Unit 4: Earth Systems and Resources

Unit 4 explains how Earth's physical systems work. It covers plate tectonics and earthquakes, how soil forms and what makes it fertile, the composition and layers of the atmosphere, the global wind patterns that move air around the planet, how solar energy varies across latitudes and seasons, and the climate patterns that result.

4.1 Plate Tectonics

Earth's lithosphere is broken into tectonic plates that drift a few centimeters per year. Where plates meet, the type of boundary decides what forms. A convergent plate boundary is where two plates push together. When oceanic crust meets continental crust, the denser oceanic plate sinks beneath the other in subduction, producing volcanoes and earthquakes like those along the Andes. Two oceanic plates colliding build island arcs like Japan. Two continental plates colliding crumple upward into mountain ranges like the Himalayas.

A divergent plate boundary is where two plates pull apart. Magma rises to fill the gap, so new crust forms here. These boundaries build mid-ocean ridges, drive seafloor spreading, and open rift valleys like the East African Rift. Volcanoes and earthquakes occur at divergent boundaries too. A transform plate boundary is where two plates slide horizontally past each other. No crust is created and none is destroyed, so the signature product is earthquakes, like those on the San Andreas Fault.

Reading plate boundary maps

Plate boundary maps show where these boundaries run around the planet. Because most volcanoes, earthquakes, island arcs, and faults sit on or near boundaries, a map of boundaries doubles as a map of geologic hazards. Learn one deliberate exception: hot spots. Hot spots like Hawaii form over stationary plumes of hot mantle rock in the middle of a plate, not at a boundary, so they will not appear where the boundary map says to look.

How earthquakes happen

An earthquake occurs when stress overcomes a locked fault, releasing stored energy. The plates on either side of a fault are stuck together while tectonic motion keeps pushing, so strain energy builds up over years or decades. When the stress finally exceeds the strength of the lock, the fault slips and the stored energy releases as seismic waves. The point underground where the slip starts is the focus, and the point on the surface directly above it is the epicenter.

4.2 Soil Formation and Erosion

Soil formation begins with parent material, the rock or sediment the soil develops from. That material is weathered into smaller pieces by physical forces like freeze-thaw cycles and by chemical reactions like dissolution, then transported by wind, water, or ice, and finally deposited as sediment that develops into soil. The process is slow. A few centimeters of new soil can take hundreds to thousands of years to form, which means eroded topsoil is effectively nonrenewable on human time scales.

Soils are described in horizons, layers defined by composition and organic content. The O horizon is mostly organic litter at the surface. The A horizon is topsoil, a mix of organic matter and mineral particles, and it holds most plant roots and biological activity. The B horizon is subsoil, where minerals leached from above accumulate. The C horizon is weathered parent material, and the R horizon is bedrock. A question about fertility or crop growth usually points at the A horizon, since that is the layer agriculture depends on.

Soil can be eroded by wind and water. Bare or disturbed soil washes away in rain and blows away in dry wind, carrying nutrients and sediment into streams and lakes. Protecting soil also protects water quality, because soil filters and cleans water that moves through it, trapping sediment and some pollutants before they reach groundwater and surface water.

4.3 Soil Composition and Properties

How much water soil holds depends on particle size. Sand grains are the largest, silt is in the middle, and clay particles are the smallest. The tiny particles of clay pack together with many small pore spaces, so clay has the greatest water holding capacity and sand the least. Loam, a blend of sand, silt, and clay in roughly balanced proportions, holds enough water for crops while still draining well, which is why it is the preferred texture for farming.

Particle size also controls two properties that students constantly mix up. Porosity is the fraction of the soil volume that is empty pore space. Permeability is how easily water moves through those spaces. Clay has high porosity because its fine particles leave many tiny pores, but low permeability because those pores are so narrow that water moves slowly. Sand is the reverse: lower porosity, higher permeability. The soil texture triangle names a soil from its percentages of sand, silt, and clay. Read each percentage on its own axis and find where the three lines meet. A soil that is about 40 percent sand, 40 percent silt, and 20 percent clay reads as loam.

Soil testing measures the chemical, physical, and biological properties of a sample. Farmers and land managers test before deciding how much to irrigate or fertilize, because adding fertilizer a soil does not need wastes money and sends excess nutrients into waterways. Testing replaces guessing about what the soil actually needs.

4.4 Earth's Atmosphere

The composition of the atmosphere is mostly nitrogen and oxygen: about 78 percent nitrogen and 21 percent oxygen, with argon making up most of the remaining 1 percent. Trace gases like carbon dioxide, methane, and ozone are present in tiny amounts but do outsized work. Carbon dioxide and methane trap heat near the surface, and ozone absorbs incoming ultraviolet radiation.

The atmosphere is divided into layers based on how temperature changes with altitude. The troposphere is the lowest layer, where weather happens and temperature falls as you go up. Above it is the stratosphere, which holds the ozone layer; temperature rises with altitude here because ozone absorbs UV. Next is the mesosphere, where temperature falls again and meteors burn up. Then the thermosphere, where temperature rises sharply and auroras occur. The exosphere is the outermost, thinnest layer, where satellites orbit and gases gradually escape into space.

4.5 Global Wind Patterns

Global wind patterns start with uneven solar heating. The equator receives the most intense solar radiation, so air there warms, becomes less dense, and rises. That rising air flows toward the poles, cools, sinks, and returns toward the equator, forming large convection cells. The Coriolis effect, caused by Earth's rotation, deflects these moving air masses: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The result is the planet's prevailing winds: the trade winds blowing toward the equator, the westerlies blowing toward the poles in the mid-latitudes, and the polar easterlies near the poles.

4.6 Watersheds

A watershed is an area of land that drains into a common body of water. Rain that falls anywhere inside the boundary eventually flows to the same river, lake, or estuary. Watersheds are described by their area, length, slope, soil, vegetation types, and the divides that separate them from adjoining watersheds. What happens on the land ends up in the water, so farming, paving, and construction anywhere in a watershed affect the water body it drains into. Managing water quality means managing land use across the whole watershed, not just at the shoreline.

4.7 Solar Radiation and Earth's Seasons

Insolation is incoming solar radiation, Earth's main energy source. It varies with latitude and season, and the variable that controls it is the solar angle, the angle at which the sun's rays strike the surface. Radiation is most intense where the rays hit perpendicularly, directly overhead, because the same amount of energy lands on the smallest area. Where the rays strike at an angle, the energy spreads over a larger area and the intensity falls.

The latitudinal radiation gradient follows directly. The equator receives the most solar radiation per unit area, and the amount decreases steadily toward the poles. This gradient is the engine behind the global wind patterns in topic 4.5 and the ocean currents that move heat around the planet. Almost every large-scale circulation on Earth traces back to the fact that the equator gets more energy per square meter than the poles.

At any one location, the solar radiation received also changes through the year. The most arrives on the longest day of summer and the least on the shortest day of winter. This seasonal variation comes from Earth's axial tilt, not from distance to the sun. The tilt points one hemisphere toward the sun for half the year and away for the other half, which changes both the solar angle and the number of daylight hours.

4.8 Earth's Geography and Climate

Weather is the short-term state of the atmosphere and climate is the long-term pattern, and both respond to more than the sun's energy. Mountains reshape precipitation: moist air forced up a slope cools and drops its moisture as rain or snow on the windward side. The air that descends on the other side is dry, leaving a rain shadow, a region made drier because the higher elevation blocked precipitation from reaching it. The arid land east of the Sierra Nevada exists for exactly this reason. Ocean temperature matters too. Warm currents warm the air above them and cold currents cool it, so coastal regions near cold currents tend to be cooler and drier than their latitude alone would predict.

4.9 El Nino and La Nina

El Nino and La Nina are the two phases of the El Nino-Southern Oscillation, or ENSO, a periodic shift in Pacific Ocean surface temperatures. In a normal year, trade winds push warm surface water westward, and cold, nutrient-rich water wells up along the coast of South America. During El Nino, the trade winds weaken, warm water sloshes back eastward, and the upwelling weakens. During La Nina, the trade winds strengthen and the eastern Pacific gets cooler than normal. Both phases shift rainfall, wind, and ocean circulation patterns around the globe.

The effects of ENSO vary by location. El Nino can bring heavy rain to the southern United States while causing drought in Australia and Indonesia, and La Nina tends to flip that pattern. Geological and geographic features like mountain ranges and ocean currents shape how strongly each region feels the shift, so the same phase can mean floods in one place and drought in another. A question that asks about ENSO impacts is really asking whether you know that the effects are regional, not uniform.

Confusions That Cost Points

Practice Questions

Original questions written for this guide in the style of the AP exam. Answers and explanations are on the next page, so complete the questions before checking them.

Answer Key

1. B. At a divergent boundary the plates pull apart, magma rises, and new crust forms, producing seafloor spreading, mid-ocean ridges, and rift valleys. A describes a convergent boundary where oceanic crust subducts. C describes a transform boundary. D describes a convergent boundary between two oceanic plates.

2. A. On the soil texture triangle, 40 percent sand, 40 percent silt, and 20 percent clay falls in the loam region, a roughly balanced blend that supports a variety of crops. B would require a higher sand percentage. C would require substantially more clay. D would require silt above about 50 percent.

3. B. The ozone layer sits in the stratosphere, where temperature increases with altitude because ozone absorbs ultraviolet radiation. A names the weather layer, where temperature falls with altitude. C is the layer where meteors burn up, also with falling temperature. D has rising temperature but contains auroras, not the ozone layer.

4. A. Seasons come from Earth's axial tilt, which angles one hemisphere toward the sun for half the year, changing both the solar angle and the hours of daylight. B is wrong because solar output does not rise and fall with the seasons. C is wrong because the greenhouse effect does not vary seasonally to produce summer. D confuses trade winds, which drive ENSO and ocean patterns, with the cause of the seasons.

One-Page Recall Check

• Name the three types of plate boundaries and state what each one produces.
• Explain how a plate boundary map predicts where earthquakes and volcanoes occur, and name the one feature it will not predict.
• Describe the earthquake mechanism from a locked fault to released energy.
• Explain how soil forms from parent material and why the process takes so long.
• List the soil horizons in order and describe what is in the A horizon.
• Distinguish porosity from permeability, using clay and sand as examples.
• Use the soil texture triangle to identify loam from sand, silt, and clay percentages.
• State the composition of the atmosphere with approximate percentages.
• List the atmospheric layers in order and describe how temperature changes in each.
• Explain how uneven solar heating and the Coriolis effect produce the global wind patterns.
• Define a watershed and list the characteristics used to describe one.
• Explain how the solar angle controls radiation intensity and produces the latitudinal gradient.
• Explain why seasons happen, using Earth's axial tilt.
• Describe the rain shadow effect and state which side of the mountain stays dry.
• Contrast El Nino and La Nina in terms of trade winds and eastern Pacific temperatures.

Key terms for this unit

Convergent plate boundaries, Divergent plate boundaries, Transform plate boundaries, Plate boundary maps, Earthquake mechanism, Soil formation, Soil horizons, Soil erosion and water filtration, Water holding capacity, Particle size and soil properties, Soil testing, Soil texture triangle, Loam, Composition of the atmosphere, Layers of the atmosphere, Causes of global wind patterns, Watershed, Insolation, Solar angle and radiation intensity, Latitudinal radiation gradient, Seasonal variation in solar radiation, Earth's axial tilt and seasons, Geographic factors in climate, Rain shadow, El Nino and La Nina (ENSO), Variable effects of ENSO.

About this guide. Written for Rycal and aligned to the College Board AP Environmental Science course framework, Unit 4. All questions and explanations are original Rycal writing. Rycal is independent and is not affiliated with or endorsed by the College Board.