Relative Dating & Deforming the Earths Crust
Folds, Faults and Deformities OH MY!! Is this the correct principle?
Uniformitarianism Geologic processes that occurred in the past canbe explained by current geologic processes Erosion and weathering that happens today musthave been the same millions of years ago Very slow, gradual, predictable process Is this the correct principle? Why was this the accepted principle?
Catastrophism Geologic change happens suddenly Mountains, canyons and oceans formed during rareevents These rare events are called catastrophes Up until the mid-1800s, this was the acceptedprinciple Why was this the accepted principle? Modern Geology Uniformitarianism Catastrophism
Can geologic processes take a very long time?(think Tectonic Plates, mountains growing) YES! Mountains forming/growing Grand Canyon forming from erosion Moving of the continents Can events happen very quickly? (Think meteor causing dinosaur extinction) Large Volcanic eruptions forming islands and causing extinctions Meteor strikes creating craters and causing extinctions Earthquakes, Tsunamis, etc Relative Dating Relative Dating a method used to determinewhether an object or event is older or younger thananother object or event Who is Older or Younger? Absolute Dating Absolute Dating any method used to determinethe age of an event or object in numbers Certain molecules called Isotopes decay over time Isotope An atom that has the same number ofprotons, but a different number of neutrons We can use this decay to measure how old somethingis (rocks, fossils, etc) Half-life the amount of time it takes for half of aradioactive substance to decay Disturbances Unfortunately not all rock layers are laid outperfectly. Some things that can make things moredifficult are Unconformity missing part of a geologic column Missing rock layer(s) due to either Erosion or Non- deposition Deformations Altering of rock layers after theyare laid down, but not removing them Deformations Deformations Tectonic plates move, causing stress to build up inthe lithosphere Stress = force (three types) Compression = squeeze together Tension = stretch apart Shearing = sliding past Stress from tectonic plates cause rock layers in thelithosphere to change Folding Definition: bending rock layers due to stress
Older rock starts on bottom Anticline = upward arch (looks like an A) Syncline = downward, U shaped (think sink) SYNCLINEANTICLINE Folding Folds can be large or small
Some measured inkilometers orcentimeters MONOCLINE Faults Definition: surface along which rocks break andslide past each other Can be seen on the earths surface Usually have a Hanging wall and Footwall Type of Stress? Which wall falls down? Are rock layers disturbed? Type of Stress? Which wall falls down? Are rock layers disturbed? Type of Stress? Which wall falls down? Are rock layers disturbed? Rock layers arent always perfect
Relative Dating Faults, Unconformities and Deformations raise avery important problem Rock layers arent always perfect So what do we do when rock layers arent perfect? What happens when they look more like This. How do we find Relative Age? Relative Dating One way scientists calculate relative age is by usinga set of rules Superposition if undisturbed, rock layers willalways be older as you go down (youngest on top) Original Horizontality Layers of rock are alwaysformed in relatively even horizontal layers Cross-Cutting Rule if there is a fault cutting throughlayers of rock, the rock must be older Deformation Rule if there is a deformation (fold,tilt, intrusion) the rock that is deformed must be older How do we find Relative Age? Relative Dating Another way geologists can figure out disturbedlayers of rock is with the Geologic Column Geologic Column a complete set of the rock layerswith all known fossils and layers in their proper order Geologic Columns are used to find Unconformities When a layer of rock is missing from Erosion or Non- Deposition the geologic column can tell geologistswhat SHOULD be there Which layers are missing from the second outcropping?