TELLING DEEP TIME
Lecture Four, September 20 th
How do we tell geologic time? Why do we care about it? Well, believe it or not, it actually
affects every single second of your life. You just don’t realize it. Steno’s principles, how they
apply to understanding the rock record and geologic time, are related to the method that we’ve
been using to extract energy from the surface of the planet for about the past 150 years. Where
does the fuel for your car come from? What kind of scientific construction is evolution? It is a
theory. Evolution is a theory. We have very well calculated geologic time for the rock record,
which allows us to measure how fast things evolve. Geologic time is important. There are two
ways that we construct the time scale.
(1) Absolute techniques: these include radiometric techniques, particularly radioactive
(2) Relative techniques: such techniques use different methods for taking different sections
of rock and determining whether they’re younger or older than each other. They cannot
give you absolute numbers. Only absolute techniques can do that. One example, amongst
many, is biostratigraphy.
We use these two methods in combination to create a giant framework of time within which we
can place rocks at any time and place.
What is the difference between two isotopes of an element? They differ in the number of
neutrons. There are protons and neutrons in the nucleus, and electrons which orbit around the
nucleus. Neutrons are neutrally charged. If you changed the number of protons in an element,
you change the element. If you change the number of neutrons, you simply change the isotope.
You can have unstable isotopes that can go down to stable daughter elements. This is the decay
series, which includes such elements as uranium decaying to lead. It is a series of steps by which
decay occurs. Now, there are some important things to note: 1. Radioactive decay is spontaneous
and occurs at a constant rate. If you know the rate at which the decay occurs, theoretically you
could look at a sample of elements and you could measure the amount of parent isotope that is
unstable to daughter isotope that is stable, and you could know that the daughter isotope is the
result of decay of the parent. If you look at a sample that is half lead and half uranium, it tells
you that half of the uranium has decayed.
The fact that you have radioactive elements in lavas/magmas is significant. As lava cools,
unstable parent elements will crystallize. So, you have a crystal that is 100% parent element at
the time of crystallization. As soon as it locks up, some of the parent element will spontaneously
decay. Remember, it is spontaneous and occurs at a constant rate. As crystals of radioactive
elements form, at the point of formation of the crystal, it’s all radioactive parent element. Now,
the millisecond after crystallization, you’ll have some decay. The older the crystal gets, the more
decay you will have. Eventually, there will no more of the parent element left. All of it will have
decayed into the daughter element. There are different types of radioactive decay:
Alpha Decay: the loss of two protons and two neutrons from the nucleus.
Beta Decay: the loss of an electron from neutron. This increases the number of protons by one.
Electron Capture: the proton captures one electron and becomes a neutron.
The decay is exponential. We measure the amount of decay from the parent to the daughter in
what is called the half-life.
What is the half life? It is the amount of time that half of a sample of a parent takes to decay to a
daughter isotope. At one half life, what percentage of the parent and daughter elements,
respectively, will be left? 50% each. After two half lives, what percentage of the parent and
daughter elements, respectively, will be left? 25% and 75%, respectively. After three half
lives...and so on and so forth. It is called an exponential decay. As you go through time, because
decay is spontaneous and constant, it is always 50 percent of the remaining parent sample that
decays. Thus, the parent element never actually decays completely into the daughter element. In
actuality, the exponential decay is not very fast. Half lives are extremely long. The half life of
uranium 238 to lead 236 is 4.5 billion years. That’s how long it will take us to get from 50%
uranium to get to 50% lead. Different radioactive parents decay at different rates. The rate at
which any one radioactive parent element decays is constant. It does not change. Different
parents decay at different rates into their respective daughter elements. We can thus use different
minerals to calculate the age of rocks.
Not all radioactive elements are useful for dating rocks. Some elements decay very fast. Some
decay very slowly. If a rock is too young, you sometimes cannot calculate its age. Any rock that
is between 100,000 and 4.6 billion years old, you can measure with potassium-argon. Knowing
the rate of decay, we can figure out how old rocks are. In truth, however, we’re only figuring out
how old those particular minerals are.
As magma cools, crystals form. It will eventually become solid igneous rock. All you can get
from the mineral is the age of that mineral.
Now, it does get tricky. There are three types of rocks: igneous, sedimentary, and metamorphic.
However, fossils are almost only found in sedimentary rock, formed from the weathering of pre-
existing rock. Organisms die, get buried in sediments, the sediments lithify and preserve the
record of life. Now, the fundamental problem is that you can only use most of the radiometric
dating techniques we’ve discussed on igneous rocks...but fossils are not found in those rocks
(with a few exceptions). There’s only one sedimentary rock that you can date with such
techniques. So, if we can’t measure the absolute ages of sedimentary rocks, how do we know
how old fossils are?
First, let’s say you’re looking at sandstone. The sandstone includes radioactive minerals of
uranium. Why can’t you use that to date the age of the sandstone? Well, because it will only tell
you the age of the mineral.
How do we figure out the ages of sedimentary rocks? Remember Steno’s law of superposition? The sedimentary unit on top must always be younger.
You have basalt, for which you can determine an absolute age. You have sandstone with a
dinosaur in it. How old is the dinosaur. Well, since we know the absolute age of basalt, and the
sandstone (which contains the dinosaur fossil) is on top of the basalt, the dinosaur fossil must be
less than 400 million years old.
The rock record is not always that easy. It is a lot more complex in the real world. A lot of rocks
undergo transformations. There is igneous rock seeping into pre-existing rocks. There is another
Radiocarbon Dating: this is a totally different method, but uses the same principles of radioactive
decay. How does it work? Well, here’s a question. What element makes up the majority of our
bodies? Carbon. Carbon has two isotopes. Namely, these are C12 and C13. We’re mostly made