Determining the Age of Rocks Relative Age of Rocks

Types of Rock Sedimentary rock forms from sediment (sediment- solid particles of rock produced by weathering and erosion by water and wind) * Igneous rock forms from volcanic lava flows * Metamorphic rock- forms from intense heat and pressure

Relative Dating of Rocks The relative age of rocks determines the age of rock layers as younger or older, but does not give the exact age. The principle of uniformitarianism states that forces that shaped the earth in the past continues to shape the earth today (volcanoes and weathering and erosion by wind and water).

Sedimentary Rocks Sedimentary rocks formed from sediment deposited millions of years ago. Sedimentary rocks are deposited in horizontal layers. (principle of original horizontality) Sedimentary rock is deposited in horizontal layers over geologic time with the oldest layer on bottom and the youngest layer at the top (principle of superposition)

Sedimentary Rock Layers Remember - sedimentary rock layers are horizontal with the oldest at the bottom and the young at the top unless the layers are disturbed by a fault or igneous intrusion

Faults and Igneous Intrusions Sometimes sedimentary rock layers are disturbed by geological forces. A crack in the rock layer is called a fault. When igneous rock (volcanic lava) intrudes or cuts through layers of sedimentary rock it is called an igneous intrusion. The principle of cross-cutting relationships states that a fault or igneous intrusion is always younger than the rock it cuts across.

Fault (Crack) in Sedimentary rock Here is a picture of a fault or crack in the rock

Igneous Intrusion in Rock Layers Remember that igneous intrusions (lava) are always younger than the rock layers they cut through. Note the igneous intrusions (volcanic rock) cutting through the rock layer

Relative Age of Rock The Relative Age of the rock can be determined by the sequence of the rock layers using the Principle of Superposition (oldest on the bottom and youngest at the top). If the sedimentary rock layer has been disturbed by a fault or igneous intrusion, the fault or intrusion is always younger than the rock layer they cut through (cross-cutting)

Finding the Relative Age of Rocks using Index Fossils Certain fossils called Index fossils help geologists determine the Relative Age of rocks. To be a useful Index fossil: - the fossil must be widely distributed different geographic areas - and represent an organism that existed only briefly

Index fossils Trilobites were a group of hard shelled animals that evolved in shallow sea more than 500 million years ago (trilobites became extinct at the end of the Paleozoic Era) Trilobites are useful as index fossils because they were widely distributed geographically and represent an organism that existed briefly.

Example of an index fossil (trilobite) in layers of sedimentary rock. How old is the bottom layer?

Continental Drift Theory All continents were once joined together in a single landmass (supercontinent) called Pangea Fossil evidence supports the Continental Drift Theory. Fossils from a fernlike plant Glossopteris have been found in Africa, South America, Australia, and Antarctica. The seeds could not have traveled across the span of the oceans

Absolute Age of Rocks To determine the Relative Age of Rocks geologists use the Principles of Superposition, Cross-Cutting Relationships, and Index fossils. To determine the Absolute Age (exact age) of rock, geologists use Radiometric Dating. How did geologist determine the Earth was 4.6 billion years old?

Radiometric Dating Radioactive elements are found in igneous rocks. Radioactive Dating can only be used for igneous (volcanic ) rock The rate of decay for radioactive elements is a constant called a half-life. A half-life is the time it takes for half of the radioactive atoms to decay. As the radioactive atoms decay, they change to another element

How old is the Earth? Radioactive dating has been used to determine the age of the Earth. The oldest rocks found on Earth are 4.0 billion years old. The moon formed when a large object collided with Earth. In 1969, Apollo 11 landed on the moon. The age of the moon rocks is 4.6 billion years old Scientists infer that the Earth is a little older than the moon – roughly 4.6 billion years old

Carbon-14 Dating A radioactive form of carbon (Carbon – 14) is used to date once living plants and animals Carbon -14 is very useful in dating plants and animals (for example a frozen Wooly Mammoth) that lived up to 50,000 years ago. Carbon -14 has a half-live of only 6,000 years and cannot be used to date ancient fossils

Virginia Fossils Fossils from Precambrian Time are not found in Virginia because there was no sedimentary rock, so, no fossils. The state fossil in Virginia is a marine scallop called Chesapecten jeffersonius, which lived in shallow waters (early Cenozoic Era) Ancient shark teeth fossils are also found on land in Virginia. Marine fossils are found in the Appalachian Mountains because rock in Virginia was once underwater (ocean).