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Making Sense of Things
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Making Sense of things
Geologists Have Observed The Natural world for a Long time.
Sometimes The layers of Rock (Strata) they are observing Are very jumbled and Confusing.
In an Effort to Read What James Hutton called "The Annals of The Former world" , The environments and Events that occurred before the present Time, Geologists have created 8 Principles to guide their understanding .
These Principles help them create logical Inferences and Plausible explanations.
Using the Principles of Relative Dating
The Principles
The Principles
Principle of Uniformitarianism
Uniformitarianism
The geological processes of the earth have been uniform throughout its history.
Another way of saying it is that we assume that
the geological processes that we can observe today are happening the same way they have happened throughout the earth's history.
This is a big assumption, and this principle's assumption is one that we base all the other principles upon. However it seems a reasonable principle
Evidence for Uniformitarianism
Evidence
By looking at the processes occurring now we can make sense of things that occur in the geological record.
Here the stratification of this sandstone seems to show the same structures (cross-bedding) that sand deposited at the bottom of a shallow river creates.
We can come up with a new explanation for the stratification we see in sandstone, or we can decide that it is more plausible that sand and rivers millions of years ago behaved very much the same way they do now.
Zoom in for another Example
Side by side of a current geological process and an ancient one that seems remarkably similar. Uniformitarianism!!!!
Using Uniformitarianism
Here are some rock strata that uniformitarianism can help us make sense of.
Each of these layers was deposited by a different flooding event.
Flood
Using the Principle
Flood
Flood
But....
Flood deposit sedimentation
Clay
small clasts
Silt
In a flooding event, or any kind of sediment deposited by water. we expect to find larger sediments on the bottom and smaller sediments towards the top.
The larger sediments are heavier and therefore settle more quickly.
The lighter smaller sediments stay suspended longer and will end up settling on top of the heavier sediment below.
We Expect to find this pattern in Rock Layers created by Flood deposits.
Small Sand
Large Sand
Pebble sized (if any)
large clasts
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An Unexpected Pattern
Here we see the Rock Strata have exactly the opposite Pattern we would expect to see from a Flood deposit.
WE Can invent a new Idea about How they Formed, or we can decide the layers formed normally (Uniformitarianism) and Think about what else might have occurred.
In This case Geologists Have decided that it is more Plausible that All these Rock Layers have been Flipped Upside down by Geological Events (which Might Also Explain the Fault on the Right).
They call Rock layers like This "Overturned Beds".
Large Sand
Small sand
silt
Clay
Original Horizontality
Sedimentary layers, Being the Product of Erosion, Are Deposited Horizontally, parallel to the surface of the earth.
If Sedimentary layers are Not Horizontal Then They Have Been Lifted by an event.
That Event Must be more Recent than the Rock layers that Are Lifted.
These Marin Headlands Chert Layers were deposited Horizontally on an Ancient Deep Ocean Floor. They've Been Lifted almost Vertically by The Geological Processes that Built California.
Original Horizontality
Superposition
Youngest Rock
Superposition
Since Sedimentary Layers are created by deposited sediment, A Given Rock layer is younger Than the Layers Below it.
If you were to float the Grand Canyon, As you moved downstream more and more Rock layers would become visible to you and the canyon walls would grow higher as the river cut deeper and deeper through the sedimentary layers below it.
You would be Moving "DownSection", as Geologists Call it, Through older and older rock as you go deeper into the canyon.
Oldest Rock
Lateral Continuity
Lateral Continuity
Since Sedimentary rock layers are deposited continously, when they are found separated and discontinous, it is concluded that the event which seperated them is more recent than the rock layers themselves.
Here we see where a giant flooding event punched through a hillside and changed the course of the Columbia river.
It's more likely that these two hillsides were once connected and were broken in this spot by a flood, than it is that they each formed independently.
Intrusive relationships
When igneous layers are found within other rock layers, the igneous layers must have "intruded" into the already existing rock layers above them.
Therefore the rock layers around the igneous intrusion must be older than the intrusion itself.
Intrusive Relationships
Double igneous intrusion
Mafic Dike
Felsic Sill
Granitic Dike
Cross-Cutting Relationships
Cross-cutting
When a fault cuts through rock layers, the fault must be more recent than the rock layers it cuts through.
If the rock layers are displaced across that fault, the event which caused the displacement must also be younger than the rock layers.
Displacement
Crosscutting Relationships
Fault
Another fault showing less pronounced displacement.
Inclusions and Components
Gabbro inclusions (xenoliths) on the second pitch of Serenity Crack in Yosemite.
If a rock contains inclusions or components (i.e. clasts in a sedimentary rock, or xenoliths in an igneous rock) those inclusions must be older than the rock that forms around them.
Inclusions and Components
Metamorphic Inclusion
Clastic inclusions
xenolith in diorite
Faunal Succession
Faunal Succesion helps us compare the ages of sedimentary rock layers by looking at the fossils in them.
For instance, if we know that blue snails(older) and red snails(younger) only overlapped for a short time, but we find an unknown fossil amongst both blue and red snails, we can conclude that it is from that period of time.
If we find that fossil in other rocks we know that those rocks could also be from the same period.
Faunal Succession
Graphic from Daniel Peppe, Nature Education 2013