Interpreting Geologic History

Unit 13

Relative Dating

• The determination of the age of a rock or event in relation to other rocks or events.

• Differs from absolute age.

Principle of Superposition

• When viewing layers of sedimentary rock or some igneous rock, always assume that the lowest layer is the oldest.

• Exceptions to the rule may occur when deformation of layers occur

Dating Intrusions, Extrusions, and Inclusions

• Intrusion- magma squeezes into preexisting rock and hardens. The intrusion is younger than the surrounding rock.

• Extrusion- lava flow, volcanic eruption. Younger than any rock before it and older than any rocks deposited above it.

• Inclusion- rock falls into magma but doesn’t melt before the magma solidifies.

Dating Rock Features

• Rocks are older than any fault

• Sedimentary rocks- sediments are older than the cement

• Igneous rocks- mineral crystals vary in age. Usually older crystals are bigger.

• Veins- watery mineral solution fills a crack in a rock. What is older?

Correlation

• Show that rocks or events from different places are the same or similar in age.

Exposed Bedrock

• Correlation can be determined by looking at the continuity of the layers.

Index Fossils

• One of the best methods.

• Fossils- remains or evidence of former living things

• Index fossils- must have lived over a large geographical area, life form must have lived for only a short time.

Volcanic Eruptions

• Volcanic ash is spread out over large areas. Each eruption has unique characteristics

Geologic Time Scale

• Based on fossil evidence, geologists have been able to divide time into divisions.

• Eons, eras, periods, and epochs. Listed from smallest to shortest.

• Precambrian- composed of the earliest eons, represents 88% of time.

Unconformities

• When reading the rock record, we find buried eroded surfaces.

• Indicates uplift occurred in the past, which led to erosion. Later, sinking occurred causing new layers to be deposited on top.

• Rocks in an unconformity are older than the ones above them and younger than the ones below them.

Uniformity of Process

• One of the basic principles used to interpret history.

• Based on the idea that geologic processes today also occurred in the past.

Absolute Dating of Rocks using Radioactive Decay

• Ages given in years before present.• Element- substance consisting of atoms

that are chemically alike.• Isotope- an element that exists in

several varieties. The only difference is in the mass of its atoms.

• All mass is generally located in the nucleus. Most nuclei are unstable.

• Unstable atoms emit particles and EE. This process is called radioactive decay. Eventually a stable atom will form.

Uranium-238

• One of the most important isotopes used. Passes through a series of decays and eventually ends up as lead-206.

Half-Life

• The time required for half the atoms in a given mass of an isotope to decay

• Each isotope has its own characteristic half-life.

• Not affected by environmental factors.

Carbon-14 Dating

• Uranium-238 is used to determine ages of rocks that are billions of years old.

• When dealing with younger rocks, carbon-14 (with a half-live of 5,700yrs), is used.

Evolution of Earth and Life

• The study of fossils and rock layers can tell us a lot about the history and evolution of life.

Variations in Fossils and Environments

• Chances for fossilization are low and only a small percentage of sedimentary rocks are studied, therefore many life forms of the past probably have not been identified.

• Comparing fossils to similar life forms today we can make inferences about past environments. EX. Ancient corals have been found in western NY, today we know that coral grow in warm shallow water, so we can assume that a portion of NY was under the ocean and in a tropical environment.

Environment Evolution

• Major reason for changes, the movement of plates.

• As they move the latitude changes, therefore…

• Colliding plates can cause local changes in environments, how?

Environmental Evolution and Rock Type

• As environments change the types of rocks that can form also change.

Fossils and the Evolution of Life

• Species- similar group of organisms that can interbreed and produce fertile young.

• Not all members are exactly alike.• Organic evolution- variations in the

genetic code give some individuals a favorable trait which gives them a better chance for survival.

• These favorable traits are passed down to other generations.

• Eventually organisms with the weaker traits will die out and become extinct.

• Fossil record provides evidence for evolution. Sometimes fossils from adjacent intervals of geologic time show a gradual transition from an older species to a newer one.

Early Evolution of Earth

• Evidence from the radio active decay of moon rocks and meteorites show that Earth formed about 4.6 billion years ago.Earth heated up and melted due to the heat from impact events.

• Earth’s zones formed due to density• First known crust formed about 4.2 billion

years ago. Earth’s first atmosphere formed.• Earth cooled and the oceans were formed.

• 3.8 billion years ago, single-cell organisms existed.

• 3.5 b.y.a. bacteria formed and release oxygen

• Most oxygen reacted with iron to form rust and the surface may have looked like Mars does today.

• 2.8 b.y.a. early ozone formed.• Life began to evolve ever since.