The Relative Age of Rocks 10-2

Place in order from youngest to oldest B A D C E F

Answers youngest to oldest F E C B A

What is the age of each? D F E C B A

Relative Dating Used to determine the order of events and age compared to surrounding materials Can not tell exact age

Law of Superposition Older layers of rock are beneath younger layers of rock in the rock record

Law of Superposition In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Igneous Rock Clues Forms when magma or lava hardens Extrusion – lava that hardens on the surface -an extrusion is always younger than the extrusion below it

Extrusion When lava hardens on the surface. It is always younger than the rock below it.

Igneous Rock Clues Intrusion – magma pushes into bodies of rock then cools and hardens - an intrusion is always younger than the rock layers around it Intrusion

Intrusion A mass of igneous rock below the surface is called an Intrusion. It is always younger than the rock layers around and beneath it.

Fault A fault is a break in the Earth’s crust. A fault is always younger than the rock it cuts through. It makes the layers not line up.

Unconformities Occur when there are missing layers in the rock record Erosion of existing layers No new deposition for a period of time (disconformity) Angular Unconformity – tilted rock layers meet horizontal rock layers indicating gaps in rock record

Unconformity The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. It is a gap in the geologic record.

Index Fossils Index Fossils help match rock layers. Must be widely distributed and represent a type of organism that existed only briefly. They tell the relative ages of the rock layers in which they occur.