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The Rock Record and Geologic Time: Studying Earth History · Overview of geologic time · Rates of geologic change (and "deep time") · Dating of geologic events · Geologic principles used in relative dating · Fossils for dating and correlation · Radiometric other types of numerical dating · The Geologic Time Scale · Some events in earth history

Geologic Time and the Age of the Universe

Geologic Time, the vast span of time over which earth processes have occurred, includes more than 4 billion years!



Illustration of the enormous scale of geologic time

Illustration of the enormous scale of geologic time - continued



The Rock Record and Geologic Time: Studying Earth History Time scales of geologic processes Days to Months to Years Human lifetime Thousands of years Millions of years Billions of years

A single eruption may be short in time, but the lifetime of an entire volcano may be thousands to millions of years. Understanding Earth



Glaciers move slowly in human terms. Their movements are almost imperceptible from one day to the next, but given enough time they can carve great valleys and strip away entire mountain ranges.


Geologic History in the Grand Canyon, Arizona

The rock layers of the Grand Canyon record a great deal of geologic time. The granite, schist, and gneiss found at the bottom of the canyon and are more than 1 billion years old. These are overlain by successively younger sedimentary rocks. The Kaibab limestone at the top of the canyon is about 250 million years old.


Several unconformities are present, and present the time they represent exceeds the time recorded by the rocks.

Understanding Earth Understanding Earth

Cross-section of part of the Grand Canyon with rock ages


Development of Geological Thought Approaches to Interpreting the rock record ­ continued

· Catastrophism

Animal tracks

Plant fossils

· Proposed by Baron Georges Cuvier, a French zoologist · Explained the both the geologic history and the biologic history of the Earth through a series of sudden, widespread catastrophes · I l d d six major catastrophic events Included i j hi · Each catastrophe produced major changes in a short period of time · Compatible with a young age for the Earth

Marine fossils Trilobites

· Geologic evidence failed to support the idea ­ For example, it was recognized that many more than six catastrophes were needed to explain observed rocks

Fig 8-13 Chernicoff, Geology 2nd edition

Development of Geological Thought Development of Geological Thought Approaches to Interpreting the rock record ­ continued

· Principle of Uniformitarianism · A central concept of modern geology · In the 1700's, James Hutton proposed the basic premise: presentday processes have occurred throughout geologic time and can be used to explain Earth history equ es ve y o d a t · Requires a very old Earth · Hutton saw geologic processes as operating in cycles: a mountain

range could be worn away, its material could be deposited elsewhere, and those sediments could be pushed upward to form another mountain range

· On the basis of his field observations and the experiments of others, Hutton recognized that igneous rocks were formed by cooling of molten rock, an idea known as plutonism · Hutton's ideas were popularized largely by Charles Lyell who published a book, Principles of Geology, in 1830

Modern concept of Uniformitarianism · Allows for the rates, intensities, and extents of processes to vary over time · e.g. volcanism · e.g. ice ages · Allows for large, catastrophic events as well as slow change · volcanic eruptions, large storms, meteorite impacts, etc. are eruptions storms impacts etc seen as normal processes · rivers, for example, shape the Earth through day-to-day sediment transport and erosion as well as the occasional flood · Used to interpret the rock record by applying our understanding of processes that we can observe today · Used to predict future geologic events (e.g. earthquakes, opening/closing of oceans, building/wearing away of mountains)




Dating Rocks and Geologic Events How do we date rocks? What principles and methods are used to determine the ages of earth features? Geologists use two types of ages to describe rocks: Relative ages · Answer questions like: - What came first? What came second? · Involves putting rocks and geologic events in the order or sequence in which they occurred Numerical ages (also called "absolute" or "specific" ages) · Answer questions like: - When did it happen? How old is it? · Involves assigning an age which indicates a number of years ago

The Rock Record and Geologic Time: Studying Earth History

Principles used for determining relative ages: Superposition Original Horizontality Cross-Cutting Relationships Inclusions Lateral continuity



Original Horizontality: Most sedimentary rock units are deposited as horizontal or nearly horizontal layers. When they are found steeply tilted or folded, as in this example, the tilting must have happened after the rocks were formed.

Superposition: Sedimentary and volcanic rock layers are deposited with the oldest rocks at Geology - Chernicoff the bottom and the youngest rocks at the top.

Understanding Earth



Cross-cutting relations: Faults, igneous intrusions, and erosion surfaces which cut across other rocks must have been formed after the rocks which they cut across.

The Rock Record and Geologic Time: Studying Earth History

Unconformities: Surfaces of erosion or non-deposition which represent missing records of earth history much lik pages missing i i d f th hi t h like i i from a book

Upper: In this example, the sedimentary rocks were formed first. The dike was intruded later. The erosion which formed the valley happened even later. (from Fig 4.16 Understanding Earth) Lower: A fault cuts through older sedimentary rocks (from Fig 2.3 Wicander and Monroe, Historical Geology)

Unconformities are grouped into three categories (angular

unconformity, disconformity, and non-conformity) according to the

relationship between the rocks above and below the erosion surface.


19 Angular unconformity in the Grand Canyon Note the horizontal layers above and the tilted layers below. The rocks above the unconformity are more than 200 million years younger than the rocks below. Fig 9.7 Understanding Earth

Deciphering the sequence of events

· Which fault came first? · What is the first event overall? · What are the last two events overall?


Layer 4 Unconformity 2 Layer 3

Layer 2

Layer 1 Igneous intrusion (dike and sill)

What is the sequence of events?

Fig 9.9 Understanding Earth

Put the following in order: __ folding __ deposition of sedimentary layers (A-J) __ intrusion of pluton and dikes __ fault movement __ "mystery" event: _____________



Methods of Numerical Dating

Numerical dates can be obtained in a variety of ways, counting tree rings or measuring lichens, for example. The most important source of numerical ages is radiometric dating. Radiometric dating A type of numerical dating that which relies upon the breakdown of unstable (radioactive) isotopes of certain elements and the formation of other isotopes in their place. Examples include:

Carbon-14 dating Uranium-lead dating Potassium-argon dating (and related argon-argon dating) Fission track dating Cosmogenic or exposure dating


The Acasta Gneiss Northwest Territories, Canada the oldest earth rocks discovered so far: 3.96 billion years old The age comes from radiometric dating di t i d ti using isotopes of uranium and lead and probably represents the timing of the metamorphism.

Fig 8-23 Geology 2nd ed Chernicoff

The age of the Earth and solar system is believed to be about the same as the age of the oldest meteorites.

Example: the ~4.6 billion year old Hoba meteorite found in Namibia, Africa.


Fig 8-37 Geology 2nd ed Chernicoff


Strong tendency to lose outermost electrons

The Periodic Table groups elements by similarities in their chemical 25 properties. Most of the known, naturally-occurring elements are present only in trace amounts.

Atomic number Chemical symbol

Tend to share, gain, or lose electrons Inert (noble) Strong tendency gases to gain electrons

Hydrogen and carbon atoms

Modified from Fig 2.2 Understanding Earth


Transition elements

Atomic number = 1

Atomic number = 6

Lanthanides Actinides


Isotopes of Hydrogen

Modified from Fig 2.2 Understanding Earth



Isotopes of carbon


Fig 2.3 Understanding Earth

Atomic number = 1 Atomic weight = 2 Atomic number = 1 Atomic weight = 1


Unstable (Radioactive)

Atomic number = 1 Atomic weight = 3

Rate and Progress of Radioactive Decay


Age = number of half lives elapsed x number of years per half life

The amount of time that it takes for half of the radioactive isotope to decay depends on the isotope. For 14C, the half life is 5,730 years. For 238U, it is 4.5 billion years.


S. Kuehn

S. Kuehn

Example: 2 half lives x 5,730 years/half life = 11,460 years




If 12.5% of the parent remains, how many half lives have elapsed? If one half life is 50,000 years, how old is the rock?

Understanding Earth

S. Kuehn


Generation of new carbon-14 occurs in the atmosphere. It is then absorbed into plants and animals. The proportion of radioactive carbon in living organisms remains constant during their life life. Over time, carbon-14 decays to nitrogen-14.

Other Numerical Dating Methods · Dendrochronology - use of tree rings for dating


Fig 8-24 Geology 2nd ed Chernicoff

Fig 8-27 Geology 2nd ed Chernicoff


Other Numerical Dating Methods · Varves

one varve


Other Numerical Dating Methods · Lichenometry

Fig 8-29 Geology 2nd ed Chernicoff


Fig. 15.27 Understanding Earth

Fig 8-28 Geology 2nd ed Chernicoff

S. Kuehn




The Geologic time scale

This time scale originally was constructed without knowing the exact ages of the boundaries, only relative ages. The divisions of time are defined based on rocks and fossils found at many locations around the world. The Geologic time scale is divided into sections of time with eons representing the ti f ti ith ti th longest intervals of time and the epochs representing much shorter intervals of time. Eons are divided into eras. Eras are divided into periods. Periods are divided into epochs.

The oldest part of the Geologic Time Scale.

The oldest known rocks belong to the Archean Eon which dates from about 4 to 2.5 billion years ago. Rocks belonging to the Hadean Eon no longer exist and are omitted from the time scale shown. They have been recycled to form younger rocks. rocks The Proterozoic refers to the time of "early life." It also is the time when molecular oxygen became abundant in the earth's atmosphere. Next is the Phanerozoic Eon, the time of "visible life" and more abundant fossils.

Modified from Fig 9.13 Understanding Earth

Fossils and the History of Life · Fossils are remnants of ancient life · Fossils include: · preserved organisms (body fossils like shells and bones) · evidence of biological activity (trace fossils like burrows and tracks) · Fossils record the development of life on earth: · th earliest forms of life were much simpler than those which the li t f f lif h i l th th hi h came later · numerous new species have evolved from earlier ones · many species have gone extinct, leaving only remnants · The ages of rock units from different places can determined by comparing fossils which they may contain · Geologists who study fossils and the history of ancient life are called paleontologists



150 million year old fossil bones at Dinosaur National Monument, Utah

Fig 8-1 Geology 2nd ed Chernicoff

Fossils - ammonite (left) and petrified wood (right)

Fig 9.4 Understanding Earth



Many different organisms have lived and died during geologic time. Rocks from different periods of time are characterized by fossils of different organisms.

Fig 7.3 NAGT lab manual



Fossils in rocks record changes in the diversity of life through geologic time. Fossils record not just the development of new types of organisms (evolution), but also the loss of species (extinction). More than once during Earth history, large numbers of species. have been lost over relatively short periods of time. Such events are called mass extinctions. Many scientists consider the Earth to be in the middle of another mass-extinction today, an extinction resulting from human activities. The greatest extinction in Earth history occurred about 250 million years ago. A more famous mass extinction occurred about 65 million years ago and marks the end of the dinosaurs.


Fossils record the development of new organisms and of others. Fossils also record the ancestry of modern plants and animals.

Fig 23.10 Understanding Earth

Extinction of the Dinosaurs (and lots of other stuff too) about 65 million years ago

What caused this mass extinction? Two of the leading candidates: - massive volcanic activity (fits better with a slower extinction over hundreds of thousands to millions of years) - meteor/asteroid collision (fits better with rapid extinction) We know that both of these things happened at the time, but which was more important?



Death due to lava (from the related climate effects)? Massive lava flows are preserved in the Deccan Traps of India and on the floor of the Indian Ocean. These were erupted during the time of the K-T extinction. The Deccan Traps are one of the largest volcanic provinces in the world, covering an area roughly the size of the states of Washington and Oregon combined. The preserved lava is only a remnant of what was once an even larger volcanic province.

Why did some organisms survive whereas others went extinct?

Photograph by Lazlo Keszthelyi

Evidence for a collision


Tektites from Thailand

Similar glass spheres have been found around the world in 65 million year old sediments. These probably were formed when rock melted by meteor impact was thrown into the air.

Fig 1-5b Geology 2nd ed Chernicoff


65 million year old iridium-bearing sediment layer at Bubbio, Italy One possible clue to what may have caused the extinction of the dinosaurs.

Chapter 23 Understanding Earth

Mineral grain shattered by meteorite impact - an example of shock metamorphism

Fig 1-5c Geology 2nd ed Chernicoff




More to explore:

Gravity map of the Chicxulub impact crater Yucatan Peninsula, Mexico

This crater may have been produced by a large meteor impact at the end of the Cretaceous Period C t P i d An impact crater on the Moon for comparison

What Killed The Dinosaurs? The K-T Extinction Dinosaurs: Facts and Fiction - USGS Origin of Birds ­ Descendents of the Dinosaurs? Winged dinosaurs and the origin of birds



More to explore:

More to explore: Advances in understanding how evolution works (examples) Evolution and the Fossil Record - AGI & The Paleontological Society (Very Good!) Understanding Evolution, a resource for K-12 teachers Frequently Asked Questions About Evolution ­ PBS The Geological Evolution of the Earth (rocks)

U.C. Berkeley Museum of Paleontology Geologic Time - U.S. Geological Survey Fossils, Rocks, and Time - USGS Dinosaur National Monument (home page) (geology) (virtual museum)



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