Objectives Vocabulary

Objectives Vocabulary
The Early Earth Objectives Describe the evidence used to determine the age of Earth. Understand why scientists theorize that the early Earth was hot. Vocabulary zircon asteroid meteorite

The Early Earth Earth’s “Birth” For about the first 4 billion years of Earth’s 4.6-billion-year existence, most of the life-forms that inhabited Earth were unicellular organisms. In 1996, the announcement that a meteorite from Mars might contain microscopic fossils of bacteria rekindled scientific interest in the search for life elsewhere in the universe. It may be possible to identify clues to the possible existence of life on other planets through rocks from those planets.

The Early Earth Earth’s “Birth” There is evidence of life’s beginnings on Earth in Precambrian rocks. Most of Earth’s history is contained within the 4 billion years that make up the Precambrian.

The Early Earth How old is Earth? We know that Earth must be at least as old as the oldest rocks in the crust. The age of the oldest rocks on Earth is between to 3.8 billion years. Evidence of 4.1- to 4.2-billion-year-old crust exists in the mineral zircon that is contained in metamorphosed sedimentary rocks in Australia. Zircon is a very stable mineral that commonly occurs in small amounts in granite.

The Early Earth How old is Earth? Meteorites have been radiometrically dated at between 4.5 and 4.7 billion years old. The oldest rock samples from the Moon are approximately 4.6 billion years old. Scientists commonly agree that the age of Earth is 4.6 billion years.

The Early Earth Earth’s Heat Sources Earth was most likely extremely hot shortly after it formed, and there were three likely sources of this heat. The first source was radioactivity. Radioactive isotopes were more abundant during the past. One product of radioactive decay is energy, which generates heat.

The Early Earth Earth’s Heat Sources The second source of Earth’s heat was the impact of asteroids and meteorites. Asteroids are metallic or silica-rich objects that are 1 km to 950 km in diameter. Meteoroids are small asteroids or fragments of asteroids. Meteorites are meteoroids that fall to Earth. Evidence suggests that collisions, which generate a tremendous amount of thermal energy, were much more common throughout the early solar system than they are today.

The Early Earth Earth’s Heat Sources The third source of Earth’s heat was gravitational contraction. As a result of meteor bombardment and subsequent accumulation of meteorite material on Earth, the size of Earth increased. The weight of the material caused gravitational contraction of the underlying zones, the energy of which was converted to thermal energy. The new material also caused a blanketing effect, which prevented the newly generated heat from escaping.

Section Assessment 1. How are meteorites evidence of Earth’s age?
The Early Earth Section Assessment 1. How are meteorites evidence of Earth’s age? Most astronomers agree that the solar system formed at the same time as Earth, and therefore, Earth and meteorites should be about the same age.

The Early Earth Section Assessment 2. What are were the three likely sources of heat on Earth shortly after it formed? The three sources of heat were likely radioactive decay, the impact of asteroids and meteorites, and gravitational contraction.

The Early Earth Section Assessment 3. Identify whether the following statements are true or false. ______ The Precambrian represents about half of Earth’s existence. ______ The oldest rocks on Earth are between 3.96 and 3.8 billion years. ______ Meteorites are asteroids that fall to Earth. ______ A meteorite could possibly provide evidence of life on another planet. false true

End of Section 1

Objectives Vocabulary Explain the origin of Earth’s crust.
Formation of the Crust and Continents Objectives Explain the origin of Earth’s crust. Describe the formation of the Archean and Proterozoic continents. Vocabulary differentiation Precambrian shield Canadian Shield microcontinent Laurentia

Formation of the Crust and Continents
Early in the formation of Earth, the planet was molten, and numerous elements and minerals were mixed throughout the magma. Over time, the minerals became concentrated in specific zones and Earth became layered. As the magma reached the surface and cooled, landmasses began to form.

When Earth formed, iron and nickel, which are dense elements, concentrated in its core. Lava flowing from the interior of Earth concentrated the less-dense minerals near the surface of Earth over time. The denser minerals, which crystallize at higher temperatures, concentrated deeper within Earth and formed the rocks that make up Earth’s mantle.

Differentiation is the process by which a planet becomes internally zoned when heavy materials sink toward its center and lighter materials accumulate near its surface.

Earth’s earliest crust most likely formed as a result of the cooling of the uppermost mantle and was similar to basalt. As sediment-covered slabs of the crust were recycled into the mantle at subduction zones, the slabs partly melted and generated magmas with different mineral compositions. These magmas crystallized to form the first granitic continental crust, which was rich in feldspar, quartz, and mica.

The formation of the majority of crustal rocks was completed by about 2.5 billion years ago. As less-dense material has a tendency to float on more-dense material, continental crust “floats” on top of the mantle below it. Basaltic crust is more dense than granitic crust, and therefore, it does not float as high on the mantle.

The Cores of the Continents
Formation of the Crust and Continents The Cores of the Continents A Precambrian shield is a core of Archean and Proterozoic rock that forms the core of each continent. Buried and exposed parts of a shield together compose the craton, which is the stable part of a continent. The Canadian Shield is the name for the Precambrian shield in North America because much of it is exposed in Canada.

The Cores of the Continents
Formation of the Crust and Continents The Cores of the Continents

Growth of Continents Microcontinents, which were small pieces of continental crust that formed during the Archean, began to collide as a result of plate tectonics early during the Proterozoic. At each of these collision sites, the Archean microcontinents were sutured or fused together at orogens. These orogens are belts of rocks that were deformed by the immense energy of the colliding continents.

Growth of Continents Laurentia, the ancient continent which was assembled 1.8 billion years ago, would become the core of modern-day North America.

Growth of Continents Near the end of the Early Proterozoic, between 1.8 and 1.6 billion years ago, volcanic island arcs collided with the southern margin of Laurentia. The final phase of Proterozoic growth of Laurentia, the Grenville Orogeny, occurred between 1.2 billion and 900 million years ago. By the end of the Proterozoic, nearly 75 percent of present-day North America had formed.

Growth of Continents By the end of the Proterozoic, all of the major masses of continental lithosphere had formed. As the lithospheric plates moved around, they periodically collided and sutured together to form Rodinia, the first supercontinent. Rodinia began to break apart at the end of the Proterozoic and continued to do so during the Early Phanerozoic.

Growth of Continents

Section Assessment 1. Match the following terms with their definitions. ___ differentiation ___ Precambrian shield ___ microcontinent ___ Laurentia A B D C A. the process by which a planet becomes internally zoned B. a core of Archean and Proterozoic rock that forms the core of continents. C. the ancient continent that formed the core of modern-day North America D. small pieces of continental crust that formed during the Archean

Section Assessment 2. What is the Canadian Shield?
Formation of the Crust and Continents Section Assessment 2. What is the Canadian Shield? The Canadian Shield is the Precambrian shield in North America. It is called the Canadian Shield because much of it is exposed in Canada.

Section Assessment 3. Why do the rocks of the earliest crust no longer exist? The rocks of the earliest crust no longer exist because they were recycled in subduction zones long ago.

End of Section 2

Formation of the Atmosphere and Oceans Objectives Describe the formation of Earth’s atmosphere and oceans. Identify the origin of oxygen in the atmosphere. Explain the evidence that oxygen existed in the atmosphere during the Proterozoic. Vocabulary cyanobacteria stromatolite banded iron formation red bed

Formation of the Atmosphere and Oceans
Earth’s early atmosphere was nothing like what it is today. The oxygen that early forms of algae produced through the process of photosynthesis affected the development of life on Earth in two very important ways. It changed the composition of the atmosphere and thus made life possible for oxygen-breathing animals. It produced the ozone layer that filters ultraviolet (UV) radiation.

The Precambrian Atmosphere
Formation of the Atmosphere and Oceans The Precambrian Atmosphere Hydrogen and helium probably dominated Earth’s earliest atmosphere but probably escaped into space due to their small masses. Gases that have greater masses, such as carbon dioxide and nitrogen, cannot escape Earth’s gravity. Considerable volcanic activity during the Early Precambrian released tremendous amounts of gases into the atmosphere through the process of outgassing.

The Precambrian Atmosphere
Formation of the Atmosphere and Oceans The Precambrian Atmosphere The most abundant gases vented from volcanoes are water vapor (H2O), carbon dioxide (CO2), nitrogen (N2), and carbon monoxide (CO). Many geologists hypothesize that outgassing formed Earth’s early atmosphere. In addition, the early atmosphere most likely contained methane (CH4) and ammonia (NH3). Argon (Ar) also began to accumulate during the Early Precambrian.

Oxygen in the Atmosphere
Formation of the Atmosphere and Oceans Oxygen in the Atmosphere There was no oxygen in the atmosphere during the Precambrian. The oldest known fossils, which are about 3.5 billion years old, are the remains of tiny, threadlike chlorophyll-bearing filaments of cyanobacteria. Ancient cyanobacteria used photosynthesis to produce the nutrients they needed to survive, giving off oxygen as a waste product.

Formation of the Atmosphere and Oceans Oxygen in the Atmosphere Oxygen Producers The abundance of cyanobacteria increased throughout the Archean until they became truly abundant during the Proterozoic. Stromatolites, which are large mats and mounds of billions of cyanobacteria, dominated the shallow oceans of the Proterozoic.

Formation of the Atmosphere and Oceans Oxygen in the Atmosphere Evidence in the Rocks Iron oxides are identified by their red color and provide undeniable evidence of free oxygen in the atmosphere. Evidence indicates that there was little or no free oxygen in the atmosphere throughout most of the Archean. Near the end of the Archean and by the beginning of the Proterozoic, photosynthesizing stromatolites in shallow marine water increased oxygen levels in localized areas, which caused banded iron formations to form.

Formation of the Atmosphere and Oceans Oxygen in the Atmosphere Evidence in the Rocks Banded iron formations are deposits which consist of alternating bands of chert and iron oxides. Red beds are sedimentary rocks that are younger than 1.8 billion years and rusty red in color. The presence of red beds in rocks that are Proterozoic and younger is strong evidence that the atmosphere by this time contained free oxygen.

Importance of Oxygen Oxygen is important because most animals require it for respiration and it provides protection against UV radiation from the Sun. Earth is naturally protected from this radiation by ozone (O3) molecules that are present in the lower part of Earth’s upper atmosphere. Oxygen in Earth’s atmosphere that was produced mainly through photosynthesis also contributes to the ozone layer. Nearly all the oxygen that is present was released into the atmosphere by photosynthesis.

Formation of the Oceans
Formation of the Atmosphere and Oceans Formation of the Oceans Oceans are thought to have originated largely from the same process of outgassing that formed the atmosphere. As the early atmosphere and the surface of Earth cooled, the water vapor condensed to form liquid water. During the Archean, tremendous amounts of rain slowly filled the low-lying, basalt-floored basins, thus forming the oceans.

Formation of the Oceans
Formation of the Atmosphere and Oceans Formation of the Oceans Dissolved minerals made the oceans of the Precambrian salty just as they make the oceans salty today. A recent hypothesis suggests that some of Earth’s water may have come from the bombardment of microcomets, or small comets made of frozen gas and water.

Formation of the Atmosphere and Oceans Oxygen in the Atmosphere Oxygen Causes Change The Precambrian began with an oxygen-free atmosphere and simple life-forms. This oxygen added by cyanobacteria not only enabled new life-forms to evolve, but it also protected Earth’s surface from the Sun’s UV rays. Oceans formed from abundant water vapor in the atmosphere and possibly from outer space. Earth was then a hospitable place for new life-forms to inhabit.

Section Assessment 1. Match the following terms with their definitions. ___ cyanobacteria ___ stromatolite ___ banded iron formation ___ red bed D B C A A. sedimentary rocks that are younger than 1.8 billion years and are rusty red in color B. large mats and mounds of cynobacteria C. deposits that consist of alternating bands of chert and iron oxides D. chlorophyll containing bacteria that may be responsible for the addition of oxygen to Earth’s early atmosphere

Section Assessment 2. Why is Earth’s current atmosphere rich in carbon dioxide and nitrogen? Gases that have greater masses, such as carbon dioxide and nitrogen, cannot escape Earth’s gravity like lighter gases such as hydrogen and helium.

Section Assessment 3. Identify whether the following statements are true or false. ______ Stromatolites currently exist on Earth. ______ Free oxygen is released during outgassing. ______ Around half of the oxygen that we breathe today was released into the atmosphere through photosynthesis. ______ There was little free oxygen in the atmosphere during the Archean. true false

End of Section 3

Early Life on Earth Objectives Describe the experimental evidence of how life developed on Earth. Distinguish between prokaryotes and eukaryotes. Identify when the first multicellular animals appeared in geologic time. Vocabulary amino acids hydrothermal vent prokaryote eukaryote Varangian Glaciation Ediacara fauna

Early Life on Earth Origin of Life on Earth Fossil evidence indicates that life existed on Earth about 3.5 billion years ago. Earth probably could not have supported life until about 3.9 billion years ago because meteorites were constantly striking its surface. This places the origin of life somewhere between 3.9 and 3.5 billion years ago.

Origin of Life on Earth Experimental Evidence
Early Life on Earth Origin of Life on Earth Experimental Evidence Molecular biologists in the 1920s also suggested that an atmosphere containing abundant ammonia and methane but lacking free oxygen would be an ideal setting for the “primordial soup” in which life may have begun. Stanley Miller and Harold Urey set up an apparatus that contained a chamber filled with hydrogen, methane, and ammonia to simulate the early atmosphere. Sparks from tungsten electrodes simulated lightning in the atmosphere.

Early Life on Earth Origin of Life on Earth Experimental Evidence Their atmospheric chamber was connected to a lower chamber that was designed to catch any particles that condensed in the atmospheric chamber. Only one week after the start of the experiment, the lower chamber contained organic molecules such as cyanide (CN), formaldehyde (H2CO), and four different amino acids. Amino acids are the building blocks of proteins, the basic substances from which life is built.

Early Life on Earth Origin of Life on Earth Experimental Evidence Continued experiments showed that 13 of the 20 amino acids known to occur in living things could be formed using the Miller-Urey method. Further experiments demonstrated that heat, cyanide, and certain clay minerals could cause amino acids to join together in chains like proteins. Miller and Urey demonstrated that however life first formed, the basic building blocks of life were most likely present on Earth during the Archean.

Origin of Life on Earth The Role of RNA
Early Life on Earth Origin of Life on Earth The Role of RNA The nucleic acids RNA and DNA are the basic requirements for reproduction, an essential characteristic of life. In modern organisms, DNA carries the instructions necessary for cells in all living things to function. RNA ribozymes, unlike DNA, can replicate without the aid of enzymes, and may have been the first replicating molecules on Earth. An RNA-based world may have been intermediate between an inorganic world and the DNA-based organic world that followed.

Origin of Life on Earth Hydrothermal Vents and the Beginnings of Life
Early Life on Earth Origin of Life on Earth Hydrothermal Vents and the Beginnings of Life Life on Earth may have originated deep in the ocean, near active volcanic seafloor rifts. Hydrothermal vents are the openings where hot water rises and is expelled from the ocean floor. All of the energy and nutrients necessary for the origin of life are present at these deep-sea hydrothermal vents. Some scientists hypothesize that during the Archean, near hydrothermal vents, amino acids joined together on the surfaces of clay minerals to form proteins.

Early Life on Earth Proterozoic Life The only evidence of life-forms that existed before the Proterozoic is the fossilized remains of unicellular organisms called prokaryotes. A prokaryote is an organism that is composed of a single cell, which does not contain a nucleus and is the simplest kind of cell. A eukaryote is an organism that is composed of a cell or cells that contain a nucleus.

Early Life on Earth Proterozoic Life The Varangian Glaciation was a widespread glaciation event that occurred between 800 and 700 million years ago that played a critical role in the extinction of many members of a group of possible eukaryotes, the acritarchs. Shortly after the ice retreated toward the poles, 700 million years ago, multicellular organisms first appeared in the fossil record.

Early Life on Earth Ediacara Fossils Fossils collectively referred to as the Ediacara fauna are the impressions of soft-bodied organisms that were discovered in Late Proterozoic rocks in the Ediacara Hills of southern Australia.

Early Life on Earth Ediacara Fossils It is generally agreed that these fossils represent animals that were composed of different types of eukaryotic cells. Scientists are unsure, however, whether the Ediacara fauna are relatives of modern animal groups or whether they were completely different types of organisms. The Ediacara fauna seem to provide fossil evidence of an ancestral stock of complex Proterozoic animals.

Early Life on Earth Ediacara Fossils Some scientists consider the similarity in shape to animals in other phyla coincidental and that the Ediacara fauna represents a virtual dead end. Ediacara fossils have been found in all parts of the world. These organisms seem to have flourished between 670 and 570 million years ago until an apparent mass extinction.

Early Life on Earth Section Assessment 1. Match the following terms with their definitions. ___ prokaryotes ___ eukaryotes ___ amino acids ___ Ediacara fauna B A D C A. organisms that are composed of cells that contain a nucleus B. an organism that is composed of a single cell, which does not contain a nucleus C. fossils of soft-bodied organisms that were discovered in Late Proterozoic rocks D. the building blocks of proteins

Section Assessment 2. When did life most likely develop on Earth?
Early Life on Earth Section Assessment 2. When did life most likely develop on Earth? The origin of life is somewhere between 3.9 and 3.5 billion years ago.

Early Life on Earth Section Assessment 3. Identify whether the following statements are true or false. ______ RNA can be easily synthesized under conditions that likely existed at the surface of the Archean Earth. ______ Ediacara fauna may not represent an ancestral stock of any modern group. ______ Life is currently being synthesized at hydrothermal vents. ______ Cyanobacteria are examples of prokaryotes. false true

End of Section 4

Chapter Resources Menu
Study Guide Section 22.1 Section 22.2 Section 22.3 Section 22.4 Chapter Assessment Image Bank Chapter Resources Menu

Section 22.1 Study Guide Section 22.1 Main Ideas Geologists have used radiometric dating to show that Earth must be at least 4.2 billion years old. Because the solar system formed all at the same time, Moon rocks and meteorites that are approximately 4.6 billion years old suggest that Earth is also 4.6 billion years old. The early Earth was a very hot place because of abundant radioactive isotopes, bombardment by meteorites, and gravitational contraction.

Section 22.2 Study Guide Section 22.2 Main Ideas Earth’s early crust formed by the cooling of the uppermost mantle. This early crust weathered and formed sediments. Sediment-covered slabs of this early crust were subducted and generated magmas that contained granitic minerals. During the Archean, microcontinents collided with one another throughout the Proterozoic and formed the cores of the continents. By the end of the Proterozoic, the first supercontinent, Rodinia, had formed.

Section 22.3 Study Guide Section 22.3 Main Ideas Earth’s early atmosphere and the oceans formed mainly by the process of outgassing. Nearly all of the oxygen in the atmosphere is a result of photosynthesis. Certain minerals oxidize, or rust, in the presence of free oxygen. Proterozoic red beds are sedimentary rock deposits that contain oxidized iron. They are the evidence that there was free oxygen in the atmosphere during the Proterozoic.

Section 22.4 Study Guide Section 22.4 Main Ideas All the ingredients were present on the early Earth to form proteins, the building blocks of life. Amino acids, the molecules that make up proteins, were likely abundant on the surface of the early Earth. Prokaryotic cells are generallly small and contain no nuclei. Eukaryotic cells contain nuclei and are generally larger and more complex than prokaryotic cells. The first evidence of multicellular animals is fossils of 2.1 billion year old eukaryotic algae.

Multiple Choice 1. Approximately how old are Earth’s oldest rocks?
Chapter Assessment Multiple Choice 1. Approximately how old are Earth’s oldest rocks? a. 2.4 billion years c. 3.9 billion years b. 3.2 billion years d. 4.6 billion years Radiometric dating has determined that the age of the oldest rocks on Earth is between 3.96 and 3.8 billion years.

Multiple Choice 2. What was the first supercontinent called?
Chapter Assessment Multiple Choice 2. What was the first supercontinent called? a. Rodinia c. Pangaea b. Laurentia d. Gondwanaland Rodinia formed approximately 750 million years ago. Laurentia was the ancient continent that today forms the core of North America. Pangaea was the most recent supercontinent that began to break apart 200 million years ago. Gondwanaland was a supercontinent that formed near the south pole during the Late Paleozoic.

Chapter Assessment Multiple Choice 3. Which organism may have been responsible for introducing mass quantities of oxygen into Earth’s atmosphere? a. stromatolites c. Ediacara fauna b. cyanobacteria d. eukaryotes Cyanobacteria were chlorophyll-containing prokaryotes that relied on photosynthesis and gave off oxygen as a by-product. Large mats and mounds of billions of cyanobacteria called stromatolites dominated the shallow waters of the Proterozoic.

Chapter Assessment Multiple Choice 4. Which of the following is likely the same age as Earth? a. zircon c. meteors b. red beds d. banded iron formations Astronomers generally agree that the bodies in the solar system, including Earth, formed at about the same time. Meteorites have been radiometrically dated at between 4.5 and 4.7 billion years old.

Chapter Assessment Multiple Choice 5. Which of the following offers the strongest evidence of abundant free oxygen in the atmosphere? a. red beds c. oceans b. active volcanoes d. none of the above Red beds would offer the strongest evidence of atmospheric free oxygen because free oxygen is needed to react with the iron causing the reddish color in the rock. Volcanoes and oceans do not add free oxygen to the atmosphere by themselves.

Short Answer 6. What is the process of differentiation?
Chapter Assessment Short Answer 6. What is the process of differentiation? Differentiation is the process by which a planet becomes internally zoned when heavy materials sink toward its center and lighter materials accumulate near its surface.

Short Answer 7. When and where did the Ediacara organisms flourish?
Chapter Assessment Short Answer 7. When and where did the Ediacara organisms flourish? The Ediacara organisms were widely distributed throughout the shallow oceans of the Late Proterozoic. They seemed to have flourished between 670 and 570 million years ago.

Chapter Assessment True or False 8. Identify whether the following statements are true or false. ______ During the Varangian Glaciation, the glacial ice advanced almost to the equator. ______ Asteroids are usually smaller than 1 km across. ______ Processes which modify a system are known as feedback. ______ DNA can replicate without the help of enzymes. ______ By the end of the Proterozoic, all of the major masses of continental lithosphere had formed. true false

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