The Rock Record Uniformitarianism A principle that geologic

The Rock Record

Uniformitarianism • A principle that geologic processes that occurred in the past can be explained by current geologic processes. • James Hutton, the Father of Geology, theorized that processes occurring in the present were the same processes that had operated in the past, and would be the processes that operate in the future. • “The present is a key to the past” • Examples: volcanism and erosion.

Earth Age • Before Hutton many people believed that earth was only 6, 000 years old, and believed that geologic features where about the same age. • Hutton started to look at the rocks around his farm and the natural processes that occurred. He then estimated that earth was one million years old.

Relative Age • History books are written about the Human history. • The history of the rocks can be written the same way, if we just look the rock layers or strata and study their placement and what separates them. • Find the older rock and the younger rock. Then compare against events that have occurred like volcanic eruptions, flooding, earthquakes, droughts, fossils, etc.

Law of Superposition • A general law stating that in any sequence of sediments or rocks, that has not been overturned, the youngest sediments or rocks are at the top of the sequence and the oldest are at the bottom.

Principle of Original Horizontality • State that sedimentary rocks left undisturbed will remain in horizontal layers

Sedimentary Structures • Graded Bedding • Cross Bedding • Ripple Marks

Unconformities A break in the geologic record created when rocks layers erode or when sediment is not deposited for long period of time. The types: • Nonconformity • Angular Unconformity • Disconformity

Index Fossils • Also know as Guide Fossils or Zone Fossils • Index fossils are used to define and identify geologic periods (or faunal stages). • The premise is that different sediments may look different depending on the conditions under which they were laid down, but they can contain the remains of the same species of fossil. • The species of the fossil and their relative life span can be used to identify when certain sediments were deposited. Fossils with relative short life spans (in geological terms, lasting a few hundred thousand years), are more valuable as different sediments can be more easily correlated. • The best index fossils are common, easy-to-identify at species level, that have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is minor.

• The best-known Index Fossils are Ammonites. Other fossils used as Index fossils are: corals, graptolites, brachiopods, trilobites, and echinoids (sea urchins). Fossilized teeth of mammals have also been used. • Geologists use both large fossils (called macrofossils) and microscopic fossils (called microfossils) for this process, known as biostratigraphy. Macrofossils have the advantage of being easy to see in the field, but they are rarer, and microfossils are very commonly used by oil prospectors and other industries interested in mineral resources when accurate knowledge of the age of the rocks being looked at is needed.