Lecture 2 - A Trip Through 1.8 Billion Years of EarthHistory

Outline

I. National Parks of the Colorado Plateau- introductory slide show and background

II. The Grand Canyon- A Geologic View of Time

 

I. National Parks of the Colorado Plateau

The first parks that we are going to discuss all lie within the geologic province known as the Colorado Plateau. This region lies within what is known as the four corners, overlapping parts of Utah, Arizona, Colorado and New Mexico. What do these 130,000 square miles share to be lumped together into one geologic province?

  1. The Colorado Plateau is covered with relatively flat lying sedimentary rocks. Sedimentary rocks are composed of fragments of other rocks or derivatives of the ocean that have been deposited by the action of wind or water. More about sedimentary rocks later.
  2. This package of sedimentary rocks which exceeds 4 miles in thickness in some regions has been uplifted between 5000 and 13,000 feet in recent geologic time without being seriously contorted and crumpled in the process like the rocks that make up the Rocky Mountains.
  3. Powerful river systems have cut deeply into the plateau producing a spectacular set of Canyons and revealing the thick layers of rock, each telling its own story of how it formed.
  4. Erosion in arid climate has produced the common mesas, buttes, pinnacles and arches of the region.

(Slide Show)

II. Geologic View of Time

Aside from the spectacular scenery, the most striking aspect of the Grand Canyon is the extent of Earth History that is spanned in its rock layers. The Earth is 4.5 Billion years old, that is 4500Million Years and the oldest rock is about 4 billion years old. The rocks of the Grand Canyon span 1.8 billion years or about 1/2 of all recorded Earth History. Time is a very important concept in Geology. The Earth's landscape, rocks, and fossil lifeforms all reflect events which occurred at different times in the past and it is the geologists job to read the rock record and to interpret when certain events occurred. On the Colorado Plateau the rock record is mostly in the form of layered sedimentary rocks laid down by a stream or in the ocean so before we try to interpret the timing of geologic events inthe Grand Canyon, we must be familiar with a few principles especially important when interpreting sedimentary rocks.

  1. Principle of Uniformitarianism- The present is the key to the past- geologic processes operating today have operated throughout Earth history and we can compare ancient rocks to similar rocks forming today and assume that they developed in the same manner. This concept was introduced by James Hutton in the late 1700's. Prior to this time people believed that Earth's features like mountains, valley, and oceans had been produced by a few great catastrophes (catastrophism).
  2. Principle of Original Horizontality- Sediments get deposited under the force of gravity and thus are laid down as flat layers
  3. Principle of Superposition- oldest sedimentary deposits are on the bottom with successively younger deposits on top
  4. Unconformities- Breaks in time in the rock sequence caused by either periods of non-deposition or erosion
    1. angular unconformity-angular relationship between tilted older rocks and flat lying younger rocks indicating that between deposition of the two units, tectonic forces tilted the older rocks so that they are no longer horizontal. A lot of missing time between rock units.
    2. disconformity- loss of time in recorded parallel rock units, implies non-deposition or erosion.
    3. nonconformity- flat sedimentary rocks on top of igneous or metamorphic rocks implies deformation and erosion. A lot of missing time between units. Igneous rocks form from cooling molten rock like that which comes out of volcanoes, and metamorphic rock is one that starts as a sedimentary rock or igneous rock and is subjected to high temperatures and pressures during deformation like mountain building. Remember campus marble is a metamorphic rock.

This gives enough information to go to the Grand Canyon and determine relative timing of events, however if we want to understand the geologic history of a larger area, like the whole Colorado Plateau, and place the relative timing of events at the Grand Canyon into a bigger picture then we need to correlate equivalent rock units from one place to the other we can do this by:

  1. Rock Correlation- hook up rocks that look a like, formed in the same setting. Relies on a distinctive rock unit. We will see that the Navajo Sandstone is a good distinctive rock unit to correlate on the Plateau.
  2. Fossil Correlation- hook up rocks that formed at the same time, possibly under different settings by matching distinctive fossils in the rock called index fossils. Good index fossils should be common, occur over a large geographical region and have lived for a short period of time.
    - In music, the Beattles or Rolling Stones would be poor index fossils because they were popular over large areas, but they spanned many decades. The Sex Pistols would be great since they were around for only 1 year, 1978-1979, but a lot of people heard of them.

Using fossils, rocks all over the world of the same age can be correlated regardless of the setting under which they formed. Through this type of correlation, a worldwide geologic column of the relative age of rocks all over the world has been produced. Now we understand something about relative time, how do we get absolute time in years?

We use the natural radioactive decay of certain elements contained in rock units. Since the decay of various elements has known rates, we can count the number of products produced and determine how long the process was occurring. Assuming that the decay began when the rock formed, we can get its absolute age in years. We will learn more about this process in the first exercise that you will do in sectionso for now lets look at the geologic time scale that has been determined for a worldwide geologic column of rock. It spans 3.8billion years and is divided into many pieces which represent major changes in lifeforms found on Earth.

 

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