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This 6 page Class Notes was uploaded by June Gottlieb on Friday October 23, 2015. The Class Notes belongs to GEOL 101 at University of Idaho taught by Staff in Fall. Since its upload, it has received 36 views. For similar materials see /class/227869/geol-101-university-of-idaho in Geology at University of Idaho.
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Date Created: 10/23/15
Physical Geology 101 1 Introduction to Geology p16 The textbookfor this course is Earth An Introduction to Physical Geology 9m Ed by Tarbuck amp Lutgens This course will follow the textbook closely so it will be a good resource to have T e TurningPoint student response system clicker is also required for this course What is Geology Geology is a science Greek quotgeoquot Earth quotlogosquot discourse So geology is the science of the Earth Age ofthe Earth Geology is the study ofthe Earth and all its natural component parts that impact on each other Although we have no reason to believe that the processes we see happening around us today were any different to the processes that have been occurring throughout Earth history we know these processes are very slow which raises the issue about exactly how old the Earth is The earliest estimates of the age ofthe Earth were not based in science at all but were determined from biblical interpretations James Ussher an Irish Anglican archbishop suggested in the early 17m century that the Earth formed a mere 6010 years ago in 4004 BC Clearly this estimate makes no sense scientifically as the geologic processes that constantly modify the Earth are far too slow for anything of any consequence to happen in so short a time Ussher s supporters however simply inferred that early geologic events such as the creation of mountains must have been extremely rapid and catastrophic an idea known as catastrophism However there is no geologic evidence to support such an idea With the advent of better scientific studies and hypotheses it became clear that the Earth is extremely ancient This idea was first promoted in the late 18m century by a Scottish physician named James Hutton often referred to as the father of modern geology He coined the famous phrase regarding the age ofthe Earth no vestige of a beginning no prospect of an end Hutton introduced the Principle of Uniformitarianism which implies that we can study modern geologic processes to understand the history of the Earth because ofthe great body of evidence that these modern processes have occurred throughout Earth history Simply stated this principle advocates the present is the key to the past In order to better understand the long complex history of our planet we must develop a stron understanding of its geologic complexity This includes the evidence for how the Earth initially formed its internal structure the material it is made out of the nature of the Earth39s surface and the natural processes that operate at the surface its dynamic characteristics such as earthquakes and volcanoes its resources and its physical chemical and biological history Q So what is Physical Geology Physical geology is concerned with the materials that make up the Earth as well as the processes that operate on those materials either at or beneath the surface ofthe Earth What materials elements minerals rocks water Physical Geology 101 What processes plate tectonics volcanic eruptions earthquakes mountain building the action of rivers glaciers oceans and wind and weathering and erosion To understand these processes and the great amounts oftime needed to see their lasting impacts we have to think about time in a whole new way in comparison to human perceptions oftime ln geology we have to think in terms of deep time in which processes may occur over millions of years That39s why geologists developed the geologic time scale to divide the long duration of Earth history into numerous time divisions dating all the way back to about 4600 million years ago Course Information The expectations and schedule are shown in the course syllabus and lecture plan Course website httpwwwuidahoedusimkatgeol101html What will we look at in this course the origin ofthe Earth and how it has changed through time the composition ofthe Earth and its interior structure movements of tectonic plates and the causes of earthquakes and volcanoes the natural processes that shape the surface of the Earth Earth39s resources that are utilized by mankind mankind39s impact on the Earth and the environment l l l l tthe natural processes that modify the interior ofthe Earth l l t the geology of other planets l the development of life on Earth and implications for life on other planets Why should we study geology We are fundamentally dependent on Earth39s resources to function as a society We need to know where to find these resources so that society can continue to function But we must also recognize that many of Earth39s resources are finite and could eventually run out So we must also know how to manage our resources and recognize how our use ofthe resources may ultimately be impacting on the planet We also need to know about geology for practical reasons like choosing a safe place to construct buildings where they will be safe from river flooding landslides earthquakes volcanic eruptions and other potential natural disasters We also need to be able to pick safe sites for building dams and bridges nuclear power stations and for disposing of society39s waste products so as not to create environmental disasters Physical Geology 101 19 Crustal Deformation p 274278 288292 Introduction Although the Earth is 46 billion years old most of the oldest rocks on Earth have long since vanished so we see more younger rocks than older rocks 80 the older rocks must have been removed by processes like weathering erosion and the effects of plate tectonics Some rocks were deeply buried and turned into metamorphic rocks Where do we see deeply formed metamorphic rocks that now form mountain ranges All over the world rocks that were once perfectly flat lying are often inclined at a range of angles or are warped They are also often intensely fractured and may contain abrupt breaks called iiiiii across which one block of rock slides past another block These different types of features are called deformation features We need to know how to describe them how they formed and what they can tell us about the geologic history of a region The field of geology that considers deformation of rocks is called iiiiiiii It is vital for understanding what happens to rocks in response to the huge forces that occur inside the Earth39s crust as a result of plate tectonics Rock Deformation When we looked at metamorphic rocks we learned why a foliation develops in a rock Foliation is produced by forces pushing against rocks but the force cannot be the same from all directions To produce a foliation we need to have forces being different from different directions this is called a iiiiiiiii This gets produced by the interactions and motions of the tectonic plates These motions produce stresses that we call tectonic stresses which are different from different directions resulting in differential stress As a result rocks get deformed Rock deformation refers to all the different ways that rocks respond to squeezing stretching or any other kind of tectonic force This deformation may involve 1 Buckling and bending of rocks which we call 7Wiiii 0r 2 Cracking and breaking of rocks which we call ii77777 Examples and Fiocks may also change their shape or volume in response to differential stresses Physical Geology 101 Pressure Stress and Strain Stress and pressure are both a measure of forces being applied to rocks inside the Earth 80 how are stress and pressure different from each other Pressure alsocalledi if 7 Stress How is stress defined in terms of force Whenever a stress is applied to a rock something happens inside the rock It starts to deform and the rock is said to have undergone iiiii Definition of strain The reason for this is that rocks are not infinitely strong If you push on it hard enough something happens inside the rock either at the grain scale or the atomic level to produce strain 80 rocks have a limit to their strength Definition of rock strength The tectonic stresses produced inside the crust are big enough to deform rocks But they are applied to rocks over such a long period of time that we rarely deformation happening in rocks What we see today happened a long time ago If a uniform pressure is applied to a rock in other words one that is the same from all directions the only strain that the rock can undergo is a decrease in volume but no change in shape Example of a typical source of uniform pressure in the Earth iii77777ii7 What would happen to a cube of rock under the influence of a uniform pressure In order for a rock to change its shape as it deforms a differential stress must be applied to the rock The three types of differential stress are i777ii7 pushes on rock and causes it to shorten or compress i777ii7 pulls on rock and causes it to stretch or extend i777ii slides the rock in opposite directions from each side and causes it to change its shape likeishearing a deck of playing cards Types of Deformation The way in which a rock responds to stress is not always the same Some rocks deform by bending whereas others responded by breaking What was different about these rocks that caused the deformational style to be so different Physical Geology 101 First let us define what the different types of deformation behavior are There are essentially three 1 iiiiiii deformation if you apply force to a rubber ball and then remove it again it deforms while the force is applied but goes back to its original shape after the force is removed This is called elastic deformation Rocks can also behave elastically They start to deform as stresses are applied but return to their original shape if the stress is removed PERMANENT or 80 is elastic deformation RECOVERABLE i777ii7 deformation this is what happens to rocks when they can no longer behave elastically They fracture Exam ples of fractures and i777ii7 deformation some rocks deform by flowing or bending instead of fracturing This is called ductile deformation or iiiiiiiiii which is how Wifff develop Controls on Deformation Whether a rock is brittle or ductile depends on the conditions under which deformation occurs A certain type of rock may be brittle under one set of conditions but ductile under another set of conditions ie temperature pressure rock type and strain rate Temperature as temperature increases do rocks become LESS BRITTLE or MOREBRITTLE So they stop fracturing and instead start to flow They become ductile Where in the Earth are rocks the most brittle Rocks get more ductile with depth due to the i77ii777ii7 Pressure greater pressures cause rocks to be more 77777 which adds to the temperature effect deeper and deeper into the Earth Rock type the composition of the rock is important because different materials behave differently at a particular temperature and pressure For example feldspar behaves in a ductile manner at higher pressures and temperatures than is needed for quartz So is feldspar more ductile than quartz deeper down in the Earth or higher up DEEPER or SHALLOWER 2 Strain rate the rate of deformation has a great impact on the type of deformation that occurs in rocks Does brittle deformation occur for fast or slow strain rates FAST or SLOW Physical Geology 101 The same material will deform differently depending on strain rate In the Earth39s crust high strain rates like iiiiiiiii and blasting with explosives causes brittle deformation or fracturing Low strain rates such as are produced by tectonic forces are more likely to cause ductile deformation Brittle Structures Brittle structures are fractures such as joints and faults which are produced because of tectonic stresses The intrusion of magma into rocks can also create brittle fractures such as dikes and sills Joints Joints are fractures that form very close to the Earth39s surface and indicate that the rock has been stretched by tensional stresses When the rocks are stretched far enough the elastic limit is exceeded and the rocks fracture The two sides of the joint simply pull apart from each other Joints can be found in most rocks They generally open by no more than iiiiiiiiiii They often form dense networks that leave rocks intensely fractured Joints are useful because they can tell us about the stresses that produced them Joints form iiiiiiiiii to the direction of maximum tensional stresses When a number of joints form parallel to each other they produce a 777777 A special type of joint called a iiiiiiiiii forms as lava cools and contracts to form an igneous rock These joints arrange themselves into polygonal shapes forming long columns of rock FINAL QUESTION What rock type commonly forms these joints as lava cools
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