Earth's Systems and Resources
Earth is a spectacular and ever-changing place, with constant activity and incredible transformations. The changes include everything from earthquakes and volcanoes to the formation of life and extinction of it. This chapter covers Earth's geologic changes and time scale, along with its atmosphere, water, and soil.
To prepare for the AP Environmental Science exam and to fully understand the concepts and workings of Earth's systems, you need to understand how the planet functions and its composition.
With a unique set of characteristics and features, Earth is the only planet in our solar system that is known to support life. It is the third planet from the sun, with the order of the planets being Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Although Pluto used to be considered a planet, it is now classified as a dwarf planet.
Geologic Time Scale
At an age of approximately 4.6 billion years, Earth has seen many radical changes throughout its eons, eras, periods, and epochs. This span of time, and the changes that have taken place in it, are grouped into the geologic time scale. On the geologic time scale, eons are the largest spans of time; they include the Archean, Proterozoic, Hadean, and Phanerozoic. The eons Archean and Proterozoic are referred to as Precambrian time with seven eras. Eons are divided into eras, which are divided into periods.
With its varying composition and dense core, Earth is layered, and each layer has its own properties. From the interior outward, Earth is composed of a core, a mantle, and a crust.
The crust, the outermost layer of the Earth and the surface on which we live, can be either continental crust or oceanic crust, depending on where it's found. Oceanic crust is denser than continental crust because it is, in large part, made up of basalt, which contains the heavier elements iron and magnesium. The continental crust has a high amount of granite, which is rich in the lighter element aluminum. Because of its brittle nature, the crust can fracture and lead to earthquakes. Continental crust is 22 to 44 miles thick, and oceanic crust is 3 to 6 miles thick.
Below the crust is the mantle, which makes up approximately 80 percent of Earth's volume. It contains the upper mantle and lower mantle. The crust and upper mantle are grouped together in a structure called the lithosphere, which is the rigid outer layer of the Earth. Below the lithosphere but still above the lower mantle is a layer called the asthenosphere, which is made up of a plastic-like substance that tends to flow. The lower mantle is semi-rigid and flows very slowly. Combined, the upper and lower mantle are 1,802 miles thick.
The dense core at the center of the Earth is similarly subdivided into an inner and outer core. The inner core is mainly made up of iron and nickel and is solid because of the extreme pressure from the other layers above it. The liquid outer core is composed mainly of iron and nickel; it is molten because of its extreme heat, which is at least 10,832°F. The outer core is about 1,429 miles thick, and the inner core is 746 miles thick.
The Earth's lithosphere (the crust and upper mantle) is broken into tectonic plates (also known as lithospheric plates). These plates are in constant motion atop the asthenosphere, which is the Earth's molten mantle layer that keeps the continents slowly moving. This movement of the continents is called continental drift.
The reason that tectonic plates are in constant motion is a process called seafloor spreading, which is the movement of the seafloor at the mid-ocean ridge. A mid-ocean ridge is the location from which magma (molten rock within the Earth) rises to the surface from the asthenosphere. It looks like a scar across the ocean floor. As the magma pushes through the crust and hardens, new seafloor is created. As new magma surfaces, it pushes away the existing seafloor, causing it to spread and move apart.
Magma rises through the crust because of convection currents in the magma below the plates. When magma is heated, it becomes less dense and rises, but as it rises it cools, causing it to become denser and sink. As the heating and cooling, rising and sinking process continues, currents are created, and it's this movement that drives the plate motion.
At plate boundaries, major geologic activity occurs, including earthquakes, volcanoes, and the formation of mountain ranges. The type of activity depends on the type of plate movement. The three types of plate boundaries are
* Transform plate boundaries: Transform plate boundaries are commonly known as faults and are found on the ocean floor. At the zone of transform plate boundaries, tectonic plates slide in a sideways motion past one another. As they slide and stick, friction and energy build up. When the pressure is eventually relieved, earthquakes can occur at these boundaries. Examples of transform boundaries exposed on land include the San Andreas Fault in California and the Alpine Fault in New Zealand.
* Divergent plate boundaries: When two or more plates pull away from one another, divergent plate boundaries are created. An example of a divergent boundary is the movement of plates at the mid-ocean ridge, where the ocean becomes wider as plates diverge. The East Africa Rift Valley is an example.
* Convergent plate boundaries: When two plates move toward one another, a convergent plate boundary is created as one plate dives under the other. With this collision, different events can follow, depending on the type of crust involved. Examples of convergent boundaries include the Andes Mountains in South America, the Cascade Mountains in the northwestern United States, and the Marianas Trench in the Pacific Ocean.
Oceanic and continental plates
Oceanic and continental plates in collision lead to subduction. Subduction of oceanic plates at plate boundaries is the cause of continental crust being older than oceanic crust. The denser oceanic plate is pushed below the lighter continental plate. When the crust is compacted and pushed up during the collision of two continental plates, mountain ranges are formed. This motion created the Himalaya Mountains, which are still growing today. When two oceanic plates collide, one plate may be pushed below the other, forming a trench or producing a volcano and allowing magma to rise. Formations created from convergent boundaries include the Andes Mountains in South America, the Cascade Mountains in the northwestern United States, and the Marianas Trench in the Pacific Ocean. Earthquakes also can occur as a result of movement at plate boundaries.
At times, pressure builds up at plate boundaries because of friction from plate movement, and stress is created. When this stress is ultimately discharged, energy is released throughout the Earth's crust, causing vibrations, or earthquakes. Often, earthquakes are caused by movement of the lithospheric plates and occur at plate boundaries. The focus of an earthquake is the location at which the earthquake originates within the Earth. Above the focus is the epicenter, which is the first place on the Earth's surface affected by the earthquake.
The types of faults from which earthquakes can occur include strike-slip, normal, and reverse. Strike-slip faults occur where the plates slide past one another horizontally. Normal faults are caused by tension from a pulling-apart motion. Reverse faults are caused from compression.
Earthquakes themselves do not generally kill people, but their effects on human-built structures do. Because an earthquake is a vibration of the Earth caused by a sudden release of energy, this movement transfers into buildings, roadways, and other infrastructures. The consequences of structural failure in constructed facilities ultimately cause harm. Tsunamis are seismic sea waves generated from undersea earthquakes or volcanic eruptions and are an exception to this statement, though. They are not dangerous while traveling through the ocean, but they can cause massive destruction once they reach a coastline, which can be thousands of miles from the location of the earthquake or volcano.
Volcanoes are openings in the Earth's surface that allow magma, gases, ash, cinder, and other volcanic material to escape from the mantle. A volcano's structure includes a magma chamber, which contains a pool of magma deep within the earth; a pipe (conduit) that brings lava, gases, and other materials from the magma chamber to the surface; and a vent, which is the opening through which lava and other material escapes. Some volcanoes also have a crater, or depression, at the mouth,
Because of the ever-changing interior of the Earth, volcanoes have various stages and remain active for a period of time. An active volcano is either presently erupting or will eventually erupt because of a large amount of seismic and thermal activity occurring within it. A dormant volcano is inactive but could potentially erupt again. An extinct volcano is not erupting and most likely will never erupt again.
Types of volcanoes
Three main types of volcanoes have been identified:
* Shield volcanoes are large with broad sides, gradual slopes, and usually a crater at the top. They typically erupt slowly, with lava oozing from the vent or multiple vents. Examples include Mauna Loa in Hawaii, Mount Wrangell in Alaska, and Skjaldbreiur in Iceland.
* Composite volcanoes (strato volcanoes) are tall, symmetrical, and steep. They're built of alternating layers of ash, lava, and cinders. Examples include Mount Hood in Oregon, Mount Lassen and Mount Shasta in California, Mount Fuji in Japan, Arenal in Costa Rica, Mount Cotopaxi in Ecuador, Mount Etna in Italy, and Mount Rainier and Mount St. Helens in Washington. Eruptions of composite volcanoes can be either explosive or lava extruding; therefore, predicting the type of eruption and its severity is difficult.
* Cinder cone volcanoes are usually made of lava that erupts in the form of cinders, which are blown into the air and then settle around the opening of the volcano, ultimately forming a small, steep-sided mountain. This is the most common type of volcano. Examples include Mount Mazama in Oregon (a destroyed volcano that is now the location of Crater Lake), Paricutin in Mexico, Mount Shasta in California, and Cerro Negro in Nicaragua.
Locations where magma emerges from within the Earth but not at plate boundaries are called hot spots. Hot spots form in the middle of tectonic plates. The magma's extreme heat burns through thin crust, and then cools and forms new crust. Over time, this new land can build up to form volcanoes in the middle of plates or islands in the ocean. Examples of places where hot spots have occurred include the Hawaiian Islands, the Galapagos Islands, Iceland, and Yellowstone National Park.
Effects of Volcanoes
Although volcanoes are natural events, they still have an impact on people's health, the environment, and other organisms. A variety of gases are released into the atmosphere during a volcanic eruption, and the effects vary, depending on the amount released, the location, the wind pattern, the height of discharge, and other factors. The most abundant gases released during an eruption include water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). Other gases released include carbon monoxide (CO), helium (He), hydrogen (H2), hydrogen chloride (HCl), hydrogen sulfide (H2S), and hydrogen fluoride (HF).
Posing the potentially most harmful effects on organisms and the environment are
* Hydrogen fluoride (HF), also called sewer gas, which can cause respiratory tract irritation, bone degeneration, and pulmonary edema in high concentrations. At lower concentrations, exposure can cause eye irritation, diarrhea, dizziness, excitement, and staggering. When HF coats grass and animals then ingest it, poisoning can occur, as can bone degeneration and even death. HF also contributes to acid rain.
* Carbon dioxide (CO2) has a density greater than that of air, so it sinks and can kill animals, people, and plants. The CO2 replaces the air, so asphyxiation can occur in areas with abundant CO2. This gas can also collect in soils, which can affect the microbial population in the soil and nutrient intake by plants.
* Sulfur dioxide (SO2) can lead to acid rain, air pollution, and smog at a local level. On a global level, it can lower surface temperatures and exacerbate depletion of the ozone layer. SO2 also can harm human health mainly by affecting the respiratory system and also irritating skin, eyes, nose, and throat.
* Hydrogen chloride (HCl) causes irritation of the eyes, throat, and respiratory system. It can lead to acid rain because of its solubility in water, as well as to loss of ozone.
Atmospheric Effects of Volcanoes
Because these volcanic gases are released into the atmosphere, the effects can be dramatic:
* Ozone can be broken down when reactions occur with HCl or SO2. Fortunately, the ozone depletion diminishes once the gases are reduced in the atmosphere.
* Volcanic gases can contribute to global warming because CO2 and water vapor trap and absorb solar energy, raising the temperature of the planet over time.
* The gases can contribute to the haze effect (smog), in which particulate matter in the atmosphere blocks out solar radiation and ultimately can lower the mean global temperature.
These effects usually are not long-term when they occur because of volcanic activity, but they're exacerbated by human activities that also release these gases into the atmosphere.
Solar Radiation, Intensity, and Seasons
Solar energy affects the entire dynamic of the planet, including climate, weather, biodiversity, and life's productivity. The amount of solar energy the Earth receives depends on the tilt of Earth's axis, its rotation around that axis, and its revolution around the Sun. One rotation equals one day, and a revolution equals a year.
Throughout the year, Earth has two equinoxes, times when day and night are equal. Toward the end of March, the vernal equinox occurs, signifying the start of spring in the Northern Hemisphere and fall in the Southern Hemisphere. The autumnal equinox, marking the beginning of fall in the Northern Hemisphere and spring in the Southern Hemisphere, occurs at the end of September.
Solstices occur when the sun is most north or south of the celestial equator. In the Northern Hemisphere, the summer solstice, when the sun is northernmost, occurs on June 21 over the Tropic of Cancer. The winter solstice occurs on December 21 over the Tropic of Capricorn and is when the sun is southernmost. In the Northern Hemisphere, the summer solstice is the longest day of the year, and the winter solstice is the shortest.
Earth's seasons are created by the tilt of Earth's axis to its orbital plane and its rotation around the sun, which is 23.5 degrees. At different times throughout the year, different parts of the Earth are facing the sun. Summer occurs when the sun's rays hit Earth's surface at the most direct angles, also giving summer the longest daylight hours. During winter the angle of the sun's rays are more oblique, giving that portion of the Earth shorter days and less solar energy. The seasons are not related to Earth's distance from the sun. The Earth is actually closest to the sun in January (perihelion) and farthest away in July (aphelion).
As a protector of Earth, the atmosphere deflects many harmful UV rays from the sun and helps to maintain a stable temperature by helping to retain heat with a natural greenhouse effect. Without the atmosphere, life as we know it would not be able to exist on this planet. It is also a dynamic aspect of Earth, changing over the 4.6 billion years of the planet's existence.