GEOLOGIC HISTORY THE RULES OF THE GAME © Copyright 2005 - 2006  M. J. Krech. All rights reserved.

Time What is time?

Time is… Either relative or absolute Time is: a measured period during which an event, process, or condition exists or continues.

One Year = the time it takes the Earth to complete one orbit around the Sun.

Geologic Time Based on the same time units Generally much greater periods of time 100,000 years 1,000,000 years

Dating Techniques One ways in which geologists look at time: Relative Dating

Relative Dating is.. …the measure of a sequence of events without knowing the exact date at which the events occurred. What happened first? What happened next?

Principles of Relative Dating Principle of Superposition Principle of Cross-Cutting Principle of Inclusions Principle of Unconformities

Principle of Superposition The youngest rocks are on the top. The oldest rocks are on the bottom.

Principle of Superposition Q#1 Which rocks are older? Which rocks are younger?

Principle of Cross-Cutting Any feature that cuts across another rock or structure is younger than what it cuts across. You can't cut something until that something exists, therefore the cut has to be younger. Volcanic vents cutting across sedimentary rock layers - Which is youngest?

Principle of Cross-Cutting Q#2 What is younger? The rocks or the fault line?

Principle of Inclusions Rock layers which contain other rocks are younger than the inclusion. The inclusion (the other rocks)  is older!

Principle of Inclusions Q#3 Which is older? The rock layer or the rocks included in the layer?

Principle of Unconformities Unconformities are surfaces that represent gaps in the geologic record.

Principle of Unconformities missing rock layer = missing time

Where's the missing sediment? Q#4 How do you know?

Where's the missing sediment? Q#5 Where's the missing sediment? How do you know?

What happened first? Q#6 How do you know?

Where is the youngest rock? Q#7 How do you know?

Where is the unconformity? Q#8 How do you know?

What rock layer is the oldest? Q#9 How do you know?

Which is younger? The fault or the rock layers? Q#10 How do you know?

Does this diagram show an unconformity? Q#11 How do you know?

Which is younger? The rock layer or the inclusion? Q#12 How do you know?

The End BONUS: What do you see here?