Earth’s HistoryEarth’s History

Geologic EventsGeologic Events

““The present is the key to the past”.The present is the key to the past”.

This statement summarizes one of the basic principles of geologic history – that the geologic processes going on today, such as weathering, erosion, volcanism, and earthquakes, also went on in the past.

This concept is known as the principle of uniformitarianism.principle of uniformitarianism.

Sequence of Geologic Sequence of Geologic EventsEvents

Relative AgeRelative Age versus Absolute Absolute AgeAge

The relative agerelative age of a rock or event is the age as comparedage as compared to other rocks or events.

The absolute ageabsolute age of a rock or event is the actual ageactual age.

Chronology of LayersChronology of Layers

There are two major principles used when interpreting geologic history:

•Principle of original horizontality;•Principle of superposition.

Principle of Original Principle of Original HorizontalityHorizontality

• The principle of original principle of original horizontalityhorizontality states that sediments are deposited in horizontal layers that are parallel to the surface on which they were deposited.

• This implies that tilted or folded layers indicate that the crust has been deformed.

Original Original HorizontalityHorizontality

Principle of SuperpositionPrinciple of Superposition

• The principle of superpositionprinciple of superposition states that, in a series of undisturbed layers, the oldest oldest layer is on the bottomlayer is on the bottom and each overlying layer is progressively younger with the youngest layer youngest layer on the topon the top.

Superposition

Evidence of EventsEvidence of Events

• Several things provide geologists with evidence of events:

• Igneous intrusions and extrusions;• Faults, joints and folds;• Internal characteristics.

Igneous Intrusions and Igneous Intrusions and ExtrusionsExtrusions

When magma forces its way into cracks or crevices in crustal rock and solidifies, it forms a mass of igneous rock called an intrusionintrusion.

When lava solidifies at the surface it forms a mass of igneous rock called an extrusionextrusion.

Since the rock thatrock that the magma moved through, or over, existed priorexisted prior to the intrusion, (or extrusion), it must be oldermust be older.

Igneous IntrusionsIgneous Intrusions

Faults, Joints, and Faults, Joints, and FoldsFolds

•Features such as faults, faults, jointsjoints, and foldsfolds must be younger than the rocks in which they are found.–A jointjoint is a crack in a rock formation, similar to a faultfault but without any displacement.

Internal CharacteristicsInternal Characteristics

•Cracks, veinsCracks, veins, and mineral mineral cementcement are younger than the rocks in which they appear.

•A veinvein is a mineral deposit that has filled a crack, or permeable zone, in existing rock.

Correlation TechniquesCorrelation Techniques• CorrelationCorrelation is the process of

matching rocks and geologic events in one location to the rocks and events in another location.

• Methods used for correlationcorrelation include:– Continuity of rocksContinuity of rocks– Fossil evidence in rocksFossil evidence in rocks– Volcanic time markers in rocksVolcanic time markers in rocks

Correlation

Bedrock and OutcropsBedrock and Outcrops• BedrockBedrock is the solid, unbroken rock

of the crust.• An outcropoutcrop is bedrock that is

exposed at the Earth’s surface.– OutcropsOutcrops provide opportunity for geologists to directly study the layers of the bedrock, tracing them from one location to another – called “walking an outcrop”.“walking an outcrop”.

Bedrock / Outcrop

Fossil EvidenceFossil Evidence

• Index fossilsIndex fossils are fossils or organisms that lived over an extensive area, preferably over the entire Earth, for relatively short periods of time.

• Index fossilsIndex fossils are useful in correlating the sedimentary rocks in which they are found.

Index FossilsIndex Fossils

Fossil CorrelationFossil Correlation

Volcanic Time MarkersVolcanic Time Markers

• Severe volcanic eruptions can deposit a thin layer of volcanic ashvolcanic ash over the surface of the entire Earth.

• These layers within a rock sequence may remain distinguishable and provide a time provide a time markermarker. (Similar to index fossils)

AnomaliesAnomalies

•AnomaliesAnomalies are differences from what is expected.

Determining Geologic Determining Geologic AgesAges

• Without the rock record there would be no geologic history.

• The older the rock the more difficult to determine absolute age.

• The geologic history of an area is determined primarily by fossil evidence, the age of the rocks, and the erosional record in the rocks.

Geologic Time ScaleGeologic Time Scale• Geologic time is subdivided divided into

units based on fossil evidence.• There are 4 major divisions:

– PrecambrianPrecambrian – represents the first 85% of Earth’s history (mostly devoid of fossils).

– Paleozoic EraPaleozoic Era – represents ~ 8.5% of Earth’s history (invertebrates, fishes, amphibians, vertebrates and land plants first appear).

– Mesozoic EraMesozoic Era – rep. ~ 3.5% of Earth’s history (dinosaurs, earliest birds, and mammals).

– Cenozoic EraCenozoic Era – rep. ~ 1.4% of Earth’s history (humanoids show up late ~0.04% of history).

Erosional RecordErosional Record

• Buried erosional surfaces, called unconformitiesunconformities, indicate gaps or breaks in the geologic time record.

• 4 steps combine in sequence to form unconformities:– Uplift, erosion, submergence and deposition

• 3 most common types of unconformities are: angular unconformities, parallel unconformities, and nonconformities.

Radioactive DecayRadioactive Decay• Radioactive decay occurs when the

nuclei of unstable atoms break down, changing the original atoms into atoms of another element.

• The rate of radioactive decay is measured in terms of half-life.– Half-life is the amount of time it takes for

half the atoms of a substance to decay into another element.

– Different substances have different half-life’s• Examples are Uranium 238 and Carbon 14.

History and Evolution of the History and Evolution of the Earth’s AtmosphereEarth’s Atmosphere

• Earth is about 4.5 billion years old.• The primary source of gases for the

earliest atmosphere are thought to be from outgassingoutgassing by volcanoes.

• Water vapor in the outer atmosphere would have been broken down into hydrogen and oxygen.

Fossil RecordFossil Record

•Fossils preserved in the rocks provide evidence that many kinds of animals and plants have lived on Earth in the past under a variety of different conditions.