HMXEarthScience

Geology: Earth's History

Geologic Time

Geologic time is broken into two eons. The precambrian eon represents about 80% of the history of the earth. The Phanerozoic eon is modern history and represents the explosion of diverse life on earth.

Eons are divided into eras, which are divided into periods, which are divided into epochs. This time scale is an example of relative dating.

The centerfold lists the biological and geological history of the earth (pages 8 and 9 of the ESRTs). The type and distribution of fossils gives us information about the properties of the past environment in that location.

The Generalized Bedrock map of New York State details the specific ages of the bedrock in different regions of New York State (page 3 of the ESRTs).

Sequence of Events Laws

By studying layers of rocks within a geologic outcrop, we can piece together the sequence of events that occurred in that region to form the outcrop. To do so, we must follow some basic geologic laws outlined below.

• Uniformitarianism- “The present is the key to the past”
• Relative Age- “This layer is older than that layer”
• Absolute Age- This layer was deposited 105 million years ago.
• Original Horizontality- Sediments are deposited in horizontal layers parallel to the earth’s surface.
• Superposition- The oldest layer is on the bottom and the youngest is on top (assuming the layers have not been overturned).
• Intrusions and Extrusions- An intrusion or extrusion is always younger than the rock it cuts through. Look for regions of contact metamorphism to determine the sequence of events.
• Faults, joints, folds, and tilts- All deformation is younger than the rocks that are deformed.
• Fragments- All fragments in a rock are older than the rock in which they appear.
• Unconformity- A buried erosional surface. Implies that uplift, weathering and erosion, and subsidence (or submergence) have occurred.

Correlation

Correlation is the process of matching rocks and events in one location with rocks and events in another location in order to determine the geologic history of the region.

• Walking the outcrop- Tracing a rock layer from one location to another by observing characteristics like color texture and sequence.
• Index Fossils- Fossils of organisms that lived over a wide geographic area but for a relatively short period of time. These are used to match up different rock layers.
• Volcanic Time Markers- Layers of ash resulting from a volcanic eruption that can be used in the same way as index fossils.

Radioactive Decay

If an isotope (forms of chemical elements that differ in the number of neutrons in their atomic nuclei) is radioactive, it will break down naturally into a lighter element called a decay product. This process occurs at a predictable rate and can be used to determine how old an object is. One half-life is the time required for half of an element's atoms in a sample to change to the decay product. In each half-life only half of the remaining radioactive atoms decay, no matter how large the sample is. Look at the diagram below which represents the radioactive decay of uranium-238. The shaded area represents the decay product which is lead-206. The half-life of uranium-238 is 4.5 billion years, since this object has gone through two half-lives it is 9 billion years old.How can we determine the number of half-lives that have occurred? Look at the decay-product ratio, which is the ratio between the mass of a radioactive element and its decay product. This will help calculate the number of half-lives that have occurred since the sample was formed. Once you know that you can find the age of the sample. For example, If an object is made up of 50 % decay product then the object has been through 1 half-life. 75% decay product equals 2 half-lives, 87.5% decay product equals 3 half-lives, 93.76% decay product equals 4 half-lives, and so on. Once you know the number of half-lives you can determine the age by using the half-life in years listed on the back of the Reference Tables.