Chapter 8 – The Rock Record
There are two ways to date rocks or sediments. Absolute age gives an age in years to the rock or rock
layer. Relative age compares the age of a rock layer to another by stating whether it is older or younger.
James Hutton was the scientist known as the “father of geology.” He studied rock layers in his native
Scotland to develop geologic principles still used today.
There are five major principles used to help date rocks:
The principle of uniformitarianism states that the processes which are shaping the Earth’s surface today
are the same processes that changed the Earth in the past. For example, glaciers, rivers, volcanoes,
earthquakes, storms, floods, wind, and waves eroded or created Earth’s surface in the past like they do
today, but the rates of change may have been different. Another way of saying this is “the present is the
key to the past.”
The principle of superposition states that the oldest layer is on the bottom and the youngest on top, if
the sequence has not been overturned. Characteristics such as graded bedding (largest particles on the
bottom and smallest on the top), cross-beds (sandy sediment forming curved beds at an angle to the
bedding plane), mud cracks (cracks that result from the surface of a muddy sediment drying out), and
ripple marks (small waves that form on the surface due to wind or water), are all clues to interpret
whether the rock sequence has been overturned or not.
The principle of cross-cutting relationships states that if an intrusion or fault cuts across rock layers, it is
younger than the rocks it cuts across. The older rock had to be there first before the younger fault or
intrusion could cut across it.
The principle of included fragments states that the fragments included in a rock layer are older than the
layer in which it is included. This often occurs when a lava flow incorporates rocks on the surface as it
flows across them.
The principle of original horizontality states that sedimentary rock layers are deposited horizontally.
Rarely is the geologic history of an area preserved in a complete geologic column. Usually there are
pieces of the puzzle missing at one location due to erosion of some of the rock layers. Geologists must
look at different locations or outcrops to find what was missing at another location. The process is
similar to reading a book with a couple of pages missing, or putting together a puzzle with some of the
pieces missing. The more pieces missing, the harder the puzzle is to solve.
Where rock layers are missing due to erosion is called an unconformity. Unconformities are shown in
diagrams by a squiggly line. There are three types of unconformities. These are determined by what is
below the erosional surface. If a “non”-sedimentary rock layer (in other words: metamorphic or igneous)
is below the erosion, it is called a nonconformity. If there are tilted or folded sedimentary layers below
the erosion, it is an angular unconformity. If there are horizontal sedimentary layers below the erosion,
it is a disconformity.
The absolute age of a rock or rock layer is determined using radioactive isotopes of certain elements
which decay naturally from “parent” isotopes to “daughter” isotopes. The half-life of one of these
elements is the time it takes for half of the parent to decay. For example, if the half-life of Uranium235 is
704 million years, a 20-gram sample of Uranium235 will have 10 grams of parent isotopes (Uranium235)
and 10 grams of daughter isotopes (Lead207) after 704 million years.
Just like tree rings can show how old a tree is, some rocks that formed on the bottom of glacial lakes
also show seasonal changes called varves. During the winter months when the pond is frozen over,
there is very little deposition and a thin dark layer forms. During the summer months when more
sediments and organic matter are rushing into the lake from meltwaters, a coarse light-colored layer
forms.
Another way geologists determine the age of rock layers, or the past environments in which they
formed, is by studying fossils. The study of fossils is called paleontology. Fossils are not found in
metamorphic or igneous rocks, as the forces that create these rocks would destroy the fossil. Fossils are
mostly preserved in an environment where the plant or animal is buried rapidly by soft sediment. It also
helps if the organism had hard parts such as shells, teeth, or bones.
Mummification is one way fossils form in dry climates where the bacteria that decompose living
organisms cannot survive.
Hardened tree sap, called amber, preserves some small insects.
Permafrost preserves some organisms because the decomposing bacteria cannot survive these frozen
conditions.
Where petroleum oozes to the surface, a tar seep is formed, which traps and preserves animals as
fossils.
Groundwater carrying dissolved minerals can sometimes replace organic matter and form a perfect
replica of a plant. This is called petrification. An example is the Petrified Forest National Park.
A carbonized imprint of leaves or fish is another type of fossil.
An animal that is buried and then decomposes can leave an open space called a mold, and when this
space fills up with groundwater minerals a cast is formed.
Fossilized waster materials, called coprolites, and digestive stones, called gastroliths, are another type
of fossil.
Trace fossils are another evidence of life forms. Examples of these include footprints and worm burrows.
Certain fossils that lived during a relatively short period of time and are found over a large geographic
area provide useful reference points when determining relative age. These are called index fossils.