TOPIC 13 – GEOLOGIC HISTORY

DATING ROCKS There are two ways to get the ‘age’ of a rock: 1.) RELATIVELY ex) Rock layer ‘A’ is older than rock layer ‘B’ 2.) ABSOLUTELY ex) Rock layer ‘A’ is 860 million years old

Dating Method #1 - RELATIVE DATING The determination of an age of a rock or event in relation to other rocks. To do this, use: PRINCIPLE OF SUPERPOSITION When rocks are layered, the bottom layer is: OLDEST And they get progressively younger as you rise through the rock column

There are exceptions… FOLDING:

FAULTING:

How can we tell this has happened? 1.) Look for surface features Ex) ripple marks, mud cracks, etc 2.) Check for vertical sorting patterns “out of whack” 3.) Check fossil patterns – are they all upside down?

DATING LEFTOVER MAGMA INTRUSION – When molten rock squeezes into pre-existing rocks and crystallizes EXTRUSION – When lava flows on Earth’s surface and solidifies. KEY – extrusions are YOUNGER than rocks below it, and OLDER than any rocks that form on top of it.

UNCONFORMITY Missing time in the geologic rock record Occurs when layers are deposited, then eroded, and new layers are later deposited.

Where could there be an unconformity in this picture?

Whenever the Regents includes an unconformity in a rock column, they ALWAYS give you a HUGE clue:

Dating Method #2 - CORRELATION The process of showing that rocks or geologic events from different places are the same or similar age

Accomplished by using: 1.) OUTCROPS – exposed layers of bedrock Seen in roadcuts, etc… SAME LAYER

2.) Similarities between rock layers: Ex) red shale red shale, large breccia large breccia 3.) Similarities in fossils:

3.) INDEX FOSSIL – an incredibly useful tool! - They must have lived over a huge area -Must have lived over a very short time -Ex) TRILOBITE Occurred in the CAMBRIAN period (500 mya) So if a rock has this fossil in it, we know how old it is

So what can be done with these fossils? 1.) If you find a rock with a trilobite in it, it is approximately 500 myo 2.) If you find rock layers at different levels in the rock column with trilobites, they had to come from the same time period. 3.) If an older index fossil is on TOP of a younger one: Then the Law of Superposition has been disturbed

Fossils typically get more complex over time:

4.) Volcanic ash layers - They are spread through a very large area – they act as an AGE MARKER (since they happened over a very short time)

5.) Impact debris - A certain element is rarely found on Earth but frequently in space: IRIDIUM If this occurs in the rock record, it was most likely from an asteroid impact – and this can be matched to other layers around the Earth.