GEOL 1110, Chapter 1, Introduction to Geology PDF

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Chapter 1. Introduction to Geology 1.1. What is Geology? Geology is the scientific study of the Earth 1.2. What does it mean by “scientific study”? Scientific study means that a certain method is used to study something, in this case the Earth. 1.3. The Scientific Method The method used in geology is the scientific method, a step-by-step way to recognize something, ask a question about it and come up with an answer. There is no universal scientific method used by everyone in science. The method we’ll use in this course includes 5 steps. Step 1. The first step is data collection. This means getting as much information as you can, or is required, about something. It may be that you collect as many rocks as possible from the bank of a river. You may not know exactly why you are collecting these rocks other than they look cool (all rocks are cool) and that they should say something about the geological processes in the area. Step 2. Step two is generalization. This step is really important, arguably the most important step in the entire scientific method. Generalization means you look at a bunch of data and recognize a pattern or patterns that say some about the information collected in step 1. Say for example you collected a bunch of rocks and after examining them you realize most are black and the rest are white. What’s more all the rocks are between 5 mm and 25 mm in size. These two generalizations say something about the origin of the rocks and the processes that have affected them.

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Step 3. The third step is a hypothesis. A hypothesis is: “A supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation”. This is where you try and explain what the information in steps 1 and 2 is telling you. A supposition or proposed explanation for your collection of rocks might be that the colours of the rocks (black and white) suggest a certain type of rock. The narrow range of rock sizes (5 to 25 mm) is because the rocks were deposited by the river and therefore say something about the behaviour of the river. What’s more you could suppose that the rocks came from a source area upstream and that they were carried to the place where you found them by the water in the river. This gives you an idea where to find the rock source. So, your hypothesis might be: The rocks collected from the river bank are derived from a source upstream of the river. They somehow ended up in the river where they were carried downstream by moving water capable of carrying rocks of this size. The water did not carry any larger rocks. If there were any smaller rocks then they were not deposited where you collected rocks. This all sounds pretty simple and obvious but it goes a long way to explaining how the Earth works. This is what scientists in general and geologists in particular do all the time. Step 4. The fourth step is a test. Here’s where you take your hypothesis and do it again to see if you can get the same results. In the case of the rocks collected by the river, you go to the same spot in the river and collect more rocks or you go to another spot in the river and collect another set of rocks. You might even go to a different river and collect rocks. In any case, the intent is to examine more rocks the same way - observe colour and size - to see if the results are the same. You could also ask someone else to collect rocks to see what they come up with. A test such as this will confirm your hypotheses is correct, completely false or is partially true and needs improvement.

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Step 5. The final step is a theory. A theory is: “A supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained.” Here is where you take your hypothesis and improve it such that it better explains you data because you have more data from conducting more investigations. You could now revise your hypothesis to say: The black and white rocks are diorite and gabbro which are igneous intrusive rock types. They came from a rock outcrop approximately 5 km upstream of where they were deposited. They were eroded, transported and deposited by the river. The velocity of the water in the river was between 1 and 3 meters per second because it takes water of this velocity to move rocks of this size. Now were know something more specific about the geology of the study area. Going back to the original explanation of a theory, what then is a general principle in the example presented in these steps. One is the behaviour of water. Water moving on the Earth’s surface will always flow downhill. The velocity of the water will dictate the size of the sediment it is capable of moving. The faster the water the bigger the piece of rock it can carry. As a final comment, a theory may become a law. With science as law is: “A statement of fact, deduced from observation, to the effect that a particular natural or scientific phenomena always occurs if certain conditions are present”. Another way to think about a law is that it’s something based on proof or that it proves something. The law of conservation of mass states that mass is neither created nor destroyed. It is based on enough observations by so many different people under so many different circumstances that it is demonstrated conclusively to be true under all conditions, it has essentially been proven. There are very few laws or proofs in science. There are lots of theories as we’ll see throughout the course. Proof implies a certainty about an explanation that is rarely achieved in science. In geology for every answer, argument or explanation there is 3

always a counter argument or alternative explanation that is just as valid. So, be careful what you say, explain your theory clearly and provide as much evidence as possible and never use the word “proves” unless you’re certain that’s the case. 1.4. Why study geology? There are five basic reasons to study geology. 1. Scientific curiosity. The need to know. What kind of rock is that and how did it get there? 2. Discovery and utilization of natural resources. What is oil, why is it only found in sedimentary rocks and how do we find it underground? 3. Environmental protection. What controls the behaviour of water underground? How do we locate and protect groundwater? What is the best way to clean it once it’s been contaminated? 4. Natural hazard awareness. Where do earthquakes come from? How often do they occur and why? How does the Earth behave during an earthquake and what can be done to minimize the effects of this ground motion? 5. For credit. Sadly, not everyone wants to be a geologist. If you’re here to get a lab science credit then welcome. Let’s hope this course opens up a whole new perspective on the Earth, how it works, how vital it is to all of us, and how we can better protect it. Remember there is no plan(et) B. 1.5. The Origin of the Earth The Earth is one of 8 planets in our solar system (Figure 1 - A) that revolve around the Sun. The Earth came about through a process explained in the following five steps. Step 1. In the beginning there was nothing but a nebula, a giant cloud of dust and gas, spread across the vastness of space. Over time this dust and gas coalesced into larger and larger concentrations of more and more mass. Concentration of mass was the result of collisions in space and gravity as the dust and gas moved.

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Earth

Figure 1 - A. Our solar system Step 2. As larger and larger masses collided in space their kinetic energy (energy of motion) was, at least partially, transformed into heat energy. Overtime, more and more heat built up in what eventually became a few large, hot objects. Most of the mass, and therefore most of the heat, collected in the center of our solar system in what is now the Sun. The Sun collected so much heat that eventually it ignited in a process called nuclear fusion to become a star. Step 3. The Earth also collected mass and heat, but not enough to become a star. However, the Earth still got hot, very hot. So hot that it started out as a molten mass of melted rock. Eventually the collisions in space stopped, except for the odd small object that still strikes the Earth. As well the accumulation of heat stopped and the Earth began to cool. Because the early Earth was made up of all sorts of different atoms, when it began to cool and solidify it did so at different rates. What this means is heavier atoms, such as iron and nickel “fell” toward the center while lighter elements “floated” upward to the surface. Eventually as the Earth became more solid, these atoms and the rocks they formed were frozen in place and the internal layers of the Earth were formed - the inner core, outer core, mantle and crust. (Chapter 1, Figure 1.7, page 12). There is still plenty of heat inside the Earth. The two primary sources of this heat are: 1) the transformation of kinetic energy to heat energy during the collisions that formed the Earth; and 2) the decay of radioactive elements inside the Earth.

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Figure 1.7, page 12. The internal structure of the Earth. Step 4. Water on the Earth is thought to be derived from two sources, at least that’s the theory. One source of water may be the degassing of the Earth’s crust as it cooled and solidified. This means that as rocks formed they did not include water and the water was released on to the surface. The second possible source of water is the impact of comets with the Earth during Earth’s early formation. A comet is a mass of frozen water and rock fragments, sometimes referred to as a “dirty snowball”, and when they impact the Earth they deliver water and rocks. It’s thought there were many more comets in the solar system during Earths early formation. Step 5. The formation of the atmosphere took place at the same time as water collected on Earth. Gases released from the crust or contributed from space were light enough to rise above the Earth’s surface and heavy enough that the Earth’s gravity was able prevent their escape into space. The first atmosphere is thought to be made up of hydrogen sulphide, methane, and carbon dioxide. Note that there was no oxygen in the first atmosphere, it was to come much later. 6

Note that this explanation of the formation of the solar system, including the Earth, is a hypothesis, a possible explanation that requires further investigation. Geologists spend a great deal of time with hypotheses trying to better our understanding of the Earth and earth-forming processes. The only thing we haven’t considered is the origin of the biosphere. What is the origin of life? Where did life come from? No one knows for sure. We’ll talk more about this when we come to the section on fossils. 1.6. The Geological Time Scale The Earth is old. By any standard it is very old. Our best estimate of the age of the Earth is 4.57 billion years. This age is based on the geological record, the sum total of information from all the rocks, minerals and fossils found on Earth. In addition geologists determine the age of rocks using radioactive atoms and hypotheses and theories on how the Earth should behave over time. The oldest rock fragment found so far is 4.375 billion years old. Anything older has not yet been found or doesn’t exist because its been recycled or never formed because long ago the Earth was very hot and rocks could not solidify. So, what do we do with all this time? How do we make sense of it? The answer is we create a calendar or, more precisely, a geological time scale. This nothing more than a list of time intervals with important events, much like a calendar for the month of December with birthdays marked on specific days. The information is illustrated like a chart for ease of use. Figure 1 - B is a typical geological time scale. It begins at the bottom at 4.6 billion years with the origin of the Earth and shows progressively younger or more recent dates until it reaches the present day at 0 years. There is no universal geological time scale, depending on which source you use it will vary. The time scale in figure 1 - B is the one we’ll use for this course. The geological time scale is broken into eons, eras, periods and epochs, much the same way that a day is broken down into hours, minutes and seconds. Most of Earth history is included in the Archean and Proterozoic eons which make up approximately 4 billion years of Earth history. Approximately 542 million years ago the Earth changed. This is reflected in the geological time scale with the end of the Proterozoic Eon and the beginning of the 7

Cambrian Explosion

Figure 1 - B. Geological time scale. 8

Phanerozoic Eon. So what happened 542 million years ago? At this point in Earth history, according to the fossil record, abundant complex life appeared on Earth. Now life first appeared on Earth approximately 3.7 billion years ago. But this early life consisted mostly of simple, single cell microscopic organisms. So something happened that allowed life to evolve into a much larger number of diverse, complex creatures resulting in what is called the ‘Cambrian Explosion’. What that was we’re not sure. The Phanerozoic Eon is separated into the Paleozoic, Mesozoic and Cenozoic eras. Each era is defined by specific events in Earth history. The beginning of the Paleozoic Era is marked by the Cambrian Explosion. At the end of the Paleozoic and beginning of the Mesozoic eras is the largest known mass extinction in Earth history. At this time, approximately 251 million years ago almost 90% of life on Earth disappeared. The end of the Mesozoic and beginning of the Cenozoic eras, approximately 65 million years ago, is when the second largest mass extinction occurred, the one that ended the dinosaurs. There is a trend or pattern in the geological time scale as we’ve examined it so far. Although the intent is to catalogue Earth history based on the geological record, this record of Earth history is, at least initially, based on something different. It is based on the history of life. In a way it is more biological then geological, a primary example of the cross disciplinary quality of geology. The newest, most recent time interval is the Holocene or Recent Epoch. This is the time interval we live in which began approximately 11,477 years ago with the retreat of the last large ice sheets. Not exactly a biological event but certainly one that significantly impacted Earth history. 1.7. What’s next? There has been significant progress made for declaring an end to the Holocene Epoch and the designation of a new, more relevant time interval, the Anthropocene Epoch. The motivation for this change is the recognition that human beings have become such a controlling influence on the Earth that we are essentially the dominant force shaping the evolution of the Earth. It’s estimated that human activity - mining, construction, etc - moves more rocks than all the natural earth moving processes - rivers, wind, glaciers, etc - put together. 9

The intent of this change is not to condemn nor celebrate the impact we humans have but to raise awareness of our collective responsibility. To this end there must be a defining rock formation or geological deposit that indicates in the geological record where one epoch ended and the next began. The end of the Mesozoic Era and beginning of the Cenozoic Era 65 million years ago is defined by a thin clay layer that is unusually rich in iridium an atom normally found in very low concentrations on Earth. What’s more this clay layer is found in many locations around the world. This geological deposit that marks a significant event in Earth history is what’s called a type section. So what then could be a good type section to mark the end of the Holocene Epoch and the beginning of the Anthropocene Epoch? It has to be easily identified, commonly found and long lasting. Various deposits have been proposed. Charcoal from early forms of farming or the industrial revolution, concrete and plastic have been suggested. The International Commission on Stratigraphy (ICS) is in favour of using the radioactive signature left in rock, sediment and soil samples by atomic bomb testing as the definitive marker for the start of the Anthropocene. It’s an obvious marker identifiable around the world and should last well into the future. It’s also very precise. The first detonation of an atomic device was 5:29 am local time, July 16, 1945.

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