Unit 4: Earth Systems and Resources

Plate Tectonics and Geological Processes

Structure of the Earth

• The Earth is composed of the crust, mantle, outer core, and inner core, each with distinct compositions and physical properties. The crust is the thin outer layer where we live, composed mainly of continental and oceanic plates. The mantle, made of semi-solid rock, drives plate movement through convection currents caused by heat from the core.
• The outer core is liquid iron and nickel, generating Earth’s magnetic field through its motion. The inner core is solid due to immense pressure, despite high temperatures. Understanding these layers is essential because tectonic activity originates from deep within the Earth’s mantle and core.

Plate Boundaries and Movement

• Earth’s lithosphere is divided into tectonic plates that move over the asthenosphere. At divergent boundaries, plates move apart, creating mid-ocean ridges and new crust. At convergent boundaries, plates collide, leading to mountain formation or subduction zones where one plate is forced beneath another.
• Transform boundaries involve plates sliding past each other, causing earthquakes without creating or destroying crust. The movement of plates is slow (centimeters per year) but has significant geological and ecological impacts over millions of years.

Soil Formation and Properties

Soil Horizons

• Soil forms through weathering of rocks and decomposition of organic matter over thousands of years, creating distinct layers called horizons. The O horizon is rich in organic material, while the A horizon (topsoil) contains a mix of minerals and organic matter critical for plant growth. Below are the B horizon (subsoil), rich in minerals, and the C horizon (weathered parent material), leading to bedrock.
• The composition of these layers determines soil fertility, water retention, and suitability for agriculture. Healthy soils support diverse ecosystems and provide essential ecosystem services such as nutrient cycling and water filtration.

Soil Properties

• Soil texture (proportions of sand, silt, and clay) influences water retention, drainage, and nutrient availability. Sandy soils drain quickly but hold fewer nutrients, while clay-rich soils retain water but may have poor aeration. Loam, a balanced mix of sand, silt, and clay, is considered ideal for agriculture.
• Other important properties include pH, which affects nutrient solubility, and cation exchange capacity (CEC), which measures a soil’s ability to hold positively charged ions essential for plant growth. Managing these properties is key to sustainable land use.

Earth’s Atmosphere Composition and Structure

Atmospheric Layers

• The atmosphere is divided into layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The troposphere, closest to Earth, is where weather occurs and contains most of the atmospheric mass. Above it, the stratosphere contains the ozone layer, which absorbs harmful UV radiation.
• Higher layers like the mesosphere and thermosphere play roles in filtering radiation and housing phenomena like the aurora borealis. Each layer has distinct temperature gradients, influencing atmospheric circulation and climate patterns.

Atmospheric Composition

• Earth’s atmosphere is composed primarily of nitrogen (~78%) and oxygen (~21%), with trace gases like argon, carbon dioxide, and water vapor. These gases are vital for life, climate regulation, and weather systems. Greenhouse gases like CO₂, methane, and water vapor trap heat, maintaining temperatures suitable for life.
• Human activities such as burning fossil fuels increase greenhouse gas concentrations, intensifying the greenhouse effect and contributing to global climate change. Understanding composition is essential for predicting and mitigating environmental impacts.

Global Wind Patterns and Atmospheric Circulation

Hadley, Ferrel, and Polar Cells

• Atmospheric circulation is driven by the uneven heating of Earth’s surface, creating large-scale convection cells. Hadley cells occur near the equator, where warm air rises, cools, and drops precipitation, creating tropical rainforests. The descending air at around 30° latitude creates deserts due to dry, high-pressure conditions.
• Ferrel cells operate between 30° and 60° latitudes, driven by interactions between Hadley and Polar cells, leading to variable weather patterns. Polar cells, near the poles, involve cold, dense air sinking and flowing toward lower latitudes. These cells are critical for distributing heat and moisture globally.

Coriolis Effect and Wind Belts

• The Coriolis effect, caused by Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection creates distinct wind belts, including trade winds, westerlies, and polar easterlies. These patterns influence ocean currents, precipitation, and storm formation.
• Understanding wind patterns is essential for predicting climate zones, seasonal weather, and how pollutants or airborne seeds disperse. They also connect directly to marine upwelling zones, which boost ocean productivity.

Earth’s Geography and Climate

Topography and Climate Interactions

• Geographic features like mountains, oceans, and latitude heavily influence climate. Mountain ranges create rain shadows, where moist air rises on the windward side, cools, and releases precipitation, leaving the leeward side dry. This explains why one side of a mountain may be lush while the other is desert-like.
• Proximity to large bodies of water moderates climate because water heats and cools more slowly than land. Coastal regions tend to have milder temperatures compared to inland areas at the same latitude, influencing agriculture and biodiversity.

Latitude and Solar Energy

• Latitude affects the angle and intensity of sunlight received, shaping climate zones. The equator receives direct sunlight year-round, creating warm, humid conditions, while the poles receive low-angle sunlight, leading to cold climates. These differences drive global atmospheric circulation and seasonal patterns.
• Seasonal changes in daylight hours are caused by Earth's tilt, influencing plant growth, animal migrations, and human activities. Latitude also determines biome distribution, linking directly to biodiversity patterns covered in earlier units.

El Niño–Southern Oscillation (ENSO) and La Niña

El Niño Conditions

• El Niño is a climate phenomenon where weakened trade winds allow warm water to shift eastward across the Pacific Ocean. This disrupts normal upwelling along the South American coast, reducing nutrient-rich water and decreasing fish populations. It also shifts global weather patterns, often bringing heavy rainfall to the Americas and drought to parts of Asia and Australia.
• These changes can lead to flooding, crop losses, and economic disruption, especially in agriculture and fisheries. The effects can last for months to over a year, showing the global reach of localized ocean-atmosphere interactions.

La Niña Conditions

• La Niña is essentially the opposite phase, where stronger-than-normal trade winds push warm water westward, enhancing upwelling along South America. This results in cooler-than-average sea surface temperatures in the eastern Pacific and typically increases marine productivity. La Niña often brings drier conditions to the Americas and heavier rainfall to Asia and Australia.
• While La Niña is generally considered beneficial for fisheries due to nutrient upwelling, it can still disrupt agriculture and cause extreme weather events elsewhere. Both El Niño and La Niña cycles are part of the ENSO system, which repeats every 2–7 years, making it a key focus of climate prediction efforts.

Watersheds and Freshwater Resources

Definition and Structure of Watersheds

• A watershed is an area of land where all water drains into a single body, such as a river, lake, or ocean. The boundaries are determined by topography, with ridgelines and high points directing water flow. Understanding watersheds is crucial for managing freshwater quality and availability because upstream actions directly affect downstream ecosystems.
• Watersheds also link terrestrial and aquatic ecosystems, transporting nutrients, sediments, and pollutants. This connection means that land use, agriculture, and urbanization in one part of the watershed can influence biodiversity, water chemistry, and flood risk far downstream.

Human Impacts on Watersheds

• Deforestation, urbanization, and agriculture can alter the hydrology of watersheds by increasing runoff, reducing infiltration, and changing sediment loads. Increased runoff can lead to flooding, erosion, and the transport of pollutants into water bodies. Restoring riparian buffers and wetland areas can help mitigate these effects.
• Watershed management often involves balancing human needs for water with ecological requirements for habitat stability. This requires integrating hydrology, geology, and ecology — concepts already introduced in earlier Earth systems topics.

Solar Radiation and Seasons

Solar Energy Distribution

• Solar radiation is unevenly distributed due to Earth’s curvature, tilt, and rotation. Equatorial regions receive more direct sunlight year-round, while polar regions receive low-angle sunlight, creating large temperature gradients. These gradients drive atmospheric circulation, influencing climate patterns and weather systems.
• The amount of incoming solar energy also varies daily and seasonally, affecting photosynthesis, primary productivity, and migration patterns. This connects directly to earlier discussions on biomes and global biodiversity distribution.

Seasons and Earth's Tilt

• Seasons are caused by the 23.5° tilt of Earth’s axis, not by changes in distance from the Sun. As Earth orbits, different hemispheres receive varying amounts of daylight and sunlight intensity. This variation affects temperature, precipitation patterns, and biological cycles like flowering and breeding.
• Increased seasonality toward higher latitudes influences agriculture, ecosystem productivity, and species adaptations. Understanding this pattern helps explain why certain crops or species are restricted to specific regions.

Climate and Biomes

Climate Determinants

• Climate is shaped by long-term patterns of temperature and precipitation influenced by latitude, altitude, ocean currents, and wind patterns. The interaction between these factors determines regional ecosystems and their productivity. For example, warm ocean currents like the Gulf Stream can make nearby climates milder than expected for their latitude.
• Altitude acts similarly to latitude in influencing climate — higher elevations tend to be cooler and receive more precipitation, affecting vegetation and wildlife. This vertical zonation creates microclimates that can support unique species.

Biome Distribution

• Biomes are large ecological regions defined by climate, vegetation, and wildlife. Tropical rainforests, deserts, tundras, and grasslands each occupy specific climate zones shaped by global atmospheric and oceanic patterns. Human activity, such as deforestation and urban expansion, is altering biome boundaries worldwide.
• Because climate influences biome structure, climate change can shift biome locations, causing disruptions in biodiversity and ecosystem services. This directly connects to earlier biodiversity and conservation discussions in Unit 2.

Common Misconceptions — APES Unit 4: Earth Systems and Resources

Misconception 1: Earth’s seasons are caused by its distance from the Sun

• Many students think that summer happens when Earth is closest to the Sun and winter when it is farthest. In reality, seasons are caused by the 23.5° tilt of Earth’s axis, which changes the angle and intensity of sunlight throughout the year. This means that when it is summer in one hemisphere, it is winter in the other, even though both hemispheres are the same distance from the Sun.

Misconception 2: All soils are equally fertile and interchangeable

• Students often assume that soil fertility is the same everywhere and can easily support crops. Soil properties — including nutrient content, pH, and texture — vary greatly depending on climate, parent material, and biological activity. For example, tropical rainforest soils can be nutrient-poor despite lush vegetation, which connects back to earlier discussions on nutrient cycling and biome differences.

Misconception 3: Global wind patterns are random

• Some believe wind patterns are unpredictable and unrelated to global systems. In fact, they are driven by consistent factors like uneven solar heating, the Coriolis effect, and pressure gradients. Understanding these patterns is essential because they influence climate zones, ocean currents, and even the distribution of biomes, tying directly to earlier biodiversity concepts.

Misconception 4: Watershed boundaries can be easily altered without ecological impact

• It is a common misunderstanding that altering river courses or drainage systems has minimal environmental effect. Changing watershed boundaries can disrupt nutrient flow, sediment transport, and aquatic habitats downstream. These impacts connect to Unit 2 topics on ecosystem health and biodiversity loss, showing how interconnected Earth systems are.

Misconception 5: ENSO events only affect local climates

• Some assume that El Niño and La Niña events only influence weather in the Pacific Ocean region. In reality, these events disrupt global climate systems, altering precipitation patterns, storm intensity, and temperature anomalies worldwide. This explains why droughts in Africa, floods in South America, and changes in fisheries in Asia can all be linked to ENSO phases.