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Cycles in Earth Systems

Cycles in Earth Systems

The teacher understands cycles in earth systems.

This competency has a number of important subthemes that contribute to the overall understanding of cycles in earth systems. The Geography4Kids website provides a basic introduction to Bio-Geo-Chemical Cycles.


The Rock Cycle

The beginning teacher understands the rock cycle and how rocks, minerals, and soils are formed.

Key Concepts:

  • Rocks are classified as igneous, sedimentary, or metamorphic depending upon the ways in which they originated.
  • Igneous rocks are rocks formed when molten material cools and hardens.
  • Plutonic rocks are a kind of igneous rock formed from molten material that cooled beneath Earth’s surface. Plutonic rocks are distinguished by a visible structure of interlocking mineral crystals. The sizes of the crystals in the rock indicate the cooling rate. Small crystals indicate rapid cooling. Large crystals indicate slow cooling. Molten material beneath Earth’s surface is called magma.
  • Igneous rocks, formed from molten material ejected on Earth’s surface, are called volcanic rocks. Volcanic rocks are sometimes called “lava,” but the term also is used to describe molten rock on Earth’s surface. Because of a very rapid cooling rate, volcanic rocks have a microscopic (cryptocrystalline) or an amorphous structure (e.g., obsidian) and/or vesicular (bubbly) texture.
  • Sedimentary rocks (e.g., sandstone, limestone, conglomerates) consist of particles of other rocks that have been broken down by weathering, transported by erosion, deposited, and cemented together. Sedimentary rocks may also be produced by the accumulation of chemical precipitates. Sedimentary rocks generally are characterized by the presence of layering and sometimes fossils.
  • The forces of weathering include gravity, wind, water, temperature, chemicals, and biological action. If the products of weathering are transported to a new location by wind, water, or ice, the transportation process is called erosion. Erosion creates many geologic structures such as canyons, mesas, and hanging valleys.
  • Metamorphic rocks are changed rocks due to the application of heat and/or pressure. Igneous and sedimentary rocks may be thrust deeply into the Earth by tectonic movements or just accumulate as sediments over millions of years. The pressure and heat at great depths alters mineral grains to produce fused minerals, distorted crystals, and banding that resembles sedimentary layering. The source rock determines the metamorphic rock that is produced (e.g., sandstone is converted into quartzite, limestone becomes marble, granite becomes gneiss).


In the video, Every Stone Has a Story, a “talking rock” provides a humorous explanation of the rock cycle and different types of Rock.

The US Geological Survey webpage on Rocks provides explanations of geologic processes. Rock types are explained and a simple rock classification chart is presented.

Pictures and explanations of common rock types are given in the Igneous Metamorphic Sedimentary Rock Gallery created by rocksandminerals4u.

Classroom of the Future presents a simplified diagram and explanation of The Rock Cycle are presented by Classroom of the Future.

The Water Cycle

The beginning teacher understands the water cycle and its relationship to weather processes.

Key Concepts:

  • Water exists on Earth simultaneously in three states: liquid, solid, and gas.
  • Water cycles among its three states over Earth’s surface and within Earth’s upper crust (atmosphere, hydrosphere, cryosphere, biosphere, and geosphere).
  • Water is a major component of Earth’s atmosphere, and many weather events and climate are characterized by the presence or absence of measurable water (rain, snow, humidity, etc.)
  • Water also is part of all living things, and cycles through living organisms as part of the water cycle. Transpiration (evaporation of moisture from the surfaces of plants) is a particularly important part of the water cycle over land.


The US Geological Survey provides a very detailed diagram of Earth’s water cycle. Tables and Graphs list the distribution and quantities of water sources world wide.

Classroom of the Future provides a simplified diagram and explanation of the water cycle.

The Nutrient Cycle

The beginning teacher understands the nutrient (e.g., carbon, nitrogen) cycle and its relationship to earth systems.

Key Concepts:

  • Earth’s surface systems involve many cycles, such as carbon, nitrogen, and oxygen, which support life. Cycles that exchange materials among living and nonliving components of the Earth are known as biogeochemical cycles.
  • Nutrients provide the raw materials for growth and energy for life.
  • The carbon cycle involves exchanges among reservoirs of carbon in the atmosphere (gases such as carbon dioxide and methane), on land (including carbon in living organisms, remains of living organisms, and non-living inorganic carbon, such as in soil), and in the oceans (dissolved inorganic carbon and carbon in or from living organisms).
  • Carbon dioxide makes up only about 0.03% of the Earth’s atmosphere, yet carbon is found in most molecules that make up the bodies of living organisms. Organic chemistry is the branch of science that studies the properties, reactions and synthesis of carbon-containing molecules.
  • Carbon enters the living components of the cycle through photosynthesis by plants, algae and some bacteria (such as cyanobacteria). Other living organisms (animals and fungi) obtain the carbon they need by consuming plants and other photosynthetic organisms. During photosynthesis, organisms use energy from the sun to synthesize sugar and other molecules from carbon dioxide and water.
  • Carbon dioxide is released into the atmosphere through cellular respiration (which is the breakdown of organic molecules to obtain energy), through the decay of living matter by fungi and bacteria, through combustion of organic materials and fossil fuels, and through volcanic eruptions.
  • Carbon dioxide also dissolves in water (oceans and freshwater), and forms bicarbonates.
  • Nitrogen is an essential component of organic compounds such as proteins and amino acids. However, most living organisms cannot obtain the nitrogen they need directly from the atmosphere (which is 78% nitrogen).
  • Certain kinds of bacteria trap atmospheric nitrogen and convert it into forms that can be used by other living organisms. This process often is called nitrogen fixation. Other bacteria break down nitrogen containing compounds from plant and animal materials and return them to the atmosphere.
  • The phosphorous cycle is typical of many other cycles in which the reservoir of a nutrient exists primarily in mineral reserves. Phosphorous, which is in ATP and nucleic acids (such as DNA), exists in soils in small amounts as phosphates. Phosphates, which are soluble in water, weather from soils into water, enter the food chain through plants, and eventually are redeposited into the soil when organisms decompose.


Illustrations of four nutrient cycles (nitrogen, phosphorus, potassium, and sulfur) are presented by the Internatl Plant Nutrition Institute.

NASA’s Earth Observatory site provides images and feature stories about planet Earth. The carbon cycle is explained in this article.

The Environmental Literacy Council has a good diagram of the nitrogen cycle, explanatory text, and several good links to additional information about the nitrogen cycle.

Natural Processes

The beginning teacher applies knowledge of how human and natural processes affect earth systems.

Key Concepts:

  • Earth has many natural control systems that sustain a vigorous biosphere. Among these systems are wild fires that renew forests and grasslands, predator/disease or predator/prey relationships that prevent over population of a particular species, the abilities of living things to adapt to new environmental conditions, and the constructive and destructive processes of the geosphere that tear down and build up new land.
  • Humans affect earth systems by altering local ecosystems to meet human needs (e.g., deforestation, monoculture farming, mining, transportation, urbanization).
  • The byproducts of human transportation and industrialization are significantly altering Earth systems such as climate, water and air quality, and introduction of alien species to ecosystems.


Learn about climate change from the the University Corporation for Atmospheric Research.

The Environmental Protection Agency offers a primer on effects of climate change on people and environments.

Many scientists believe that we have begun a new geologic era, called the Anthropocene, because humans are changing Earth on a global scale in many ways.

Dynamic Interactions Between Cycles

The beginning teacher understands the dynamic interactions that occur among the various cycles in the biosphere, geosphere, hydrosphere, and atmosphere.

Key Concepts:

  • Planet Earth is the sum total of all of its components and systems. Over its more than four billion year history, Earth has naturally adjusted to the interactions of its systems to create an environment unique in the solar system and the Milky Way Galaxy.
  • None of the systems within the biosphere (living systems), geosphere (solid parts of the Earth, including the lithosphere or uppermost layers), hydrosphere (mass of water found anywhere on Earth), and atmosphere (layer of gases surrounding the planet) operate independently of the others.


Earth System Science is described in this page provided by the Wheeling Jesuit College. The interactions of different Earth systems is described.

Our Ocean Planet from Texas A&M University provides a very detailed chart showing the interactions of Earth systems.

HISD Project REACH U.S. Department of Education