How do scientists construct the geologic history of an area?

The rocks to the right were found in areas miles away from each other. Observe the rocks. What inferences can you make about their formation?

Today’s Standard1.2j: Geologic history can be reconstructed by observing sequences of rock types and fossils to correlate bedrock at various locations. . The characteristics of rocks indicate the processes by which they formed and the environments in which these processes took place. . Fossils preserved in rocks provide information about past environmental conditions. . Geologists have divided Earth history into time units based upon the fossil record. . Age relationships among bodies of rocks can be determined using principles of original horizontality, superposition, inclusions, cross-cutting relationships, contact metamorphism, and unconformities. The presence of volcanic ash layers, index fossils, and meteoritic debris can provide additional information.

Correlation

process of showing that rocks occurring at different locations are the same

What information do scientists look for when correlating rock layers?

A. EXPOSED BEDROCK -"Walking the outcrop." Following a layer or formation from one location to another. B. SIMILARITY OF ROCKS - Observe similarities between rocks in different locations. (Layers with matching colors, composition, grain size) C. VOLCANIC ASH - Since the composition of eruptions is different, it is possible to identify layers of ash from different eruptions. Ash from an eruption may be deposited over large areas of Earth's surface in a very short time.

D. METEORITE DEBRIS – impact meteorite throws dust into the atmosphere, which settles over a wide area. E. FOSSIL EVIDENCE – Index fossils - organisms that lived for a brief period of time and were found all over the world can be used to tell the age of rocks.

What are the different types of fossils?Types: molds, casts, impressions, amber, etc.

Formation- quick burial and hard body parts

Importance-*Fossils preserved in rocks provide information about past environmental conditions.*Fossil evidence indicates that a wide variety of life forms has existed in the past.

*Geologists have subdivided geologic time into units, based on fossil evidence.

What is an index fossilThe fossil remains of an organism that lived in a particular geologic age, used to identify or date the rock or rock layer in which it is found.A fossil that lived for a short period of time.

SEE ESRT p. 8.[Show how to read Eon, Era, Period, Epoch, Life on Earth, Time Distribution of fossils]Look under “Life on Earth” during the Cambrian Period. – Earliest Trilobites.Look under “Life on Earth” at the end of the Permian Period – Extinction of many kinds of marine animals, including trilobites.Look at the fossils on the top of the page. WHICH THREE FOSSILS LOOK LIKE SPECIES OF TRILOBITES? (A, B, and C) [Note: there are many more species of trilobites than these three]. They came in many sizes from 1” to 1’.DURING WHICH PERIOD DID ELLIPTOCEPHALA (fossils A) LIVE? CambrianDURING WHICH PERIOD DID CRYPTOLITHUS (fossil B) LIVE? Ordovician.DURING WHICH PERIOD DID PYACOPS (fossil C) LIVE? Devonian.ARE THESE TRILOBITES GOOD INDEX FOSSILS? (Yes, different species lived for relatively short periods of time)

Work Time: Lab 11-2 Matching rock layers

Closing activity: Discuss index fossils and their importance in determining relative age of rocks.