Geologic Time

Text Chapter 8

KEY POINTS: Thinking About Time

• Geologic time is measured in time intervals spanning thousands, millions, even billions of years (= “Deep Time”).

• Earth is estimated to be 4.6 billion years old.• The change from a biblical age for Earth to the

concept of geological time is an example of a paradigm shift.

KEY POINTS: The History of Relative Time

• The concept of relative time deals with placing events in the order in which they occurred.

• Several rules used to determine the sequence of geologic events are: original horizontality, superposition, cross-cutting relationships and inclusions.

• The fossil record is also used to determine the order of events.

Archbishop James UssherIn 1650, an Irish Anglican archbishop, James Ussher, who attempted to correlated ages of biblical figures with real time. He concluded that the God created the Earth, beginning on 22 October 4004 B.C., or 6,000 years ago.His chronology was included in early King James versions of the Bible and was widely circulated.

Nicholas Steno (1638-1686)Steno, a Danish physician and priest, initiated the determination of Earth’s age from a scientific standpoint in the late 1600s, not long after Bishop Ussher’s pronouncement of 4004 BC.After dissecting a shark’s head, he believed the shark teeth found in seaside cliff sediments were real teeth deposited ages ago.

Steno’s 1st Principle:Original Horizontality

• Steno hypothesized that objects like sharks teeth were preserved when they sank to the bottom of a body of water and were covered up by sediment.

• He reasoned that if rock layers were deposited in water, they must have originally been horizontal.

• This observation –that sedimentary rocks are deposited in nearly horizontal layers—became known as the principle of original horizontality.

• Sedimentary rocks that are no longer horizontal must have gone through an episode of deformation after forming (such as uplifting via mountain building).

Steno’s 1st (B, C, D) and 2nd Principle (A): Superposition

(In a series of sedimentary rock layers that are not deformed, the rocks at the bottom of the stack are oldest, rocks at the top are youngest.)

Principle 2 Applies to Volcanic ProcessesVolcanic eruptions can produce layers (strata) from lava flows and when ash and other ejecta fall back to Earth.

James Hutton (1726-1797)

Hutton’s Principles

• Hutton developed our modern concept of geologic time.• He suggested that same slow-acting geologic processes

operating today must have operated in the past.• So it must take a very long time for those processes (e.g.

weathering, erosion) to produce and significant change in the shape of the Earth’s surface (longer than 6000 years).

• This process is know as the uniformitarian model.• Secondly, Hutton proposed that all land should be worn flat

unless some process acts to renew the landscape by forming new mountains, which then renews the slow cycle of destruction.

Siccar Point• Siccar Point, in Scotland, is

where Hutton discovered nearly flat rocks overlaying nearly vertical layers. He hypothesized that the lower layers of rock formed first (superposition) as flat layers (original horizontality) and were later pushed up and tilted by mountain uplift processes.

• These layers were then worn down by erosion and buried beneath new horizontal deposits of sediment.

• Hutton had found an illustration of his idea of cyclical processes.

Hutton called the rock formation at Siccar Point an “Unconformity” in this 1796 Book, The

Theory of the Earth.

Relative Time

• The study of “relative time” involves placing events in the order in which evidence indicates they occurred.• In the context of geology, it means

describing the order or sequence of geologic events.

Investigating the Origin of the Grand CanyonA one-armed Civil War veteran, John Wesley Powell led two expeditions down the Colorado River through the Grand Canyon (1869 and 1871).He was the first person to make a detailed record of landscape formation.Powell interpreted the rocks in the canyon walls to be the result of long-lived geological processes that had been operating “through ages too long for man to compute.”

Grand Canyon. Rock layers exposed represent intervals of time stretching back millions of years. The

canyon is 19 miles across in the well-layered sedimentary rocks but is much narrower in the

metamorphic and igneous rocks of the inner gorge.

Grand Canyon: Oldest to Youngest LayersSchist, a metamorphic rock, is the lowest and oldest (once was sedimentary).Granite (an igneous rock) cuts across the schist and is therefore younger, but still second oldest.Third oldest is the Super-group, a series of sedimentary rocks. Uplift tilted them after formation.

Grand Canyon: Oldest to Youngest LayersFourth up is a progression of sandstone, to shale, to limestone, typical of the sequence of rocks formed in a shallow sea along continental margins, followed by uplift.Next are a series of sedimentary layers indicative of shallow marine and delta conditions, formed as sea level rose and fell.The Coconino sedimentary layer was formed by windblown sand and drying desert conditions.

Wind Erosion Can Shape Sedimentary Formations

Grand Canyon: Oldest to Youngest LayersThe youngest layer (top layer), the Kaibab Formation, is a series of sedimentary (sea bottom) deposits.After this came a period of uplift, raising the land some 6,500 feet, permitting the Colorado River to cut the canyon.The last geologic event was the formation of a series of cinder cone volcanoes right at the top (north rim) after the canyon was formed.