How old is old?

Earth Processes take a really, really, really (really) long time….. So, the question is – How do we know how old something is? Especially if no one was around to see it formed? We apply scientific principles. The Principle of Uniformitarianism (James Hutton, late 1700’s) states that the same processes that are occurring today also occurred in the past. This means that since we can see volcanoes erupting now, we assume that volcanoes also erupted millions of years ago.

We have 2 methods to determine the age of something. Relative Age This puts events into order from oldest to youngest. For example, Johnny was born before Susie, therefore Johnny is older. Absolute age This gives a specific number to an age. For example, Johnny is 15 and Susie is 12.

Relative Age Scientists use several ways to determine relative ages. They include: original horizontality superposition cross-cutting relationships inclusions.

original horizontality all sediment is originally deposited in water formed in horizontal layers sediments will settle to bottom and blanket the sea floor

Law of superposition If the rocks have not been disturbed, than the oldest rocks are at the bottom and youngest at the top youngest oldest

cross-cutting relationships an intrusion or fault is younger than the rock it cuts across The rock layers had to be there first so that the intrusion or fault could cut through them.

inclusions fragments of other rocks contained in a body of rock must be older than the host rock For example 1. Xenoliths (chunks of other rock) in granite are older than granite 2. pieces of rock in conglomerate are older than conglomerate

Let’s figure this out together. In what order did events have to occur for the rock to look like this? 1. Sediment layers 4 thru 1 had to be deposited first. Number 4 came first, then 3, 2 and The igneous intrusion #6 came next. We know this because #5 cuts through #6. 3. Igneous intrusion #5 occurred last.

relative age: correlation Correlation is the matching of unique rock outcrops or fossils exposed in one geographic region to similar outcrops exposed in other geographic regions. Coconino Sandstone

Correlation using rock layers Through correlation of many different layers of rocks, geologists have determined that Zion National Park, Bryce Canyon, and the Grand Canyon are all part of one layered sequence called the Grand Staircase.

Correlation using fossils Correlating fossils from rock layers in one location to rock layers in another location shows that the layers were deposited during roughly the same time period, even though the layers are of different material

Complete Activity in your workbook

Absolute Age Scientists can use several methods to determine absolute age: Radioactive decay Radiocarbon dating Tree ring dating Ice cores Varves

Radioactive decay The emission of radioactive particles and the resulting change into other isotopes over time is called radioactive decay. This decay happens at a constant rate, so it can be used to determine the age of the minerals in a rock. For sedimentary rocks, scientists date layers of igneous rock or volcanic ash above and below the sedimentary layers

Radiocarbon dating This is used to determine the age of carbon material, or things that were alive. The amount of carbon decreases over time which allows scientists to determine how long ago the object died.

Dendrochronology – Tree Ring dating Every year, a tree produces two rings (a dark-colored and a light- colored ring). Counting the rings allows scientists to know how old the tree was when it died. Used in combination with carbon- dating, this can help determine the age of a tree.

Ice Cores Ice cores contain a record of past environmental conditions in annual layers of snow. Geologists use ice-cores to study glacial cycles.

Varves Bands of alternating light- and dark-colored sediments of sand, clay, and silt are called varves. Using varved cores, scientists can date cycles of glacial sedimentation over periods as long as 120,000 years.

Geologic Time Scale p.591 Activity p. 167 Instead of drawing the scale, write the name of the period next to the questions for numbers 4 – 15 at the top.