Early History of the Earth
The earth and the rest of the solar system were formed about 4.57 billion years ago from an enormous cloud of fragments of both icy and rocky material which was produced from the explosions (super novae) of one or more large stars It is likely that the proportions of elements in this material were generally similar to those shown in the diagram below. Although most of the cloud was made of hydrogen and helium, the material that accumulated to form the earth also included a significant amount of the heavier elements, especially elements like carbon, oxygen, iron, aluminum, magnesium and silicon. As the cloud started to contract, most of the mass accumulated towards the center to become the sun. Once a critical mass had been reached the sun started to heat up through nuclear fusion of hydrogen into helium. In the region relatively close to the sun – within the orbit of what is now Mars – the heat was sufficient for most of the lighter elements to evaporate, and these were driven outward by the solar wind to the area of the orbits of Jupiter and the other gaseous planets. As a result, the four inner planets – Mercury, Venus, Earth and Mars are “rocky” in their composition, while the four major outer planets, Jupiter, Saturn, Neptune and Uranus are “gaseous”.
As the ball of fragments and dust that was to eventually become the earth grew, it began to heat up – firstly from the heat of colliding particles – but more importantly from the heat generated by radioactive decay (fission) of uranium, thorium, and potassium (figure below). Within a few hundred million years the temperature probably rose to several thousand degrees, hot enough to melt most things. This allowed the materials to be sorted out so that the heavier substances sank towards the center, and the lighter substances floated towards the surface.
To begin with, much of the iron and magnesium would have combined with silicon and oxygen to form heavy silicate minerals such as olivine: (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3. Most of the remaining iron (along with some nickel and sulphur) would have migrated towards the center – forming a very heavy metallic core. Meanwhile, much of the aluminum, sodium and potassium would have combined with oxygen and silicon to form minerals such as quartz (SiO2)and feldspar (NaAlSi3O8) that would have floated towards the surface to form the crust (see figure below). We will look more closely at the characteristics of the core, mantle and crust later on. The original material that formed the earth included some hydrogen, oxygen, carbon and nitrogen, and these would have been brought to the surface during volcanic eruptions as molecules such as water, carbon dioxide, methane and nitrogen gas. By around 4 billion years (b.y.) ago it is likely that the earth had an atmosphere rich in carbon dioxide and nitrogen along with lots of water vapor. Initially the earth’s surface and atmosphere were probably too hot for the water to come down as rain, but as the crust cooled and hardened it kept most of the earth’s internal heat inside, and eventually the atmosphere cooled enough for rain to fall and create bodies of water on the surface.
These conceptual topics of General Geology To assist you learn more as self study, here some outlines
• How does physical geology differ
from historical geology?
• What is the fundamental didifference
between uniformitarianism and
• What is relative dating? What are
some principles of relative dating?
• How does a scientific hypothesis
differ from a scientific theory?
• What are Earth’s four major
• Why can Earth be regarded as a
• What is the rock cycle? Which geologic interrelationships are illustrated by the cycle?
• How did Earth and the other planets in our solar system originate?
• What criteria were used to establish
Earth’s layered structure?
• What are the major features of the
continents and ocean basins?
• What is the theory of plate tectonics? How do the three types of
plate boundaries differ?
The Science of Geology
The subject of this text is geology, from the Greek geo, “Earth,” and logos, “discourse.”
It is the science that pursues an understanding of planet Earth. Geology is traditionally divided into two broad
areas—physical and historical. Physical geology examines the materials composing Earth and seeks to understand the many
processes that operate beneath and upon its surface (Figure 1.1). The aim of historical geology, on the other hand, is to
understand the origin of Earth and its development through time. Thus, it strives to establish a chronological arrangement of the multitude of physical and biological changes that have occurred in the geologic past. The study of physical geology logically precedes the study of Earth history because we must first understand how Earth works before we attempt to unravel its past. It should also be pointed out that physical and historical geology are divided into many areas of specialization. Table 1.1 provides a partial list. . To understand Earth is challenging because our planet is a dynamic body with many interacting parts and a complex history. Throughout its long existence, Earth has been changing. In fact, it is changing as you read this page and will continue to do so into the foreseeable future. Sometimes the changes are rapid and violent, as when landslides or volcanic eruptions occur. Just as often, change takes place so slowly that it goes unnoticed during a lifetime. Scales of size and space also vary greatly among the phenomena that geologists study. Sometimes they must focus on phenomena that are submicroscopic, and at other times they must deal with features that are continental or global in scale. Geology is perceived as a science that is done in the out of doors, and rightly so. A great deal of geology is based on measurements, observations, and experiments conducted in the field. But geology is also done in the laboratory where, for example, the study of various Earth materials provides insights into many basic processes. Moreover, the development of sophisticated computer models allows for the simulation of many of our planet’s complex systems. Frequently, geology requires an understanding and application of knowledge and principles from physics, chemistry, and biology. Geology is a science that seeks to expand our knowledge of the natural world and our place in it.
During the seventeenth and eighteenth centuries the doctrine of catastrophism strongly influenced people’s thinking about Earth. Briefly stated,catastrophists believed that Earth’s landscapes had been shaped primarily by great catastrophes. Features such as mountains and canyons, which today we know take great periods of time to form, were explained as having been produced by sudden and often worldwide disasters produced by unknown causes that no longer operate. This philosophy was an attempt to fit the rates of Earth processes to the then current ideas on the age of Earth.The Pakistan flooding, July–November 2010 (DFO event 3696) caused close to 2000 fatalities, displaced 20,000,000 inhabitants for weeks to many months, and was 7.5 on a duration–area affected–intensity scale that compares flood magnitudes on a global basis (Chorynski et al., 2012; Brakenridge, 2012). Flooding along the Indus River began in mid- to late-July following unusually heavy monsoonal rain in northern Pakistan and was sustained in downstream areas through the end of 2010. Exceptional damage was inflicted on crops and cropland and on agriculture support systems such as canals and levees; 4,500,000 mainly agricultural workers lost their employment for 2010–2011 (Khan, 2011).
The Birth of Modern Geology
Against this backdrop of Aristotle’s views and Earth created in 4004 B.C., a Scottish physician the and gentleman farmer named James Hutton published Theory of the Earth in 1795. In this work Hutton put forth a fundamental principle that is a pillar of geology today: uniformitarianism. It states that the physical, chemical, and biological laws that operate today also operated in the geologic past.In other words, the forces and processes that we observe shaping our planet today have been at work for a very long time. Thus, to understand ancient rocks, we must first understand present-day processes and their results. This idea is commonly stated as the present is the key to the past. Prior to Hutton’s Theory of the Earth, no one had effectively demonstrated that geological processes can continue over extremely long periods of time. Hutton persuasively argued that forces that appear small could, over long spans of time, produce effects just as great as those resulting from sudden catastrophic events. Hutton carefully cited verifiable observations to support his ideas. For example, when he argued that mountains are sculpted and ultimately destroyed by weathering and the work of running water, and that their wastes are carried to the oceans by processes that can be observed, Hutton said “We have a
chain of facts which clearly demonstrates . . . that the materials of the wasted mountains have traveled through the rivers”; and further, “There is not one step in all this progress . . . that is not to be actually perceived.” He then went on to summarize this thought by asking a question and immediately providing the answer: “What more can we require? Nothing but time.”
Exploring this series of exhibits will take you on a journey through the history of the Earth, with stops at particular points in time to examine the fossil record and stratigraphy. Although Hutton and others recognized that geologic time is exceedingly long, they had no methods to accurately determine the age of Earth. However, in 1896 radioactivity was discovered. Using radioactivity for dating was first attempted in 1905 and has been refined ever since. Geologists are now able to assign fairly accurate dates to events in Earth history.For example, we know that the dinosaurs died out about 65 million years ago. Today the age of Earth is put at about 4.5 billion years. The concept of geologic time is new to many non geologists. People are accustomed to dealing with increments of time that are measured in hours, days, weeks, and years. Our history books often examine events over spans of centuries, but even a century is difficult to appreciate fully. For most of us, someone or something that is 90 years old is very old, and a 1000-year-old artifact is ancient.
Fossils, the remains or traces of prehistoric life, were also essential to the development of a geologic time scale. Fossils are the basis for the principle of fossil succession, which states that fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. This principle was laboriously worked out over decades by collecting fossils from countless rock layers around the world. Once established, it allowed geologists to identify rocks of the same age in widely separated places and to build the geologic time scale shown in Figure 1.9. Notice that units having the same designations do not necessarily extend for the same number of years. For example, the Cambrian period lasted about 56 million years, whereas the Silurian period spanned only about 28 million years. This situation exists because the basis for establishing the time scale was not the regular rhythm of a clock, but the changing character of life forms through time. Specific dates were added long after the time scale was established. A glance at Figure 1.9 also reveals that the Phanerozoic eon is divided into many more units than earlier eons even though it encompasses only about 12 percent of Earth history. The meager fossil record for these earlier eons is the primary reason for the lack of detail on this portion of the time scale. Without abundant fossils, geologists lose a very important tool for subdividing geologic time.
A view such as the one in Figure 1.13A provided the Apollo 8 astronauts as well as the rest of humanity with a unique perspective of our home. Seen from space, Earth is breathtaking in its beauty and startling in its solitude. Such an image reminds us that our home is, after all, a planet—small, self-contained, and in some ways even fragile. As we look more closely at our planet from space, it becomes apparent that Earth is much more than rock and soil. In fact, the most conspicuous features in Figure 1.13A are not continents but swirling clouds suspended above the surface and the vast global ocean. These features emphasize the importance of water to our planet.
The closer view of Earth from space shown in Figure 1.13B helps us appreciate why the physical environment is traditionally divided into three major parts: the water portion of our planet, the hydrosphere; Earth’s gaseous envelope, the atmosphere; and, of course, the solid Earth, or geosphere. It needs to be emphasized that our environment is highly integrated and is not dominated by rock, water, or ESSENTIALS OF GEOLOGY A. B. air alone. Rather, it is characterized by continuous interactions as air comes in contact with rock, rock with water, and water with air. Moreover, the biosphere, which is the totality of all plant and animal life on our planet, interacts with each of the three physical realms and is an equally integral part of the planet. Thus, Earth can be thought of as consisting of four major spheres: the hydrosphere, atmosphere, geosphere, and biosphere. The interactions among Earth’s four spheres are incalculable. Figure 1.14 provides us with one easy-to-visualize example. The shoreline is an obvious meeting place for rock, water, and air. In this scene, ocean waves that were created by the drag of air moving across the water are breaking against the rocky shore. The force of the water can be powerful, and the erosional work that is accomplished can be great.
Earth is sometimes called the blue planet. Water more than anything else makes Earth unique. The hydrosphere is a dynamic mass of liquid that is continually on the move, evaporating from the oceans to the atmosphere, precipitating back to the land, and running back to the ocean again. The global ocean is certainly the most prominent feature of the hydrosphere, blanketing nearly 71 percent of Earth’s surface to an average depth of about 3800 meters (12,500 feet). It accounts for about 97 percent of Earth’s water. However, the hydrosphere also includes the freshwater found underground and in streams, lakes, and glaciers. Moreover, water is an important component of all living things. Although these latter sources constitute just a tiny fraction of the total, they are much more important than their meager percentage indicates. Streams, glaciers, and groundwater are responsible for creating many of our planet’s varied land forms, as well as the fresh water that is so vital to life on land.
Earth is surrounded by a life-giving gaseous envelope called the atmosphere. When we watch a high-flying jet plane cross the sky, it seems that the atmosphere extends upward for a great distance (Figure 1.15). However, when compared to the thickness (radius) of the solid Earth (about 6400 kilometers or 4000 miles), the atmosphere is a very shallow layer. One-half lies below an altitude of 5.6 kilometers (3.5 miles), and 90 percent occurs within just 16 kilometers (10 miles) of Earth’s surface. Despite its modest dimensions, this thin blanket of air is an integral part of the planet. It not only provides the air that we breathe but also acts to protect us from the Sun’s intense heat and dangerous ultraviolet radiation. The energy exchanges that continually occur between the atmosphere and the surface and between the atmosphere and space produce the effects we call weather and climate.
The biosphere includes all life on Earth. Ocean life is concentrated in the sunlit surface waters of the sea. Most life on land is also concentrated near the surface, with tree roots and burrowing animals reaching a few meters underground and flying insects and birds reaching a kilometer or so above Earth. A surprising variety of life forms are also adapted to extreme environments. For example, on the ocean floor where pressures are extreme and no light penetrates, there are places where vents spew hot, mineral-rich fluids that support communities of exotic life forms(Figure 1.16). On land, some bacteria thrive in rocks as deep as 4 kilometers (2.5 miles) and in boiling hot springs. Moreover, air currents can carry microorganisms many kilometers into the atmosphere. But even when we consider these extremes, life still must be thought of as being confined to a narrow band very near Earth’s surface. Plants and animals depend on the physical environment for the basics of life. However, organisms do not just respond to their physical environment. Indeed, the biosphere powerfully influences the other three spheres. Without life, the makeup and nature of the geosphere, hydrosphere, and atmosphere would be very different.
Lying beneath the atmosphere and the oceans is the solid Earth, or geosphere. The geosphere extends from the surface to the center of the planet, a depth of 6400 kilometers, making it by far the largest of Earth’s four spheres. Much of our study of the solid Earth focuses on the more accessible surface features. Fortunately, many of these features represent the outward
expressions of the dynamic behavior of Earth’s interior. By examining the most prominent surface features and their global extent, we can obtain clues to the dynamic processes that have shaped our planet. A first look at the structure of Earth’s interior and at the major surface features of the geosphere will come later in the chapter. Soil, the thin veneer of material at Earth’s surface that supports the growth of plants, may be thought of as part of all four spheres. The solid portion is a mixture of weathered rock debris (geosphere) and organic matter from decayed plant and animal life (biosphere). The decomposed and disintegrated rock debris is the product of weathering processes that require air (atmosphere) and water (hydrosphere). Air and water also occupy the open spaces between the solid particles. Anyone who studies Earth soon learns that our planet is a dynamic body with many separate but interacting parts or spheres.The hydrosphere, atmosphere, biosphere, and geosphere and all of their components can be studied separately. However, the parts are not isolated. Each is related in some way to the others to produce a complex and continuously interacting whole that we call the Earth system.
WHAT IS A SYSTEM?
Most of us hear and use the term system frequently. We may service our car’s cooling system, make use of the city’s transportation system, and participate in the political system.A news report might inform us of an approaching weather system.Further, we know that Earth is just a small part of a larger system known as the solar system which in turn is a subsystem of the even larger system called the Milky Way Galaxy. Loosely defined, a system can be any size group of interacting parts that form a complex whole. Most natural systems are driven by sources of energy that move matter and/or energy from one place to another. A simple analogy is a car’s cooling system, which contains a liquid (usually water and antifreeze) that is driven from the engine to the radiator and back again. The role of this system is to transfer heat generated by combustion in the engine to the radiator, where moving air removes it from the system. Hence, the term cooling system.
Earth System Science
A simple example of the interactions among different parts of the Earth system occurs every winter as moisture evaporates from the Pacific Ocean and subsequently falls as rain in the hills of southern California, triggering destructive landslides. A case study in Chapter 8 explores such an event (see p. 189). The processes that move water from the hydrosphere to the atmosphere and then to the solid Earth have a profound impact on the plants and animals (including humans) that inhabit the affected regions. Figure 1.17 provides another example. Scientists have recognized that to more fully understand our planet, they must learn how its individual components (land, water, air, and life forms) are interconnected. This endeavor, called Earth system science, aims to study Earth as a system composed of numerous interacting parts, or subsystems.Rather than looking through the limited lens of only one of the traditional sciences—geology, atmospheric science, chemistry, biology, and so forth—Earth system science attempts to integrate the knowledge of several academic fields.
Using this interdisciplinary approach, we hope to achieve the level of understanding necessary to comprehend and solve many of our global environmental problems. Scientists have determined that since 1906 global average surface temperature has increased by about 0.7°C (1.3°F). By the end of the twenty-first century, globally averaged surface temperature is projected to increase by an additional 1.7 to 3.9°C (3 to 7°F).
ENERGY FOR THE EARTH SYSTEM. The Earth system is powered by energy from two sources. The Sun drives external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface. Weather and climate, ocean circulation, and erosional processes are driven by energy from the Sun. Earth’s interior is the second source of energy. Heat remaining from when our planet formed, and heat that is continuously generated by decay of radioactive elements, power the internal processes that produce volcanoes, earthquakes, and mountains.
CYCLES IN THE EARTH SYSTEM. A more familiar loop or subsystem is the hydrologic cycle.It represents the unending circulation of Earth’s water among the hydrosphere, atmosphere, biosphere, and geosphere. Water enters the atmosphere by evaporation from Earth’s surface and by transpiration from plants. Water vapor condenses in the atmosphere to form clouds, which in turn produce precipitation that falls back to Earth’s surface. Some of the rain that falls onto the land sinks in to be taken up by plants or become groundwater, and some flows across the surface toward the ocean.
Before going ahead with Rock Cycle we should know What is Rocks?
The study of geology is the study of the Earth, and so is ultimately the study of rocks. Geologists define a rock as:A bound aggregate of minerals, mineraloids, or fragments of other rocks.
There are three major groups of rocks:
- Igneous rocks are those that have formed by the cooling and crystallisation of magma, either at the Earth’s surface or within the crust;
- Sedimentary rocks are those that have formed when eroded particles of other rocks have been deposited (on the ocean floor, stream/lake beds, etc) and compacted, or by the precipitation of minerals / mineraloids from water;
- Metamorphic rocks are those that have formed when existing rocks have undergone pressure and / or temperature changes so that their original mineralogy has been changed.
Each of these rock groups contains many different types of rock, and each can be identified from its physical features.
Being able to describe and name rocks is one of the fundamental skills of a geologist. Important information regarding the nature of rocks is communicated through concise, accurate descriptions. This information allows the geologist to identify the rock, and, in the process, to learn about its history and the geological environment in which it was formed.
A knowledge of field relationships between different rock units is fundamental to the study of rocks. It is gained from mapping and observing rocks in the field. In depth analysis of rocks using a microscope or sophisticated analytical laboratory equipment provides important information on their composition. In between these extremes is the observation and description of hand specimens. The term hand specimen refers to an easily manageable piece of rock that can be picked up and easily transported back to the geologist’s base for further investigation.
The Rock Cycle:
One of Earth’s Subsystems
Rock is the most common and abundant material on Earth. To a curious traveler, the variety seems nearly endless. When a rock is examined closely, we find that it consists of smaller crystals or grains called minerals. Minerals are chemical compounds (or sometimes single elements), each with its own composition and physical properties. The grains or crystals may be microscopically small or easily seen with the unaided eye. The nature and appearance of a rock is strongly influenced by the minerals that compose it. In addition, a rock’s texture—the size, shape, and/or arrangement of its constituent minerals—also has a significant effect on its appearance. A rock’s mineral composition and texture, in turn, are a reflection of the geologic processes that created it (Figure 1.20). The characteristics of the rocks in Figure 1.21 provided geologists with the clues they needed to determine the processes that formed them. This is true of all rocks. Such analyses are critical to an understanding of our planet. This understanding has many practical applications, as in the search for basic mineral and energy resources and the solution of environmental problems.
This means that our planet consists of many interacting parts that form a complex whole. Nowhere is this idea better illustrated than when we examine the rock cycle (Figure 1.22). The rock cycle allows us to view many of the interrelationships among different parts of the Earth system. Knowledge of the rock cycle will help you more clearly understand the idea that each rock group is linked to the others by the processes that act upon and within the planet. You can consider the rock cycle to be a simplified but useful overview of physical geology.
The paths shown in the basic cycle are not the only ones that are possible. To the contrary, other paths are just as likely to be followed as those described in the preceding section. These alternatives are indicated by the blue arrows in Figure 1.22.
Igneous rocks, rather than being exposed to weathering and erosion at Earth’s surface, may remain deeply buried. Eventually these masses may be subjected to the strong compressional forces and high temperatures associated with mountain building. When this occurs, they are transformed directly into metamorphic rocks. Metamorphic and sedimentary rocks, as well as sediment, do not always remain buried. Rather, overlying layers may be stripped away, exposing the once buried rock. When this happens, the material is attacked by weathering processes and turned into new raw materials for sedimentary rocks. Where does the energy that drives Earth’s rock cycle come from? Processes driven by heat from Earth’s interior are responsible for forming igneous and metamorphic rocks. Weathering and the movement of weathered material are external processes powered by energy from the Sun. External processes produce sedimentary rocks.