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Transcript
The Earth 1 Data Location – The Earth is the third planet from the Sun, with an average distance of approximately 93 million miles. Size – The Earth is the fifth largest planet within the solar system, measuring 7909 miles in diameter. Orbital Period – 1 year (365.25636 days). Rotational Period – 1 day (23 hours 56 minutes 4 seconds). Number of Satellites – 1. Atmosphere Composition – The Earth’s atmosphere is composed of about 78% nitrogen (N2) and 21% oxygen (O2). Most of the remaining 1% is composed of argon. However, there are two other gases comprising this one percent that have significant importance: 1. Water Vapor (H2O) – Though the average water vapor content of the Earth’s atmosphere is less than 1%. The amount varies from 0.1% in cold, dry regions to 3% in warm, wet regions. Water is an essential ingredient for life on Earth. In cold, dry regions life is usually sparse, whereas in warm, wet regions life is usually abundant. 2. Carbon Dioxide (CO2) – The amount of carbon dioxide in the Earth’s atmosphere is about 0.04%, but it is increasing. This increase is due to the burning of fossil fuels (oil, natural gas, coal). This causes a warming of the lower atmosphere, referred to as a greenhouse effect, and produce global climatic changes. If conditions are left unchanged, the amount of carbon dioxide in the Earth’s atmosphere is predicted to double in the second half of this century. Pressure – The force of the Earth’s gravity and the amount of gas cause a sea-level pressure of approximately 14 lbs/ in2 or 1 bar. This pressure decreases rapidly with increasing elevation; that is, at a higher elevation, the amount of gas is less, causing a lower atmospheric pressure. Temperature – The average temperature near the surface of the Earth is about 60º F. This regulation of temperature is a result of the reflection of incoming solar radiation by the atmosphere coupled with the blanketing effect, caused by greenhouse gases (carbon dioxide and water vapor). 1 Layers – The atmosphere of the Earth is estimated to be about 600 miles thick. However, because there is no set boundary where the atmosphere ends, this estimation is somewhat arbitrary. Nevertheless, the Earth’s atmosphere can be divided into several distinct layers identified using temperature changes, chemical composition, movement, and density. Troposphere – The lowest level of the Earth’s atmosphere is referred to as the troposphere. It contains about 90% of the atmosphere’s mass. Moreover, this layer is characterized by convection currents. These convection currents are generated by the Sun’s radiation heating the ground and lower atmosphere. The warmer air near the surface expands and rises, and is replaced by downdrafts of cooler air from above. Convection currents in the troposphere generate nearly all weather phenomena. Stratosphere – The layer of the atmosphere directly above the troposphere is the stratosphere. Unlike the troposphere, the stratosphere has very little vertical mixing. The temperature is nearly uniform at approximately -75º F, and therefore very dry (water is almost entirely frozen out). Ozone Layer – In the upper part of the stratosphere, the Sun’s ultraviolet light creates a layer of ozone through a process called photochemistry. Ozone is a form of oxygen containing three atoms per molecule (O3). Importance and Concern 2 – The ozone layer blocks ultraviolet light from reaching the Earth’s surface, and therefore, protecting life from irreparable harm. Since the early 1980’s, the ozone layer has shown signs of severe damage by industrial chemicals called chlorofluorocarbons (CFC’s). A large hole in the ozone layer has formed over the Antarctic and has been depleted in the temperate latitudes. Mesosphere – The atmosphere reaches its coldest temperature of around -170° F in the mesosphere. This is also the layer in which a lot of meteors burn up while entering the Earth's atmosphere. Thermosphere – The thermosphere is a region in which temperatures again increase with height. This temperature increase is caused by the absorption of energetic ultraviolet and x-ray radiation from the Sun. Ionosphere – Within the lower portion of the thermosphere is a region referred to as the ionosphere. Here many atoms are ionized, that is broken apart into positively charged ions and negatively charged electrons. The solar wind brings charged particles to the upper levels of Earth’s atmosphere. They move towards the north and south poles, and while the particles collide with the ionized gas, colorful formations of light are produced – the Aurora Borealis (the Northern Lights) 3 and the Aurora Australis (the Southern Lights). 2 Circulation 4 – The Earth is heated more near the equator than at the poles. Because of this, the atmosphere (and the oceans) redistributes much of the solar energy from warmer regions to cooler ones, which causes the formation of a global atmospheric circulation. This circulation is usually represented in a simplified form, but in actuality, the system is quite complicated. There are many factors that contribute to the complexity of the system, including: 1. Landmasses (continents) obstructing circulation patterns. 2. The tilt of the Earth’s axis causing seasonal variations. 3. Coriolis Effect 5 – The Earth’s rotation generates the deflection of large air masses. This phenomenon is known as the Coriolis Effect and is responsible for rotating weather systems such as hurricanes 6. Weather and Climate – Weather is the state of the atmospheric system at a given place and time and its short-term variations, whereas climate is the average atmospheric conditions within a geographic region over a long period of time. Climate Changes – Climate on Earth does change. The most dramatic climatic changes are those associated with the ice ages over the past million years. At intervals of about 100,000 years, the average temperature of the Earth has dropped by 2 or 3 degrees. This decrease in temperature is sufficient enough to produce vast ice sheets (up to 2 miles thick) over much of the northern hemisphere landmasses. There are number of causes associated with climatic change: 1. Changes in the Earth’s orbit and tilt of its rotation axis (due to gravitational forces of other planets) are thought to be the primary cause of the great ice ages. 2. Variations in the amount of dust in the Earth’s atmosphere. The primary source of atmospheric dust is large volcanic explosions 7. Though rare, impacts of large asteroids or comets can also significantly increase the amount of atmospheric dust 8. Evolution – The Earth’s atmosphere did not always consist of the same relatively stable mixture of gases that we breathe today. The present atmosphere is the result of very gradual changes that began soon after the Earth formed. Primordial Atmosphere – The first atmosphere of the Earth was composed of hydrogen, helium, methane (CH4), ammonia (NH3), water vapor and low amounts of oxygen. This atmosphere was swept away by intense solar winds – vast streams of particles composed primarily of electrons and nuclei of hydrogen and helium emitted by the Sun. 3 Secondary Atmosphere – The present atmosphere of the Earth is the result of the release of gases dissolved in molten rock, a process called outgassing (similar to gases observed in present-day volcanic eruptions). The principal components of this “new” atmosphere were probably water vapor, carbon dioxide and nitrogen. However, it did not contain oxygen. The oxygen of the Earth’s atmosphere was slowly added, primarily by green plants through the process of photosynthesis. As the Earth continued to cool, clouds formed and great rains occurred. These torrential rains eventually filled the ocean basins. This event not only diminished the amount of water vapor in the atmosphere, but also carried away most of the carbon dioxide as well. Eventually, large amounts of the carbon dioxide within the oceans were chemically incorporated into carbonate rock. Surface Hydrosphere – The hydrosphere covers approximately 75% of the Earth’s surface. It includes freshwater lakes and streams, the polar ice caps, snow capped mountains, groundwater, etc. However, the main component is the oceans. Salt Content (Salinity) – Ocean water is not pure H2O. It contains “salts”, which make up on the average 3.5% (35‰) of its mass. Six elements constitute more than 90% of the salts dissolved in the oceans: chlorine, sodium, magnesium, sulfur, calcium, and potassium. The concentration of each of these elements in seawater is a balance between inflow (primarily from weathering and erosion of continental rock) and precipitation into ocean sediment. Gas Content – A number of gases are dissolved in the oceans. The most important of these are oxygen and carbon dioxide. Oxygen – The oxygen in the oceans allows for the existence of advance forms of life, such as fish, which use gills to extract the oxygen they require from the water. Carbon Dioxide – The amount of carbon dioxide in the oceans is about 60 times greater than within the atmosphere. As a result, the oceans regulate the amount of carbon dioxide in the atmosphere, and therefore influence the climate of the Earth. Temperatures – The average temperature of the oceans is about 39º F. The surface temperature varies from about 85º F near the equator to below freezing within the polar ice caps. However, the temperature a half-mile or more below the surface changes very little with location and season. Currents 9 – Like the atmosphere, areas of oceans are heated unequally and produce circulation patterns referred to as currents. These currents are affected in similar ways as the atmosphere: location of landmasses, seasonal variations and the Coriolis Effect. Moreover, currents heat and cool adjacent landmasses. For example, the Gulf Stream is responsible for England having a warmer climate than is normal for a landmass of its latitude. 4 Lithosphere 10 – The Earth’s rigid outer shell, which comprises the continental crust, oceanic crust and upper mantle (about 60 miles in thickness), is referred to as the lithosphere. Composition – Respectively, oxygen and silicon are the two most abundant elements comprising the lithosphere, and therefore most minerals and rocks contain these elements. As a whole, these minerals and rocks are called silicates. Continents and Ocean Basins – The Earth’s lithosphere is divided into two principal regions, continents and ocean basins. These divisions differ in elevation, geologic history, rock types (chemical composition), density and age. Continents – Geologically and topographically, the continents are exceedingly complex and variable in detail, yet certain large-scale structural and topographic features are common to all: 1. They have two basic components 11: (1) Cratons – areas that are stable and relatively inactive and (2) Orogenic Belts – areas that are unstable and active. Earthquakes, mountain belts and volcanoes are found within orogenic belts. 2. Major Rock Type – Granite 3. Average Density – 2.7 g/cm3. As a result, continental rock “floats” higher than the denser oceanic rock. 4. Age – Continental rocks are old, some as old as 3.8 billion years. They are found in the areas of cratons. Ocean Basins – Ocean basins cover 70% of the Earth’s surface, with the following common properties: 1. Major Rock Type – Basalt 2. Average Density – 3.0 g/cm3. Oceanic rock is denser than continental rock due to the higher abundance of ferromagnesium minerals. 3. Age – The rocks of the ocean floor are young in a geologic time frame; most are less than 150 million years old. Affect of Internal Processes – In the 1960s, the science of geology underwent a dramatic change that altered the basic assumption of the field. Until then, almost all geological science has been based on the idea that the major features of the Earth (the continents and ocean basins) remained in a fixed location. However, the Earth is an active planet. It exhibits dynamic interaction between its inner and outer parts, which puts the Earth’s surface into a state of lateral and vertical motion. Continental Drift – The term continental drift applied to early theories supporting the possibility that the continents are in motion over the Earth’s surface. In the early 1900s, German meteorologist Alfred Wegener was the first to collect substantial evidence in support of the theory of continental drift, though this data did little to convince his colleagues: 5 1. Jigsaw Puzzle Fit 12 – The shapes of continents fit together like pieces of a jigsaw puzzle, suggesting that they were once joined together, and have split and moved apart from one another. For example, the east coast of South America fits well with the west coast of Africa. Pangea 13, 14 – Alfred Wegener presented evidence for all the continents having been joined in a single super-continent referred to as Pangea about 225 million years ago. Another Pangea 15 – Computer models show that in approximately 250 million years in the future, the continents will be again in close proximity to each other. 2. Fossil Evidence – The remains of particular species of animals and plants. For example, fossil remains of Mesosaurus, a freshwater reptile, are found in South America and Africa. Because of its bone structure, it would not have been able to swim a large ocean, therefore suggesting that these two continents were once joined as one landmass. 3. Rock Matches 16 – Rock matches between continents. For example, layers of sandstone, shale and clay with coal seams located in Brazil, match with layers of identical composition and structure in South Africa. 4. Paleoclimatic Evidence – Evidence from ancient climates. Glacial deposits and other data indicate that in the Devonian period (360 to 408 million years ago), a polar ice cap covered the Sahara, while eastern North America lay near the equator. 5. Polar Wandering – The apparent movement of the Earth’s magnetic poles over time with respect to the continents. Wegener found evidence indicating that ancient poles were in different positions than the present poles. He believed that this was due to the changing positions of the continents. Mechanism – Alfred Wegener describe the drift of continents as rigid bodies moving through a yielding seafloor. He believed that the forces causing continental drift were a combination of centrifugal force from the Earth’s rotation and the gravitational forces that causes tides. Geophysicists correctly objected. The sea floor is rigid and not viscous and that the forces involved in Wegener’s mechanism were insufficient to cause continental drift. Sea-Floor Spreading – In 1962, Harry Hess proposed a new hypothesis called seafloor spreading. This hypothesis was able to explain a mechanism for continental drift by proposing that the sea floor is moving, too 17. One theory suggests that convection within the mantle of the Earth (behaving similar to a conveyor belt) may be responsible for continental drift. Convection is a circulation pattern driven by temperature differences. Hot material being less dense rises and cold material being denser sinks 18. There is considerable evidence to support the convection theory: 6 1. Geologic Features 18 – Hot molten rock from the mantle rises and cools to form new oceanic crust and what is referred to as an oceanic ridge. As molten rock continues to rise beneath the ridge, the convective circulation pattern splits and diverges near the surface. The newly formed crust moves horizontally away from the ridge on both sides (sea-floor spreading). As the newly formed oceanic crust continues to move horizontally away from the oceanic ridge, it gradually cools and contracts, becoming denser. Eventually, it becomes dense enough to sink back into the mantle (where it is melted) forming an oceanic trench (a deep, narrow trough). 2. Paleomagnetism – Paleomagnetism is studying the Earth’s magnetic field in the past. Small crystals of magnetic minerals in cooling molten rock act like tiny compass needles, preserving a record of the Earth’s magnetic field 19. Moreover, the Earth’s magnetic field has at times reversed its polarity. Such a change in the polarity of the Earth’s magnetic field is referred to as a magnetic reversal. (Magnetic reversals may be due to variations in circulation patterns in the liquid outer core where the Earth’s magnetic field originates.) For this reason, the magnetism of old rock can be measured to determine the strength and direction of the Earth’s magnetic field in the past. In the mid 1960’s, deviations from an average reading of the Earth’s present magnetic field were noted within rock of the sea floor. These deviations are called magnetic anomalies. They showed to be arranged in bands that lie parallel to an oceanic ridge 20. Moreover, the pattern on one side of the ridge is a mirror image of the pattern on the other side. This banded pattern of magnetic anomalies originates from the following process: (1) There is constant opening of tensional cracks within an oceanic ridge. (2) These cracks are filled by molten rock from the mantle. (3) As the molten rock cools, the magnetic minerals within the rock preserve the Earth’s magnetism at the time. When the Earth’s magnetic field has a normal polarity (the present orientation), the cooling rock is normally magnetized. Rock that cools when the Earth’s field is reversed is reversely magnetized 21. (4) Sea-floor spreading then cracks the rock in two, and the two halves are carried away in opposite directions down the sides of the ridge. In this way, a system of normally magnetized rock and reversely magnetized rock forms parallel to the oceanic ridge 22. 3. Oceanic Sediments – Dated by fossil plankton they contain, sediments can be no older than the surface on which they came to rest. Studies show that sediment near an oceanic ridge is relatively young and becomes progressively older moving away from the ridge. More specifically, there are no sediments older than 200 million years on the sea floor, indicating that this sediment is conveyed to oceanic trenches, and dragged back down into the mantle. 4. Benioff Seismic Zones 23 – A Benioff seismic zone is a distinct area of earthquake activity that begins at an oceanic trench and slopes landward and downward into the Earth at an angle of about 30 to 60, indicating where the lithosphere is plunging back into the mantle. 7 5. Negative Gravity Anomalies – Gravity measured over oceanic trenches is lower than in areas adjacent to the trenches. These lower gravity values are caused by the low-density rock making up the plunging lithosphere. 6. Direct Measurements – Lithospheric motion has been directly measured using satellites, radar and lasers. Plate Tectonics – The concept of plate tectonics was introduced in the late 1960s by combining two preexisting theories, continental drift and sea-floor spreading. Tectonics – Tectonics refers to forces within the Earth, which cause the Earth’s crust to move. This movement may cause the Earth’s crust to deform (change shape). Plate 24 – Plate refers to the Earth’s lithosphere being divided into segments. There are eight large plates plus a few dozen smaller plates. A plate may be: (1) entirely sea floor, like the Nazca Plate, (2) entirely continental rock, such as the small plate that roughly coincides with the country of Turkey, and (3) both sea floor and continental rock, like the North American plate, South American plate and African plate. Plate Boundary Interaction – Plates are in continuous motion and their interactions are responsible for most of the geologic activity. Though plate interiors are relatively geologically inactive, plate boundaries can be very active. There are three main types of plate boundary interactions 25: 1. Divergent Boundary – Where two plates are moving apart. This results in the formation of an oceanic ridge 26. Oceanic Divergence – Where two oceanic plates are moving apart. For example, the Nazca and Pacific Plates. This area of divergence is called the East Pacific Rise 27. Continental Divergence 28 – A divergent boundary can form in the middle of a continent in the following manner: (1) The crust thins and a rift valley forms due to tension 29. For example, the East African Rift Valleys 30. (2) The continent tears in two 31. Volcanic activity forms oceanic crust. Seawater may flood into the linear basin. For example, the Red Sea 32. (3) The new ocean widens and an oceanic ridge develops 33. For example, the Mid-Atlantic Ocean Ridge 34. 2. Convergent Boundary – Where two plates move toward each other. Ocean-Ocean Convergence 35 – Where two oceanic plates come together, with one plate sliding (subducting) beneath the other. The subduction leads to the formation of an oceanic trench. For example, the Mariana trench 36. Magma, generated by the subduction, works its way upward to erupt as an island arc – a curved line of volcanoes that form a string of islands parallel to the oceanic trench. 8 Ocean-Continent Convergence 37 – Where an oceanic plate and a continental plate come together, followed by the subduction of the oceanic plate under the continental plate, thus forming an oceanic trench. For example, the Peru-Chile Trench 38. Magma, generated by the subduction can form: (1) island arcs at sea, such as the Aleutian Islands, (2) continental volcanoes, such as the cascade volcanoes in the Pacific Northwest, and/or (3) mountain belts, such as the Andes Mountains in South America. Continent-Continent Convergence 39 – Where two continental plates come together, causing the subduction of the ocean basin between them. After collision the denser oceanic lithosphere breaks off the continental lithosphere and continues to sink, leaving the continent behind. The two continents are welded together along a dipping suture zone that marks the old site of subduction. The convergence gives rise to the formation of a mountain belt in the interior of a continent. For example, the Himalaya Mountains in Central Asia. 3. Transform Boundary – Where one plate slides horizontally past another plate. The boundary is marked by a single fault or on a group of parallel faults. For example, The San Andreas fault in California 40, 41. Earthquakes (Seismicity) 42 – Earthquake activity (the result from the release of stresses associated with rock breakage) has provided important evidence in support of plate tectonics. Earthquakes are concentrated along plate boundaries where rock is being created (oceanic ridges) or destroyed (oceanic trenches), compressed (orogenic belts) or extended (rifts), and masses are sliding past each other (fault lines). Volcanism 43 – Most volcanic activity occurs along plate boundaries. There are thousands of known volcanoes scattered over the Earth’s surface. However, only about 800 are either active or known to be historically active. Volcanism occurs in three tectonic settings: 1. Convergent Boundaries – Volcanism of this setting are located in the Circum-Pacific region. This region is commonly known as the ring of fire. 2. Oceanic Ridges – Approximately 80% of volcanic rocks are produced at the oceanic ridges. Iceland is the most active volcanic region on Earth. It is in a continuous state of tension as the eastern and western portions are moved apart by sea-floor spreading. 9 3. Intraplate Volcanism – Volcanic activity also occurs at locations associated with the interior regions of lithospheric plates. Intraplate volcanoes are immense features that may rise to about 6 miles above the ocean floor, dimensions greater than the largest mountains on the continents. Magmas that built these volcanoes are derived from a relatively fixed magma source in the upper mantle called a hot spot. Because the magma source lies beneath the plate and because the plate is moving laterally, seamounts and/or volcanic islands are created in a linear pattern known as a chain of seamounts and/or islands 44. For example, the Hawaiian-Emperor chain in the Pacific 45. Affect of External Processes – External processes are also responsible for shaping the surface of the Earth (rounded hills and valleys). Weathering – The natural breakdown of rocks and their minerals. The are two main types of weathering: 1. Mechanical Weathering – The physical breakdown of rock into smaller pieces. For example, frost action is the mechanical effects of freezing water on rocks. 2. Chemical Weathering – The breakdown of rock resulting from exposure to water and atmospheric gases. For example, limestone goes into solution when exposed to carbonic acid (formed by carbon dioxide dissolving in water), which can result in the formation of caves. Erosion – The picking up or physical removal of rock particles over the surface of the Earth. Erosional agents include: running water (the primary erosional agent), glacial ice, waves, wind and/or gravity. Deposition – The settling or coming to rest of transported material. Sediment is deposited when running water, glacial ice, waves or wind loses energy and can no longer transport its load. Impacts – Like all other solid planetary bodies, the Earth was once pockmarked with thousands of craters. The period of intense bombardment of the planets by meteorites was an early event, and the number of impacts has decreased exponentially during the last 4 billion years. Because of the Earth’s atmospheric and geologic processes, most craters have been erased. However, over 100 craters have been identified, which show how impact processes modify the Earth’s surface even now. Barringer Crater 46 – Located near Winslow, Arizona, Barringer crater is nearly 1 mile across and 600 feet deep. It was formed by the impact of 100,000 tons of iron about 50,000 years ago, making it the youngest crater of its size. Manicougan Crater 47 – Manicougan crater in northern Quebec is over 40 miles in diameter and was caused by an impact about 200 million years ago. 10 Interior Direct Study – The Earth’s interior is very inaccessible to direct study. The deepest that people have traveled beneath the Earth’s surface is approximately 2.5 miles (TauTona gold mine in South Africa). The deepest well ever drilled (the Kola scientific well in Russia) has penetrated a depth of over 7.5 miles. Remote Study 48 – Vibrational waves associated with earthquakes are our main source of information about the Earth’s interior. The shaken Earth responds like a giant bell, and its interior structure determines the “tones” that will be detected at different places on the surface. By measuring the vibrations, scientists can reconstruct different aspects of the Earth’s interior (pressure, temperature, phase differences, various layers). Density – The average density of the Earth is 5.52 g/cm3. This density is derived by averaging the densities of the layers that comprise the Earth’s interior, from 3.0 g/cm3 at the surface to 12.0 g/cm3 at the core. The density of a material suggests it composition. A low density indicates a composition of lighter elements, while a higher density indicates a composition of heavier elements. Differentiation 49 – Because the Earth’s interior is segregated according to changes in density it is referred to as being differentiated. Differentiation is a process by which the denser portions of a planetary object will sink to the center, while less dense materials rise to the surface. Pressure – The internal pressure of the Earth increases with increasing depth. This is caused by the increasing amount, and therefore weight, of overlying rock. At the center of the Earth, the pressure reaches nearly 4 million bars. Temperature – The internal temperature of the Earth increases with increasing depth, reaching a maximum value of greater than 9,000 F at its center. The Earth’s internal heat is quite evident in deep mines and molten rock (lava) from volcanoes and fissures. This heat is generated by three main sources: (1) gravitational energy converted to heat during formation and accretion of the Earth, (2) frictional energy from the Earth’s tides, and (3) the decay of radioactive elements of potassium, uranium and thorium (the primary heat source of the Earth’s interior). Layers 50 – The Earth’s interior is comprised of four primary layers: 1. Crust – The crust ranges in thickness from an average of approximately 4 miles beneath the oceans to more than 24 miles beneath the continents. 2. Mantle – The mantle is about 1800 miles in thickness, making it the largest layer of the Earth’s interior. The rock comprising the mantle is similar to the crust, but of higher density. Most of the tectonic activity seen on the Earth’s surface is caused by convection currents in the upper mantle (asthenosphere). 11 3. Outer Core – The outer core is about 1200 miles in thickness. The boundary between the mantle and core marks a sharp change in density (5.0 g/cm3 to 10.0 g/cm3). The sudden increase in density is related to a change in composition. The outer core is believed to be composed of iron-nickel. Magnetic Field – The outer core is thought to be the source of the Earth’s magnetic field. Scientists believe for a planetary object to generate a magnetic field it must possess an internal layer with three properties: (1) metal or a material with metallic properties, (2) in a liquid phase, and (3) convective flows caused by the planetary object’s rotation (referred to as a dynamo). The Earth’s magnetic field forms a shield around the Earth referred to as the magnetosphere 51. 4. Inner Core – The inner core is about 900 miles in thickness, and is associated with the center of the Earth. Like the outer core, it is composed of iron-nickel, but due to the higher pressure it is in a solid phase. Life Biosphere – The biosphere is that part of the Earth that consists of self-replicating molecules, a property referred to as life. The biosphere continues to evolve. Propagation – It is still not clear how life was created. However, there are some definite criteria for the origin and continued existence of life on Earth. They include a relatively stable surface temperature, an atmosphere of a suitable composition, and the availability of liquid water. Beginnings – The oldest sedimentary rocks known on Earth contain chemical hints that life made its appearance 3.8 billion years ago. Rocks 3.5 billion years old exhibit visible structures thought to have been formed by primitive forms of life called stromatolites 52. Niches – The biosphere is not distributed evenly over the Earth, but is concentrated in various environmental niches. Most life exists in a range extending from the depth to which sunlight penetrates the oceans (about 600 feet) to the snow line of mountain ranges about 3.5 miles above sea level. Diversity – The biosphere consists of about 8.7 ± 1.3 million species. 12