Relative Dating & Deforming the Earth’s Crust Folds, Faults and Deformities… OH MY!!

Is this the correct principle? Uniformitarianism Geologic processes that occurred in the past can be explained by current geologic processes Erosion and weathering that happens today must have 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 rare events These rare events are called catastrophes Up until the mid-1800’s, this was the accepted principle 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 determine whether an object or event is older or younger than another object or event Who is Older or Younger?

Absolute Dating Absolute Dating – any method used to determine the age of an event or object in numbers Certain molecules called Isotopes decay over time Isotope – An atom that has the same number of protons, but a different number of neutrons We can use this decay to measure how old something is (rocks, fossils, etc…) Half-life – the amount of time it takes for half of a radioactive substance to decay

Disturbances Unfortunately not all rock layers are laid out perfectly. Some things that can make things more difficult 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 they are laid down, but not removing them

Deformations

Deformations Tectonic plates move, causing stress to build up in the lithosphere Stress = force (three types) Compression = squeeze together Tension = stretch apart Shearing = sliding past Stress from tectonic plates cause rock layers in the lithosphere 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)

SYNCLINE ANTICLINE

Folding Folds can be large or small Some measured in kilometers or centimeters

MONOCLINE

Faults Definition: surface along which rocks break and slide past each other Can be seen on the earth’s 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 aren’t always perfect Relative Dating Faults, Unconformities and Deformations raise a very important problem… Rock layers aren’t always perfect So what do we do when rock layers aren’t 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 using a set of rules Superposition – if undisturbed, rock layers will always be older as you go down (youngest on top) Original Horizontality – Layers of rock are always formed in relatively even horizontal layers Cross-Cutting Rule – if there is a fault cutting through layers 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 disturbed layers of rock is with the Geologic Column Geologic Column – a complete set of the rock layers with 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 geologists what SHOULD be there

Which layers are missing from the second outcropping?