Geology A Peak at Earth’s History

The Key to the Past is the Present Uniformitarianism –is the assumption that the natural processes operating in the past are the same as those that can be observed operating in the present –What does it Mean? By looking at the processes that are going on today we can get an idea of what happened in the past

Volcanoes Today we can observe a volcano erupt lava, that lava forms black rocks called Basalt –When we find basalt, we can assume it was formed by cooling lava

Erosion and sediments Today we can observe erosion by water, carrying sediment down stream, down cutting canyons and arroyos. We can see the soil and sediments being deposited down stream when the water slows down.

Sediments Larger particles will be dropped from the water first, smaller particles stay suspended in the stream, until it slows down. Sand is deposited on the inside of stream bends. When we find narrow sections of sandstone, we might determine they were deposited along a stream channel

Deltas In Deltas where rivers carry sediments into lakes and oceans we see this pattern –Sand closest to shore –Then silt –Then the smallest particles of clay

Ripples & Mud cracks We can see ripples in the sediments on the bottoms of lakes and ponds caused by the waves We can assume was rocks with ripples formed under water We see Mud Cracks when wet soil drys out We can assume rocks with cracks were formed after an area dried out

Mud Cracks Recent mud Cracks (Below) Fossilized Mud Cracks

Limestone Limestone is formed when marine organisms deposit their calcium carbonate shells or microscopic tests on the bottom of the ocean When we see limestone today we can assume that it was formed under the ocean. Sandia Limestone 

Stratigraphy Okay, if you are like me, your desk piles up with papers over the course of a day or week. The papers that were there first are on the bottom and the newest papers are on top OR you go to the grocery store and you throw a bunch of food in your cart and fill it up. The stuff on the bottom would be from the first isle you go down and the stuff on the bottom would be from the first isle.

Stratigraphy Sedimentary rocks from in the same way. Oldest layer are on the bottom, and the youngest layers are on top… Ever been to the Grand Canyon? Oldest rocks are along the bottom, youngest rock are on top Unless some thing happens to disturb them…

Like a meteor impact Meteor crater in AZ Some layers of strata Were flipped during the impact

Erosion Layers can be eroded away…. Leaving “Gaps” in the geological record Imagine if a volcano erupted and dropped a layer of ash on this outcrop in the Painted Desert (AZ)

Grand Canyon Oldest layer are on the bottom Youngest layers are on top Can tell relative age by position ie which is older, which is younger Can not tell absolute or exact age by position

Index fossils Organisms that were short lived in geologic time make good index fossils Used to help age sediments

Absolute dating Radiometric dating Certain elements give off neutrons at a certain rate called a half-life By comparing the ratios of the parent element to the Daughter product we can determine a rock’s absolute age