A fault is a rock fracture along which one side has moved relative to the other.
Fault scarps are cliffs formed by faulting.
Folding occurs when rock strata are compressed by slow, continuous tectonic movements of the crust.
Large-scale crustal movements (called tectonic changes) involve the rising, falling, tilting, or warping of whole continents or large parts of them.
15-2. Mountain Building
Mountains form in a variety of ways.
Mountains can form due to volcanic activity.
Some mountains are blocks of the earth's crust raised along faults.
The great mountain ranges have a long, complex history involving sedimentation, folding, faulting, igneous activity, repeated uplifts, and deep erosions.
In mountains having a history of sedimentation:
The sedimentary layers are usually thicker than similarly aged sedimentary layers under adjacent plains.
The sedimentary rocks have a complex structure resulting from intensive compressional forces.
15-3. Continental Drift
The German meteorologist Alfred Wegener proposed the theory of continental drift early in this century.
The evidence Wegener cited in support of continental drift included:
The occurrence of similar fossils on widely separated landmasses
Data on prehistoric climates that indicated that continents had changed position in latitude with time
Wegener suggested that the continents were originally part of a single, huge landmass called Pangaea.
About 200 million years ago, Pangaea broke apart into two supercontinents called Laurasia and Gondwana.
Laurasia comprised what is now North America, Greenland, and most of Eurasia.
Gondwana comprised what is now South America, Africa, Antarctica, India, and Australia.
Laurasia and Gondwana were separated by the Tethys Sea.
Both Laurasia and Gondwana broke up to give rise to today's continents.
15-4. Lithosphere and Asthenosphere
The lithosphere is a shell of hard rock that consists of the crust and the upper mantle.
The asthenosphere is a layer of hot, soft rock lying just below the lithosphere.
15-5. The Ocean Floors
Methods used to study the ocean floors include:
Echo sounding
Core sampling
Determination of magnetic properties of ocean floor rocks
Four findings about ocean floors have clarified our understanding about crustal evolution.
The ocean floors are geologically very young (200 million years or less in age).
A worldwide system of ocean ridges and rises runs across the ocean floors.
A system of deep trenches rims much of the Pacific Ocean. Some trenches have volcanic island arcs on their landward sides.
Ocean-floor rocks are magnetized in the same direction in strips parallel to the midocean ridges; however, the direction is reversed from strip to strip going away from a ridge on either side.
15-6. Ocean-Floor Spreading
In the early 1960s, the American geologists Harry H. Hess and Robert S. Deitz independently proposed that the ocean floors are spreading.
During the process of ocean-floor spreading, molten rock rises up along a midocean ridge and pushes apart the lithosphere on either side of the ridge. The molten rock hardens and becomes new crust.
The discovery of normal and reverse magnetic strips on either side of an ocean ridge is strong evidence for ocean-floor spreading.
15-7. Plate Tectonics
According to the theory of plate tectonics, the lithosphere is divided into seven huge plates and a number of smaller ones.
New lithosphere is created where plates move apart at midocean ridges.
There are three kinds of plate collisions:
Ocean-continental plate collision occurs where a plate bearing dense oceanic crust slides underneath a plate bearing lighter continental crust in a region of contact called a subduction zone.
Oceanic-oceanic plate collision occurs where a plate bearing oceanic crust slides underneath a second plate bearing oceanic crust in a subduction zone.
Continental-continental plate collision occurs when the plate edges are pushed together and buckle, forming a mountain range.
A transform fault is a type of plate boundary where the edges of two plates slide past each other.
The San Andreas Fault is an example of a transform fault that has formed between the boundary of the Pacific and North American plates.
The San Francisco earthquakes of 1906 and 1989 were caused by movement along the San Andreas Fault.
Two plausible mechanisms for plate motion have been proposed.
Convection in the asthenosphere
Subduction of oceanic plate margins
Thirty million years from now, if present plate motions continue:
The Atlantic Ocean will widen.
The Pacific Ocean will narrow.
Part of California will detach from North America.
The Arabian peninsula will become part of Asia.
The West Indies will become a land bridge between the Americas.
The western Pacific islands will increase in extent.
The earth's surface will likely continue to change in the future.
15-8. Principle of Uniform Change
Seventeenth-century theologian Bishop Ussher, using stories in the Bible, determined that earth was created at 9 o'clock in the morning of October 12, 4004 B.C.
The German geologist Abraham Gottlob Werner believed that all rocks were sedimentary rocks and that the geologic history of the earth consisted of three sudden precipitations from an ancient ocean that were followed by the disappearance of the water.
The French biologist Georges Cuvier regarded the earth's history as a succession of catastrophes. Cuvier based his ideas of earth's history on the study of fossils, the remains or traces of organisms preserved in rocks.
The Scottish geologist James Hutton proposed that earth's history could be understood in terms of processes under way in the present-day world.
The English geologist Charles Lyell (1797-1875) modified and expanded Hutton's ideas and proposed the principle of uniform change, which stated that geologic processes in the past were the same as those in the present.
Charles Darwin's theory of evolution showed that changes in living things as well those in the inorganic world of rocks could be explained in terms of processes operating all around us.
15-9. Rock Formations
It is often possible to reconstruct past geologic events in terms of processes still at work reshaping the earth's surface.
The science of geology is faced with two fundamental problems:
To arrange in order the events recorded in the rocks of a single outcrop or small region
To correlate events in various regions of the world to give a connected history of the earth
Basic principles of historic geology include:
In a sequence of sedimentary rocks, the lowest bed is the oldest and the highest bed is the youngest.
Sedimentary beds were originally deposited in more or less horizontal layers.
Tectonic movement took place after the deposition of the youngest bed affected.
An igneous rock is younger than the youngest bed it intrudes.
An unconformity is a buried surface of erosion and involves at least four geologic events:
The deposition of the oldest strata
Tectonic movement that raises and perhaps tilts the existing strata
Erosion of the elevated strata to produce an irregular surface
A new period of deposition that buries the eroded surface
15-10. Radiometric Dating
Two methods are used to determine the worldwide sequence of events that have shaped the earth's surface:
Radioactive dating
Fossil identification
The decay of a particular radionuclide proceeds at a steady rate, and the ratio between the amounts of that nuclide and its stable daughter in a rock sample indicates the age of the rock.
The carbon isotope 14C, called radiocarbon, makes it possible to evaluate the ages of ancient objects (wooden implements, cloth, leather, charcoal from campfires, etc.) and remains of organic origin.
15-11. Fossils
The most common fossils are the hard parts of animals (shells, bones, teeth, etc.). Plant fossils are relatively scarce.
Some fossils are trails or footprints left in soft mud and covered by later sediments.
Fossils are preserved under special conditions of burial.
Rock layers from different periods can be recognized by the kinds of fossils they contain, making possible the arrangement of strata in a relative time sequence.
Fossils help to reconstruct the environment in which the organisms lived.
15-12. Geologic Time
The past 570 million years of earth's history are divided into three time divisions called eras.
Cenozoic Era (began 65 million years before present)
Mesozoic Era (began 225 million years before present)
Paleozoic Era (began 570 million years before present)
The nearly 4 billion years before the Paleozoic is usually considered as a single long division called Precambrian time.
The fossil record reveals a number of biological extinctions that have been used to divide geologic history into periods.
Periods are subdivided into shorter time spans called epochs.
15-13. Precambrian Time
Precambrian rocks often show considerable metamorphism and have been greatly deformed.
Geologic processes occurring during Precambrian time resemble those of today.
Precambrian rocks are exposed over a broad area covering most of eastern Canada and adjacent parts of the United States.
Indirect evidence suggests some type of life existed at least 3.7 billion years ago.
Primitive blue-green algae (cyanobacteria) were among the earliest organisms. Their photosynthetic activity provided the atmosphere with its oxygen.
Fossils of multicellular algae and fungi have been discovered in the 1.9-billion-year-old Gunflint Formation in Ontario, Canada.
15-14. The Paleozoic Era
Marine invertebrates (animals lacking internal skeletons) are the oldest fossils of the Paleozoic Era.
Fossils in late Paleozoic rocks show the first evidence of land-dwelling organisms, including primitive plants (tree ferns, land snails, primitive insects).
The first amphibians appeared in the Devonian Period; the first reptiles in the Carboniferous.
Reptiles became the dominant land creatures during the Permian Period.
The end of the Paleozoic Era was a time of intense tectonic activity. The Appalachian trough was compressed and uplifted into a mountain chain.
15-15. Coal and Petroleum
During the Carboniferous Period, much coal was formed in the present eastern United States and Europe.
Coal beds represent the sites of ancient Paleozoic swamps.
Coal was formed when accumulated plant matter (largely cellulose) underwent slow bacterial decay.
Decay took place underwater and in the absence of air.
Heat and pressure resulting from burial beneath later sediments converted the plant carbon into coal.
Petroleum originated from marine life such as plankton and algae.
Three steps are involved in the conversion of organic material into petroleum:
Bacterial decay in the absence of oxygen
Burial and modification by low-temperature chemical reactions
"Cracking" of complex hydrocarbons to straight-chain alkane hydrocarbons (at higher temperatures natural gas forms instead of petroleum)
Petroleum and natural gas can migrate through porous rocks and become available for recovery when trapped by anticlines or beneath layers of impermeable clays or shales.
15-16. The Mesozoic Era
Pangaea split into Laurasia and Gondwana (followed by their own breakup) during the Mesozoic Era.
During the early Mesozoic Era, North America separated from Laurasia.
The dinosaurs were the dominant land animals during the Mesozoic Era.
Flowering plants first appeared in the mid-Mesozoic.
The first true birds arose from reptilian ancestors in the Jurassic Period.
Mammals first appeared in the Triassic Period.
The dinosaurs disappeared during a time of mass extinctions that divides the Mesozoic from the Cenozoic.
Theories for the mass extinctions include:
Comet or meteorite impact
Intense volcanic activity
15-17. The Cenozoic Era
Widespread volcanic activity and tectonic disturbance characterize Cenozoic time.
The Alps, Carpathians, and Himalayas were uplifted in the mid-Tertiary Period.
Late Tertiary saw the formation of the Cascade range.
The Appalachians, Rockies, and Sierra Nevada were repeatedly uplifted and eroded, creating their present topography.
In the Cenozoic, the continents continued their earlier drifts, resulting in the present configuration of the continents.
After the extinction of the dinosaurs, mammals multiplied and evolved rapidly.
By the end of the Tertiary both the physical and organic worlds more closely resembled their present aspect.
In the Cenozoic, the biological isolation of the continents increased.
The Pleistocene Epoch saw the formation of ice caps in Canada and northern Europe.
In North America, the ice spread outward from three centers of accumulation in Canada.
15-18. Human History
Fossil evidence indicates that the human species had its origin in eastern Africa.
Humans entered North America at least 15-20,000 years ago.
Human activities and overpopulation endanger the future.
Overpopulation is the main hazard facing the world.
Global population can be stabilized at a reasonable level, but action must be taken now while there is still time.
