8 Earth History - An Introduction to Geology PDF

An Introduction to Geology Free Textbook for College-Level Introductory Geology Courses 8 Earth History Spider Rock, within Canyon de Chelly National Monument, not only has a long human history with the Diné tribe, but also has a long geologic history. The rocks are Permian in age, and formed in t...

An Introduction to Geology Free Textbook for College-Level Introductory Geology Courses 8 Earth History Spider Rock, within Canyon de Chelly National Monument, not only has a long human history with the Diné tribe, but also has a long geologic history. The rocks are Permian in age, and formed in the desert conditions that dominated North America toward the end of the Paleozoic through the middle Mesozoic. Erosion of the canyon occurred in the Cenozoic.KEY CONCEPTS By the end of this chapter, students should be able to: Explain the big-bang theory and origin of the elements Explain the solar system’s origin and the consequences for Earth. Describe the turbulent beginning of Earth during the Hadean and Archean Eons Identify the transition to modern atmosphere, plate tectonics, and evolution that occurred in the Proterozoic Eon Describe the Paleozoic evolution and extinction of invertebrates with hard parts, fish, amphibians, reptiles, tetrapods, and land plants; and tectonics and sedimentation associated with the supercontinent Pangea Describe the Mesozoic evolution and extinction of birds, dinosaurs, and mammmals; and tectonics and sedimentation associated with the breakup of Pangea Describe the Cenozoic evolution of mammals and birds, paleoclimate, and tectonics that shaped the modern world Earth history is not just one field. It is many fields. It includes paleogeography, paleontology, paleoecology, paleoclimatology, and paleotectonics – respectively, past landscapes, past organisms, past ecosystems, past environments, and past tectonism. Entire courses and careers have been based on these and other aspects of Earth’s past. These are dynamic fields, and during Earth’s long history, change has been the norm for all of them. Looking back in time, an untrained eye would see many unfamiliar landscapes, life Geologic time on Earth, represented circularly, to show forms, ecosystems, continents, and oceans. We the individual time divisions and important events. Ga=billion years ago, Ma=million years ago. cannot cover everything, so we will focus on the major physical and biological events that occurred in each of the different Eons and Eras. We will start by looking at the origin of the universe and our solar system, and then continue to the present day. 8.1 Origin of the Universe The universe appears to have an infinite number of galaxies and solar systems and our solar system occupies a small section of this vast entirety. The origins of the universe and solar system set the context for conceptualizing the Earth’s origin and early history. The Hubble Deep Field. This image, released in 1996, is a composite long-exposure picture of one of the darkest parts of the night sky. Every light on this image that does not have diffraction spikes is believed to be an entire galaxy, with hundreds of billions of stars, demonstrating the immense size and scope of the universe. 8.1.1 Big-Bang Theory The mysterious details of events prior to and during the origin of the universe are subject to great scientific debate. The prevailing idea about how the universe was created is called the big- bang theory. Although the ideas behind the big- bang theory feel almost mystical, they are supported by Einstein’s theory of general relativity. Timeline of expansion of the universe Other scientific evidence, grounded in empirical observations, supports the big-bang theory as well. The big-bang theory proposes the universe was formed from an infinitely dense and hot core of material 13.8 billion years ago. The bang in the title suggests there was an explosive, outward expansion of all matter and space that created atoms. Spectroscopy confirms that hydrogen makes up about 74% of all matter in the universe. Since its creation, the universe has been expanding for 13.8 billion years and recent observations suggest the rate of this expansion is increasing. SPECTROSCOPY Spectroscopy is the investigation and measurement of spectra produced when materials interact with or emit electromagnetic radiation. Spectra is the plural for spectrum which is a particular range of wavelengths from the electromagnetic spectrum. Common spectra include the different colors of visible light, X-rays, ultraviolet waves, The electromagnetic spectrum and properties of light across the microwaves, and radio waves. spectrum. Each beam of visible light that we see is a mixture of wavelengths that combine to make the color we see. The light wavelengths are created or absorbed by atoms, and each wavelength signature matches a specific element. Even white light from the Sun, which seems like an uninterrupted continuum of wavelengths, has gaps in some wavelengths. The gaps correspond to elements present in the Earth’s atmosphere that act as filters for specific wavelengths. These missing wavelengths were famously observed by Joseph von Fraunhofer (1787–1826) in the early 1800s, but it took decades before scientists were able to relate the missing wavelengths to atmospheric filtering. Spectroscopy shows that the Sun is mostly made of hydrogen and helium. Applying this process to light from distant stars, scientists can calculate the abundance of elements in a specific star and visible universe as a whole. REDSHIFT Spectroscopic information can be used as an interstellar speedometer.The Doppler effect is the same process that changes the pitch of the sound of an approaching car or ambulance from high to low as it passes. When an object emits waves, such as light or sound, while moving toward an observer, the wavelengths get compressed. In sound, this results in a shift to a higher pitch. When an object moves away from an observer, the wavelengths are extended, producing a lower pitched sound.  The Doppler effect is used to study light emitted from stars and galaxies to determine their speed and direction of travel. Scientists, including Vesto Slipher (1875–1969) Click to animate. This animation demonstrates how the Doppler effect is and Edwin Hubble (1889–1953), examined galaxies both heard as a car moves. The waves in front near and far and found that almost all galaxies outside of of the car are compressed together, our galaxy are moving away from each other, and us. making the pitch higher. The waves in the back of the car are stretched, and and Because the light wavelengths of receding objects are the pitch gets lower. extended, visible light is shifted toward the red end of the spectrum, called a redshift. In addition, Hubble noticed that galaxies that were farther away from Earth had the greater amount of redshift, and thus, they were traveling faster away from us. The only way to reconcile this information is to conclude the universe is still expanding. Hubble’s observation forms the basis of big-bang theory. Example of Doppler Shift using car horn COSMIC MICROWAVE BACKGROUND RADIATION Cosmic microwave background radiation also provides evidence that a big-bang occurred. Cosmic radiation was accidentally discovered by Arno Penzias (1933–) and Robert Woodrow Wilson (1936–) when they were trying to eliminate background noise from a communication satellite. Heat map, showing slight variations in They discovered very faint traces of energy, or background heat in the universe, which is related heat, that are omnipresent across the universe. to cosmic background radiation. This energy was left behind from the big bang, like an echo. 8.1.2 Stellar Evolution Astronomers think the big bang created lighter elements, mostly hydrogen and smaller amounts of elements helium, lithium, and beryllium. Another process must be responsible for creating the other 90 heavier elements. The current model of stellar evolution explains the Origin of the elements on the periodic table, showing the important role the star life cycle plays. origins of these heavier elements. BIRTH OF A STAR Stars start their lives as elements floating in cold, spinning clouds of gas and dust known as nebulas. Gravitational attraction or perhaps a nearby stellar explosion causes the elements to condense and spin into disk shape. In the center of this disk shape a new star is born as mass gathers because of the force of gravity. The spinning whirlpool concentrates material in the center, warming as it does, and the increasing gravitational forces collect even more mass. Eventually, the immensely concentrated mass of material reaches a critical point of such intense heat and pressure it initiates Section of the Eagle Nebula known as “The Pillars of Creation.” fusion. FUSION  Fusion is not a chemical reaction. Fusion is a nuclear reaction in which two or more nuclei, the centers of atoms, are forced together and combine creating a new larger atom. This reaction gives off a tremendous amount of energy, usually as light and solar radiation. An element such as hydrogen combines or fuses with other hydrogen atoms in the core of a star to become a new element. Most commonly, two hydrogen atoms fuse to produce helium. Another product of this process is energy. Fusion in the sun produces solar radiation that leaves the Sun and comes to the Earth as light and heat. Fusion is a steady and predictable process, which is why we call this General diagram showing the the main phase of a star’s life. During its main phase, a star series of fusion steps that occur in turns hydrogen into helium. Since most stars contain plentiful the sun. amounts of hydrogen, the main phase may last billions of years, during which their size and energy output remains relatively steady. The giant phase in a star’s life occurs when the star runs out of hydrogen for fusion. If a star is large enough, it has sufficient heat and pressure to start fusing helium into heavier elements. This kind of fusion is more energetic and, as it occurs, higher energy and temperature expand the star to a larger size and brightness. This giant phase is predicted to happen to our Sun in another Two main paths of a star’s life cycle, depending on mass. few billion years, growing the radius of the Sun to Earth’s orbit, which will render life impossible. The mass of a star during its main phase is the primary factor in determining how it will evolve. If the star has enough mass and reaches a point at which the primary fusion element, such as helium, is exhausted, fusion continues using new, heavier elements. This occurs over and over in very large stars, forming progressively heavier elements like carbon and oxygen. Eventually, fusion reaches its limit as it forms iron and nickel. This progression explains the abundance of iron and nickel in rocky objects, like Earth, within the solar system. At this point, any further fusion absorbs energy instead of giving it off, which is the beginning of the end of the star’s life. DEATH OF A STAR The death of a star can range from spectacular to other-worldly (see figure). Stars like the Sun eventually form a planetary nebula, which comes from the collapse of the star’s outer layers in an event like the implosion of a building. In the tug- of-war between gravity’s inward pull and fusion’s outward push, gravity instantly takes over when fusion ends, with the outer gasses puffing away to form a nebula. More massive stars do this as well but with a more energetic collapse, which starts another type of energy release mixed with element Hubble space telescope image of the Crab Nebula, creation known as a supernova. In a supernova, the the remnants of a supernova that occurred in collapse of the core suddenly halts, creating a 1054 C.E. massive outward-propagating shock wave. A supernova is the most energetic explosion in the universe short of the big bang. The energy release is so significant the ensuing fusion can make every element up through uranium. The death of a star can result in the creation of white dwarfs, neutron stars, or black holes. Following their deaths, stars like the Sun turn into white dwarfs. White dwarfs are hot star embers, formed by packing most of a dying star’s mass into a small A black hole and its shadow have been captured and dense object about the size of Earth. Larger in an image for the first time in 2019, a historic stars may explode in a supernova that packs their feat by an international network of radio mass even tighter to become neutron stars. telescopes called the Event Horizon Telescope (Source: NASA) Neutron stars are so dense that protons combine with electrons to form neutrons. The largest stars collapse their mass even further, becoming objects so dense that light cannot escape their gravitational grasp. These are the infamous black holes and the details of the physics of what occurs in them are still up for debate. Did you get it? Click here to find out. 8.2 Origin of the Solar System: The Nebular Hypothesis The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets. According to this hypothesis,our solar system formed at the same time as our Sun. However, the spinning nebula collected the vast majority of material in its center, which Small protoplanetary discs in the Orion is why the sun accounts for over 99% of the mass in our Nebula solar system. 8.2.1 Planet Arrangement and Segregation As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water. This temperature differentiation resulted in the inner four planets of This disk is asymmetric, possibly because of a large gas giant planet orbiting relatively far from the solar system becoming rocky, and the outer the star. four planets becoming gas giants.  Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming protoplanets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid Image by the ALMA telescope of HL Tauri and its core. Rocky planets built more rock on that core, protoplanetary disk, showing grooves formed as while gas planets added gas and ice. Ice giants planets absorb material in the disk. formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases. The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and This artist’s impression of the water snowline helium, which could not be collected by weaker around the young star V883 Orionis, as detected gravity of the smaller planets. with ALMA. Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together. These collisions created the asteroid belt, an unfinished planet,  located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to outer edge of the solar system. A polished fragment of the iron-rich Toluca Meteorite, with octahedral Widmanstätten Pattern. PLUTO AND PLANET DEFINITION The outermost part of the solar system is known as the Kuiper belt, which is a scattered collection of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. The Kuiper Belt and Oort cloud are where most comets form and continue to orbit, and objects found there have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region Eight largest objects discovered past Neptune. of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria: 1) enough mass to have gravitational forces that force it to be rounded, 2) not massive enough to create fusion, and 3) large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets. Did you get it? Click here to find out.