The Age of Rocks Relative vs Absolute Age

The Age of Rocks Relative vs. Absolute Age Unconformity Index Fossils and Pangaea

Types of Rocks § Igneous Rock (formed when magma cools) § Metamorphic Rock (other rock types transformed by heat and pressure) § Sedimentary Rock (rock formed from minerals deposited near bodies of water; forms rock layers) Fossils form in Sedimentary Rocks

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

Law of Superposition § Which layer is the oldest? § Which layer is the youngest?

Relative Age vs. Absolute Age § Relative Age The age compared to the age of other rocks. § Absolute Age The number of years since the rock has formed.

Age Activity § First, separate into 2 or 3 groups based on your age. Let’s compare the relative age of the groups. § Then, line up in order based on your exact age, youngest to oldest, along the lab counters. Let’s compare the absolute age of everyone.

Intrusions and Faults § Sometimes an igneous intrusion or fault will cut across or into the rock layers. § The intrusion or fault will always be younger than the rocks being intruded or displaced.

What is an Intrusion? When magma moves up into the rock layers creating an igneous rock formation.

What is a Fault? When layers of the Earth’s crust shift and move.

Relative Age

Unconformity § Unconformity – Erosion causes new rock layers to meet older rock layers. § This leads to a “gap in the geologic record” (missing rock layers and fossils).

Steps of Unconformity 1. Sedimentary rock layers form. 2. The rock layers fold and tilt. 3. Erosion moves rock layers at the surface away. 4. New rock layers form on the surface and creates the Unconformity. The missing rock layers and fossils are the Unconformity!

Index Fossils § Index Fossils - Fossils that are widely distributed in the world and are used to tell the relative age of rocks.

Index Fossils § Index Fossils also support the idea that Pangaea (one super-continent) once existed.

Video Clips for Monday 4/1 § Plate Tectonics and Continental Drift § https: //www. youtube. com/watch? v=Wq 9 k. Lzm 36 h 0 § Who is Alfred Wegener? § https: //www. youtube. com/watch? v=nb. U 809 Cyrao