end product (the Ar) the
daughter product)
Consider a granite that crystallized from a magma some time in the past. How long ago
did this happen?
> well, the newly-formed K-feldspar in the granite would start its life with no Ar, since
Ar is a gas that would be boiled off when the granite was a magma
>> once crystallized, however, the feldspar can trap any Ar produced by the decay of the
K (in other words, the clock starts at "time zero" when the mineral forms and starts to
trap the daughter product)
>>> over time, the 40K decays at a fixed rate to 40Ar; clearly, the longer the rock sits
around (i.e., the older it is), the greater will be the ratio of 40Ar to 40K
>>>> in other words, the age of the mineral is proportional to the ratio 40Ar/40K
Concept of HALF-LIFE:
> the decay of the 40K is exponential in form, and we can talk about the HALF-LIFE of
the decay
>> this is simply the time needed for half of a given amount of 40K to decay to 40Ar
> we can measure experimentally the HALF-LIFE for the decay or radioactive isotopes;
for the decay of 40K, the half-life is 1.3 billion years
> if you sampled the feldspar from a granite and found that the feldspar had exactly equal
amounts of 40K and 40Ar, then you would know that one half-life has passed, and that the
granite is 1.3 billion years old
(Note: the concept of half-life in the exponential decay of 40K is the same idea as the
exponential increase in population, but in the case of the increase, we talk about the
doubling time)
Now, it turns out that the K-Ar clock is not the only clock around in rocks.
> there are many, many others (e.g., U decaying to Pb; Rb decaying to Sr; Carbon-14
decaying to Nitrogen-14; etc.)
> this means we can check these clocks against each other, and if several different clocks
give the same age for the rock (which we do find to be the case), then we have
confidence in the technique
Now, we have a problem though: radiometric dating like we have just described is not
very successful at dating the age of sedimentary rocks, because such rocks can contain
fragments and crystals from a whole range of rocks of different age, since the fragments
were derived by weathering from older rocks
> hence, K-Ar dating of a whole bunch of grains from a sedimentary rock would give a
range of ages
SO, HOW DO WE PUT AGES ON TO THE RELATIVE TIME SCALE TO MAKE IT
AN ABSOLUTE SCALE?
> this seems a problem, given that the Relative Time Scale was built up from studying
fossils in SEDIMENTARY rocks, and we can't successfully date such rocks
ANSWER: the key is that we go to places where we have igneous rocks in contact with
fossiliferous sedimentary rocks, and then we date the igneous rocks by geochronology,
which allows us to bracket the ages of the fossiliferous layers
> this was done for sedimentary and igneous rocks from around the world until a wellcalibrated ABSOLUTE Time-Scale was produced (see the one in your lab manual, or in
your textbook)
EARTH PROCESSES: Change and Rates of Change
(1) First Point: geochronology showed us that there is an immensity to earth time
> the earth is ca. 4,500 million years old
> this led the writer John McPhee to coin the phrase DEEP TIME to give a sense of the
immensity (it is deep like an ocean is deep, and also philosophically deep)
> also, with so much time, even processes with very slow rates can have a profound
effect
>> e.g., "continental drift" at a slow 5 cm/year (the rate your nails grow) would allow an
ocean like the Atlantic to open up in only 100 to 200 million years (that is only 1/50th of
the earth's history!!)
(2) Second Point: have things taken place at the same rate in the past as they do now?
> this can be summarized as the debate between those believing in
UNIFORMITARIANISM versus those believing in CATASTROPHISM
Uniformitarianism: "the present is the key to the past". The idea is that, to understand
events and rates in the past, we just need to look at current processes.
Catastrophism: The idea is that there may have been different rates and/or events at
different times in the past, with occasional catastrophies
A good COMPROMISE is: scientific laws are the same in the present in the past, and this
can result in both UNIFORM processes and CATASTROPHIC events
>> consider, for example, the uniform deposition of sediments at a delta from the normal
day-by-day sedimentation, compared to the annual 5 day flood event that is
"catastrophic" (more sediment can be carried in those five days than in the rest of the year
combined). Or what of the mega-flood that happens once every 100 years? Etc., etc.
>> or consider the slow change of life forms that might happen over millions of years by
gradual changes in ecosystems versus the catastrophic mass extinctions that took place at
the end of the Cretaceous (65 million years ago), almost certainly related to a major
comet/meteorite impact that formed a 200 km-wide crater and drastically, almost
instantaneously, severely altered the environment
>>> none of these are against any scientific laws
CRITICAL POINT ABOUT CHANGE: What is the rate of change of the process?
> this is particularly important for species (like humans!)
>>> for example, the meteorite/comet impact 65 Million years ago created such a rapid
change in climate that many species could not survive
> let's look at the burning of fossil fuels by humans in this light. We are burning C at an
unprecedented rate, with the production of CO2 that goes into the atmosphere
>> is this a problem? We know that CO2 is a green-house gas, and so is the burning of
fossil fuels increasing the rate of global warming?
First: is CO2 in the atmosphere actually increasing as a result of burning fossil fuels, or is
it trapped down in the oceans (dissolved) as fast as we produce it?
> since 1950, actual measurements of atmospheric carbon dioxide has shown an increase
from ca. 0.031 % in 1950 to 0.034% today
>> but was this just a natural increase independent of humans?
> it seems to be clearly human-induced, as we can look back at CO2 levels in the
atmosphere even farther back
>>How do we do this? We take cores of ice from places like the Greenland ice-sheet. The
ice traps bubbles of air when it forms, and we can measure the carbon dioxide levels in
these bubbles. Furthermore, we can date the age of the ice layers because there are
distinctive annual layers that we can count
>>> in the time period from 1500 A.D. to the industrial revolution, the levels were about
0.028% carbon dioxide, but since 1850 or so there has been an exponential rise to the
current levels of 0.034%
So, YES, humans do seem to be responsible for a CO2 increase in the atmosphere
Second: has the increase in carbon dioxide in the past 150 years led to an actual increase
in average global temperature?
> this is a tougher question to answer
>> measurements from 1880 to today show a lot of ups and downs (some of the lower
temperatures can be directly related to major volcanic eruptions, such as Tambora in
1815 and Pinatubo in 1991, which ejected so much ash into the atmosphere that it cut
down the solar flux), but there appears to be a general trend of increasing temperature
(almost a 1 degree centigrade increase over that time period)
>>> so, PROBABLY humans are responsible for some of that increase
Third: how do we determine if the rate of temperature increase that we MIGHT be
responsible for is unusually rapid (i.e., potentially so rapid that some biota can't survive
the dramatic change)?
>> one way is to look back at the temperature variations over the recent geological past
to see if what is happening now is unusual
>>> we can do that by looking back at the recent geological record (the last 150,000
years) in ice cores and muds on lake bottoms
>>>> instead of looking at carbon dioxide content, we look at the ratio of "heavy"
oxygen isotopes (18O) to "light" oxygen isotopes (16O) in the ice and muds
>>>>> it turns out that this ratio will vary with the temperature in the atmosphere, and so
we can use it to see how rapidly temperature has changed in the recent past
>> in the past 150,000 years, the temperature has fluctuated between ca. 12 degrees
Centigrade and 17 degrees centigrade
>>> but it is not yet clear whether the current rate of increase is unprecedented.
Hopefully, further studies will provide an answer.
(3) Third Point: are there some processes that have changed progressively with time?
> such changes are called SECULAR changes
> for example:
(a) there has been a decrease in heat flow from inside the earth over time (the earth will
eventually cool right off until it is a dead body)
(b) there has been a decrease in intensity of meteorite bombardment over time (it was
particularly intense in the first 500 million years or so)
(c) there has been a change in the nature of life forms over time (in fact, it is that feature
that was used to develop the Relative Time-Scale)
(d) there has been a change in earth's atmospheric composition
Let's amplify on (d):
Change in Earth's Atmosphere Over Time:
How do we know that the Earth's atmosphere has changed over time?
> first, we know from looking at sedimentary rocks that have formed over earth history
that there must have been free water at the surface of the earth for at least the last 4
billion years
>second, astrophysicists who study the evolution of stars tell us that the sun's luminosity
has increased progressively, from about 70% of its present value at the beginning of the
solar system, to its current value
1. If we calculate what the temperature of the earth's surface would have been without an
atmosphere (based on the solar luminosity values), we find that it would have been about
MINUS 35 degrees Centigrade 4 billion years ago, increasing to about MINUS 25
degrees today.
>> in other words, any H2O would have been ice through earth history, which we know
to be false from our study of sedimentary rocks
>> (Note: the earth's surface warmed over time because of the increase in solar
luminosity over time)
2. What if the earth's atmosphere was always like today's atmosphere?
>> again, using the solar luminosity values and assuming a uniform atmospheric
composition over time, we find that the surface temperature 4 billion years ago would
have been about MINUS 25 degrees Centigrade, warming up over time until it reached
the current value of ca. PLUS 15 degrees Centigrade
>>> but the temperature would have stayed BELOW ZERO degrees Centigrade until
only about 1.5 billion years ago, which again we know to be false from our study of
sedimentary rocks
Therefore, we conclude that the atmosphere must have changed over time!
It must have changed in such a way as to maintain the surface temperature between 0 and
100 degrees Centigrade for most of earth history
>> studies of other planets (e.g., Venus and Mars), and probable earth-atmosphere
compositions, suggest the following:
Early Atmosphere: the earth's early atmosphere was CO2 - rich and O2 - poor
> we know it was oxygen-poor from our studies of Banded Iron Formation (see earlier
notes on sedimentary rocks)
> carbon dioxide is the most reasonable gas to have been higher in concentration in the
early history, because it is an abundant gas during volcanic activity, but most importantly
because it is a very efficient greenhouse gas
>> it would have acted in the early history to increase the temperature of the surface of
the earth to keep it warm enough for surface water to exist
Later atmosphere: the later atmosphere became CO2 - poor and O2 - rich
> we that by 2.2 billion years there was sufficient oxygen in the atmosphere to cause the
formation of Banded Iron Formation, and we certainly know that the oxygen level is high
now (20%)
> what about carbon dioxide ?
>> well, as the solar luminosity increased over time, there was a danger that the
greenhouse effect would lead to the surface temperature rising significantly over 100
degrees Centigrade (look at Venus, with its carbon dioxide rich atmosphere: it has surface
temperatures of ca. 500 degrees Centigrade).
>>> so something regulated the CO2 levels (i.e., progressively reduced them)to moderate
the temperatures
>>>> life is the answer! Plant life-forms began to take in CO2 in order to extract C to
build their organic parts, and this put O2 back into the atmosphere. This reduced the
carbon dioxide levels in the atmosphere, thus keeping the surface from getting too hot as
the solar luminosity increased
Consider the CARBON CYCLE in this regard:
> there is a constant cycle of carbon around the four main reservoirs: (1) atmosphere; (2)
biosphere; (3) hydrosphere; and, (4) sediments and rocks.
>> plants extract CO2 from the atmosphere and store the C in the biosphere (oxygen is
given off to the atmosphere in the process)
>> some of the CO2 is recycled from the plants when they die and decay at the earth's
surface
>> however, some of the dead organisms are buried in sediments, where they may remain
trapped for millions or hundreds of millions of years as kerogen, coal, oil and gas; there is
recycling of CO2 back into the atmosphere when oil leaks to surface and is oxidized, or
when we burn fossil fuels
>> there is some cycling of CO2 back and forth between the atmosphere and the
hydrosphere (CO2 will dissolve to a certain extent in the oceans; the amount is
temperature-dependent)
>> life-forms in the sea extract some of the CO2 from the sea-water, and combine it with
Ca to build shells of calcite (CaCO3 ); when the shells collect on the sea-floor to make up
the rock limestone, the CO2 is trapped down and buried in this solid rock (which can even
turn to marble if metamorphosed)
>> when limestone or marble are brought to the earth's surface, weathering will liberate
CO2 again to the atmosphere
>> finally, there is release of CO2 to the atmosphere from volcanoes when they erupt
This carbon cycle has maintained a delicate balance of CO2 in the atnosphere such that
the surface temperature of the earth has stayed in the magic window of 0 to 100 degrees
Centigrade
>> basically, life has moderated the temperature!! If the temperature gets too cold, plants
don't do so well and more die, which releases carbon dioxide into the atmosphere to
warm things up. If it gets very warm, plants flourish, extracting higher amounts of carbon
dioxide from the atmosphere, thus cooling things off. And so it goes, moderating the
temperature!
Last revision: 17 November 2000
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