Geology Chapter 8

What is meant by “geochronology”?
This is known as the study of “Earth Time”. Determining the timing of geological events and the ages of geological materials is under the subject of geochronology.

What is the approximate age of the Earth?
The Earth is approximately 4.6 billion years old.

Briefly explain the difference between relative and numerical dating.
Relative dating compares two or more entities to determine which is older and which is younger. Geologists can figure out the relative age by comparing certain characteristics. Numerical dating tells you how long ago in years a particular rock formed or a geologic event occurred. This is also referred to as “Absolute dating”.

Principle of uniformitarianism
Principle of uniformitarianism- The scientific law stating that the geological process taking place in the present operated similarly in the past and can therefore is used to explain past geological events.

Principle of original horizontality-
Principle of original horizontality- The scientific law stating that sediments settling out from bodies of water are deposited horizontally or nearly horizontally in layers that lie parallel or nearly parallel to the Earth’s surface.

Principle of superposition-
Principle of superposition- The scientific law stating that in any unaltered sequence of rock strata, each stratum is younger than the one beneath it and older than the one above it, so that the youngest will be at the top of the sequence and the oldest at the bottom.

Principle of cross-cutting relationships-
Principle of cross-cutting relationships- The scientific law stating that a pluton (intrusive rock) is always younger than the rock that surrounds it.

What are fossils?
From the Latin fossilus meaning “something dug up”. Fossils consist of the remains of ancient organisms, or other evidence of their existence, that became preserved in geological material.

Describe the “principle of faunal succession” and how it is used to determine relative ages of rocks.
The scientific law stating that specific groups of animals have followed, or succeeded, one another in a definite sequence through Earth history. Thus, this means we can find fossils in the rocks and determine relative ages of rocks. An example is that rocks containing ammonite fossils are younger than rocks containing trilobites.

What is an “unconformity”?
Unconformity is a boundary separating two or more rocks of noticeably different ages, making a gap on the geological records. This occurs where erosion has removed rock layers or where no rocks were produced during a period of time.

What is “correlation,” and what do geologists look for when they try to correlate rock units?
Correlation is the process of determining that two or more geographically distant rocks or rock strata originated in the same time period. Geologists look for paleontological or other geological similarities between the units. Also, the farther apart two rock formations are in terms of geography, the less likely they will possess identical rock sequence. In addition, they also look at individual index fossils and they look for the same key bed.

What is a “key bed”?
It’s a distinctive stratum that appears at several locations. It records a geological event of short duration that affected a wide area. An example of this would be the eruption of Mount Mazama.

What is a “half-life”?
Half-life is the time necessary for half of the atoms of a parent isotope to decay into the daughter isotope.

Name three parent-daughter isotopic dating systems, and give the half lives of each parent isotope as well as the minerals or materials that are most likely to be dated by each.
a. Rubidium-strontium dating system:
-parent isotope is Rb-87 with a half life of 47 billion years
-the materials commonly dated are potassium-rich minerals such as biotite, potassium muscovite, feldspar, and hornblende; volcanic and metamorphic rocks
b. Thorium-lead
-parent isotope is Th-232 with a half life 14.1 billion years
-the materials commonly dated are zircons, uraninite
c. Carbon-14
-parent isotope is C-14 with a half life of 5730 years
– the materials commonly dated are any carbon-bearing material, such as bones, wood, shells, charcoal, cloth, paper, animal droppings, also water, ice, cave deposits.

This is a method of numerical dating that uses the number of tree rings found in a cross section of a tree trunk or branch to determine the age of the tree.

Varve chronology
Varve chronology -Varves form underlying a lake where a thick, light colored layer is deposited over the summer and in the winter a thin, dark colored layer is deposited. Geologists count and look at the nature of the varve to see how long ago the lake formed and identify events like landslides. This can help us indicated when landslide occurred and is helpful in planning safe development by finding unstable slopes

Name the three eras comprising the Phanerozoic Eon and give their time spans.
a. Paleozoic 545-245 (300)
b. Mesozoic 245-66.4 (180)
c. Cenozoic 66.4-0

In what eon, period, and epoch are we currently living?
We are living in the Phanerozoic Eon, Quaternary period, and Recent or Holocene epoch.

What is the distinction in characteristics of life forms that marks that boundary between the Proterozoic and Phanerozoic Eons?
The distinction is that at the beginning of the Phanerozoic Eon (“visible life”) it was the first point at which we find abundant fossil evidence of marine creatures possessing hard shells, prominent external spines, and internal skeletons.

When did the Phanerozoic Era begin?
The Phanerozoic Era began 545 million years ago.

Was dominated by marine invertebrates (sea creatures without backbones, such as corals, clams, and trilobites). Fish amphibians, insects, and land plants rose to prominence during the latter part of this era.

Was dominated by marine and terrestrial reptiles, including dinosaurs. The first birds, mammals, and flowering plants appeared during this era.

Is recent life, thus dominated by a rich variety of mammals. Basically early primates to modern humans dominate this era.

According to scientific evidence, how old is the Earth?
According to scientific evidence the Earth is 4.6 billion years old.

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