• Plate Tectonics

  • Origins of Plate Tectonic Theory
  • Did you know?

    Did you know that fossils of sea creatures are found on Earth's highest mountain peaks? Scientists used to think that the Earth contracted as it cooled after it formed, forcing mountains up like wrinkles. Now we understand that plate tectonics explains why these mountains are there, why there are sea organisms on top of those mountains, and why the continents of the world look like a super-sized jigsaw puzzle.

    Summary

    The theory of continental drift was the first step toward plate tectonic theory, which became the foundation upon which modern geology is built. This module describes how the work of Alfred Wegener, Harry Hess, and others led to our understanding of plate tectonics. It explains plate tectonics as the driving force behind ongoing changes on Earth.

    Key Concepts
    • The idea that continents can move was proposed by Wegener in 1915 on the basis of fossil evidence, the way in which coastlines seemed to fit together, and other features, but it was not widely accepted at the time.
    • Evidence that led to the development of plate tectonic theory in the 1960s came primarily from new data from the sea floor, including topography and the magnetism of rocks.
    • Seafloor spreading was proposed as a mechanism to drive the movement of the continents on the basis of symmetrical patterns of reversed and normal magnetic rocks on the sea floor.

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  • Plates, Plate Boundaries, and Driving Forces
  • Did you know?

    Did you know that earthquakes and volcanic eruptions do not happen in random places? Both are concentrated along the boundaries of tectonic plates and provide evidence for the theory of plate tectonics. Earth is a dynamic planet, and nowhere is this more evident than along the plate boundaries.

    Summary

    Earthquakes and volcanoes can reveal a lot about plate boundaries. This module looks at the nature of tectonic plates and discusses the different boundary types that exist between them – convergent, divergent, and transform. Forces that drive the push and pull of these landmasses are explored.

    Key Concepts
    • Earthquakes and volcanoes occur primarily along plate boundaries; the frequency and type of events vary with the type of boundary.
    • Plates interact with one another at boundaries in one of three ways: they diverge, converge, or slide past one another.
    • Plates are made up of two types of crust – oceanic and continental; oceanic crust is thinner and denser than continental crust. A single plate can have both continental and oceanic crust.
    • Gravity and mantle convection are two driving forces for the movement of plates.

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  • Earth Structure
  • Did you know?

    Did you know that although earthquakes can be very destructive, they provide a wealth of information about Earth's interior? Miners, geologists, and others have always wondered what lies below the surface of Earth, but heat and pressure make it impossible to explore deep into its interior. However, seismic waves produced by earthquakes reveal the structure and composition of our planet.

    Summary

    Earth's interior structure is composed of layers that vary by composition and behavior. Using principles of physics like gravity and wave motion, this module explains how scientists have determined Earth's deep structure. Different types of seismic waves are discussed. The module details both compositional and mechanical layers of Earth.

    • NGSS
    • HS-C3.2, HS-ESS2.A2, HS-PS4.A4
    Key Concepts
    • Our knowledge about the structure Earth's interior comes from studying how different types of seismic waves, created by earthquakes, travel through Earth.
    • Earth is composed of multiple layers, which can be defined either by composition or by mechanical properties.
    • The crust, mantle, and core are defined by differences in composition.
    • The lithosphere, asthenosphere, mesosphere, and outer and inner cores are defined by differences in mechanical properties.

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  • Earth Cycles

  • The Rock Cycle
  • Did you know?

    Did you know that interactions between natural cycles produce the dynamic landscapes we see across the globe – and can even change global climate? One important cycle on Earth is the rock cycle, which has neither beginning nor end. Rather, through the rock cycle, Earth’s materials just change from one form to another.

    Summary

    Earth’s materials are in constant flux. Some processes that shape the Earth happen quickly; others take millions of years. This module describes the rock cycle, including the historical development of the concept. The relationship between uniformitarianism, the rock cycle, and plate tectonics is explored in general and through the specific example of the Cascade Range in the Pacific Northwest.

    • NGSS
    • HS-C5.2, HS-C7.1, HS-ESS2.A3
    Key Concepts
    • The rock cycle is the set of processes by which Earth materials change from one form to another over time.
    • The concept of uniformitarianism, which says that the same Earth processes at work today have occurred throughout geologic time, helped develop the idea of the rock cycle in the 1700s.
    • Processes in the rock cycle occur at many different rates.
    • The rock cycle is driven by interactions between plate tectonics and the hydrologic cycle.

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  • The Hydrologic Cycle
  • Did you know?

    Did you know that rising sea levels do not mean that the amount of water on Earth is increasing? Rather, the amount of water on Earth remains the same, but with global warming, glaciers diminish and sea levels rise. With warmer temperatures comes more evaporation, which causes an increase in precipitation and extreme weather across the planet.

    Summary

    Powered by the sun, water constantly cycles through the Earth and its atmosphere. This module discusses the hydrologic cycle, including the various water reservoirs in the oceans, in the air, and on the land. The module addresses connections between the hydrologic cycle, climate, and the impacts humans have had on the cycle.

    • NGSS
    • HS-C5.2, HS-C6.1, HS-ESS2.C1
    Key Concepts
    • Though the amount of water on Earth remains constant, it is regularly cycling through the ecosystem through various processes.
    • Earth's water supply is stored in a variety of ways, from ice sheets to oceans to underground reservoirs.
    • Like other processes occurring on Earth, the hydrologic cycle is affected by global warming and, as a result, influences climate and weather patterns.

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  • The Carbon Cycle
  • Did you know?

    Did you know that scientists have been able to study climate data from hundreds of thousands of years ago? By looking at air bubbles trapped in glaciers, tree rings, and sediment on the ocean floor, scientists have measured an increase in carbon dioxide in the atmosphere due to changes in the global carbon cycle, which results in global climate change over time.

    Summary

    Carbon, the fourth most abundant element in the universe, moves between the atmosphere, oceans, biosphere, and geosphere in what is called the carbon cycle. This module provides an overview of the global carbon cycle, one of the major biogeochemical cycles. The module explains geological and biological components of the cycle. Major sources and sinks of carbon are discussed, as well as the impact of human activities on global carbon levels.

    • NGSS
    • HS-C1.5, HS-C5.2, HS-C7.2, HS-ESS2.A1, HS-ESS2.A3, HS-ESS2.D3, HS-LS2.B3
    Key Concepts
    • The presence of carbon determines whether something is organic or inorganic; all living things require carbon to live.
    • Carbon cycles through the ecosystem in various ways, from photosynthesis and respiration to weathering and other geologic processes.
    • Many factors, such as seasons and human activities, influence the concentration of carbon in the global atmosphere.

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  • The Nitrogen Cycle
  • Did you know?

    Did you know that all organisms need nitrogen to survive? While the atmosphere is full of nitrogen, it is in a form that can’t be used by living things. Processes within the nitrogen cycle convert nitrogen from the atmosphere into a form that plants and animals can use. Humans alter and influence the nitrogen cycle, primarily through the use of fertilizers, which can have serious environmental consequences.

    Summary

    Although the majority of the air we breathe is N2, molecular nitrogen cannot be used directly to sustain life. This module provides an overview of the nitrogen cycle, one of the major biogeochemical cycles. The five main processes in the cycle are described. The module explores human impact on the nitrogen cycle, resulting in not only increased agricultural production but also smog, acid rain, climate change, and ecosystem upsets.

    • NGSS
    • HS-C5.2, HS-ESS2.A1, HS-ESS3.C1, HS-LS1.C3
    Key Concepts
    • The nitrogen cycle is the set of biogeochemical processes by which nitrogen undergoes chemical reactions, changes form, and moves through difference reservoirs on Earth, including living organisms.
    • Nitrogen is required for all organisms to live and grow because it is the essential component of DNA, RNA, and protein. However, most organisms cannot use atmospheric nitrogen, the largest reservoir.
    • The five processes in the nitrogen cycle – fixation, uptake, mineralization, nitrification, and denitrification – are all driven by microorganisms.
    • Humans influence the global nitrogen cycle primarily through the use of nitrogen-based fertilizers.

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  • The Phosphorus Cycle
  • Did you know?

    Did you know that plants and animals cannot live without phosphorous, and yet it is one of the most difficult elements for living things to get from nature? Phosphorous is the basis of our teeth and bones and even provides the structure of our DNA. Other important elements are readily available from the atmosphere, but phosphorus occurs only in a liquid or solid state at normal temperatures, so there is less available for organisms to use.

    Summary

    All living organisms need phosphorous to survive and grow. This module describes forms that phosphorous takes in nature and how the element cycles through the natural world. A historical journey highlights how we came to understand this vital element. The Experimental Lakes Project shows the harmful effects of too much phosphorous on the environment as a result of human activities.

    • NGSS
    • HS-C5.2, HS-ESS2.A1, HS-ESS3.C1, HS-LS1.C3
    Key Concepts
    • The phosphorus cycle is the set of biogeochemical processes by which phosphorus undergoes chemical reactions, changes form, and moves through different reservoirs on Earth, including living organisms.
    • The phosphorus cycle is the only biogeochemical process that does not include a significant gaseous phase.
    • Phosphorus is required for all organisms to live and grow because it is an essential component of ATP, the structural framework holding DNA and RNA together, cellular membranes, and other critical compounds.
    • Agricultural runoff, over-fertilization, and sewage all increase the amount of phosphate available to plants and can cause significant ecological damage.

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  • Rocks and Minerals

  • Defining Minerals
  • Did you know?

    Did you know that identifying minerals is what led scientists to conclude that there was water on Mars? Understanding of the specific conditions necessary for different minerals to form helps scientists understand the history of Earth and can even shed light on the search for extraterrestrial life.

    Summary

    The study of minerals provides a window into the history of Earth and other planets in our solar system. This first module in a three-part series describes the history of our understanding of minerals and then defines a mineral, focusing on chemical composition and structure.

    • NGSS
    • HS-C6.2, HS-PS1.A3
    Key Concepts
    • Minerals have specific chemical compositions, with a characteristic chemical structure.
    • Minerals are solids that are formed naturally through inorganic processes.
    • Chemical composition and crystal structure determine a mineral's properties, including density, shape, hardness, and color.
    • Because each mineral forms under specific conditions, examining minerals helps scientists understand the history of earth and the other planets within our solar system.

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  • Properties of Minerals
  • Did you know?

    Did you know that even though thousands of minerals have been named, only about a dozen are common in the Earth’s crust? Sophisticated laboratory equipment exists for determining the exact chemical composition of minerals, yet sometimes the most essential tools in geology are a magnifying lens and a penknife. Using just these tools, scientists can identify about 90% of what they encounter in the field.

    Summary

    Minerals are classified on the basis of their chemical composition, which is expressed in their physical properties. This module, the second in a series on minerals, describes the physical properties that are commonly used to identify minerals. These include color, crystal form, hardness, density, luster, and cleavage.

    • NGSS
    • HS-C6.2, HS-PS1.A3
    Key Concepts
    • Properties that help geologists identify a mineral in a rock are: color, hardness, luster, crystal forms, density, and cleavage.
    • Crystal form, cleavage, and hardness are determined primarily by the crystal structure at the atomic level.
    • Color and density are determined primarily by the chemical composition.
    • Minerals are classified on the basis of their chemical composition.

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  • The Silicate Minerals
  • Did you know?

    Did you know that silicates like quartz and clay are among Earth’s most important natural resources? Imagine a world without glass, bricks, pottery, or computers – all of these rely on silicate minerals. These valuable materials make up 95% of the Earth’s crust.

    Summary

    Understanding the structure of silicate minerals makes it possible to identify 95% of the rocks on Earth. This module covers the structure of silicates, the most common minerals in the Earth's crust. The module explains the significance of the silica tetrahedron and describes the variety of shapes it takes. X-ray diffraction is discussed in relation to understanding the atomic structure of minerals.

    • NGSS
    • HS-C6.2, HS-PS1.A3
    Key Concepts
    • Silicate minerals are the most common of Earth's minerals and include quartz, feldspar, mica, amphibole, pyroxene, and olivine.
    • Silica tetrahedra, made up of silicon and oxygen, form chains, sheets, and frameworks, and bond with other cations to form silicate minerals.
    • X-ray diffraction (XRD) allows scientists to determine the crystal structure of minerals.
    • The physical properties of silicate minerals are determined largely by the crystal structure.

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  • Atmosphere and Oceans

  • History of Earth’s Atmosphere I
  • Did you know?

    Did you know that scientists can figure out what elements are found on other planets without collecting actual samples? The universe provides clues to the history of Earth’s atmosphere, a 100 kilometer-thick layer of many different gases that are kept close to Earth by gravity. One important clue to the past is found in neon, an element that is a billion times more abundant in the universe than on Earth.

    Summary

    This module looks at how the Earth's atmosphere has changed since the planet came into existence. Starting with clues provided by neon gas, the module traces how scientists have pieced together the story of Earth’s atmosphere. Techniques are described for determining the concentration of elements found on Earth as well as those on planets and stars that are too far away to allow scientists to collect samples.

    Key Concepts
    • Earth’s early atmosphere had a different composition than the modern atmosphere.
    • Earth’s modern atmosphere evolved over billions of years due to many different geologic processes.
    • Our knowledge about Earth’s primordial atmosphere comes from studying the atmospheres of other planets and the composition of stars, as well as clues from the rock record.

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  • History of Earth’s Atmosphere II
  • Did you know?

    Did you know that Earth’s atmosphere hasn’t always contained oxygen gas? In fact, for almost half of the planet’s 4.6 billion-year history, oxygen atoms could only be found bound up in water molecules or minerals, not as free oxygen in the air. The story of how Earth’s atmosphere has changed over time is closely tied to how the planet and life evolved over geologic time.

    Summary

    The composition of Earth’s atmosphere has evolved over time. This module examines how Earth came to be the only planet in the universe known to contain oxygen gas. The module explores the advent and rise of oxygen in Earth’s atmosphere. Evidence described includes the rock record, bands of iron in sediment, microscopic fossils, and isotopes of sulfur.

    Key Concepts
    • Earth’s early atmosphere had a different composition than the modern atmosphere and contained very little oxygen gas.
    • The rise of oxygen gas in Earth’s atmosphere was dependent upon the evolution of photosynthesizing bacteria in the oceans.
    • Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.

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  • Composition of Earth's Atmosphere
  • Did you know?

    Did you know that without the atmosphere, Earth's surface would be covered with meteor craters and life on this planet would be non-existent? Protecting us from meteorites, regulating temperature, and providing the air we breathe are only some of the ways that the atmosphere makes Earth the home it is.

    Summary

    Earth's atmosphere contains many components that can be measured in different ways. This module describes these different components and shows how temperature and pressure change with altitude. The scientific developments that led to an understanding of these concepts are discussed.

    Key Concepts
    • Earth's atmosphere is made up of a combination of gases. The major components of nitrogen, oxygen, and argon remain constant over time and space, while trace components like CO2 and water vapor vary considerably over both space and time.
    • The atmosphere is divided into the thermosphere, mesosphere, stratosphere, and troposphere, and the boundaries between these layers are defined by changes in temperature gradients.
    • Pressure decreases exponentially with altitude in the atmosphere.
    • Our knowledge about the atmosphere has developed based on data from a variety of sources, including direct measurements from balloons and aircraft as well as remote measurements from satellites.

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  • Factors that Control Earth's Temperature
  • Did you know?

    Did you know that Earth sits in a narrow band of space called the Goldilocks Zone? Just as the porridge in the story about Goldilocks and the Three Bears was neither too hot nor too cold, Earth’s conditions are “just right” to support life. Several factors work together to make the planet habitable. Among these are distance from the Sun and the composition of Earth’s atmosphere.

    Summary

    Based on how much sunlight hits Earth versus how much is reflected, Earth’s average temperature should be well below freezing. Fortunately, there are other factors that affect the planet’s temperature. This module explores the effects of those factors, including distance from the sun, aerosol particles floating in the air, and greenhouse gases. Topics introduced include insolation and albedo. Also explored is how a planet’s climate can be modeled by taking account of energy in, energy lost, and energy transferred.

    • NGSS
    • HS-C2.1, HS-C3.2, HS-ESS1.B2, HS-ESS2.A1, HS-ESS2.D4
    Key Concepts
    • The Sun is the primary source of energy that influences any planet's temperature, including Earth. The amount of energy received from the Sun is called insolation; the ratio reflected is called the albedo.
    • The composition of a planet's atmosphere also influences its temperature, particularly the concentration of greenhouse gases present.
    • The Earth converts solar radiation in the visible spectrum to infrared radiation, which it emits; greenhouse gases absorb infrared radiation and warm the atmosphere.
    • Aerosols usually act to cool the Earth on relatively short timescales.
    • Any planet's climate, including Earth's, can be modeled very simply by calculating fluxes of energy.

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  • Factors that Control Regional Climate
  • Did you know?

    Did you know that “trade winds” were so named because seafaring merchants relied on them to drive their vessels in search of spices and other goods? One of the earliest explorers known to diligently record and use wind direction, wind speed, and ocean current in his voyages was Christopher Columbus. Along with other factors, the large-scale circulation of air and ocean currents plays an important role in determining climate in different locations around the world.

    Summary

    Although weather can change every day, climate is the average of daily weather conditions over decades. This module presents factors that influence climate around the world, such as the shape, tilt, and orbit of Earth. Starting with observations of early ocean travelers and progressing through others’ ideas over later centuries, the module describes how we came to understand Earth’s climate. Also discussed is the imbalance of energy from incoming vs. outgoing radiation and its effect on wind and ocean currents.

    Key Concepts
    • Earth’s spherical shape and its tilt of 23.5° result in uneven heating from the sun; low latitudes near the equator receive more incoming energy than high latitudes near the poles.
    • Large-scale circulation in Earth’s atmosphere below the tropopause is caused by the combination of two factors: uneven heating from the sun and Earth’s rotation.
    • The Hadley cell is the primary circulation cell in the atmosphere. It forms a band of warm temperatures, low pressure, calm winds, and heavy precipitation called the Intertropical Convergence Zone near the equator and a band of high pressure and low precipitation around 30° N and S.
    • The Ferrel and polar cells circulate air at higher latitudes, creating dry, high-pressure areas over the poles and wetter regions around 60° N and S. Most of our early understanding of circulation in the atmosphere came from sailors, who recorded their observations in journals as they explored the oceans.

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