EARTH SYSTEM – EARTH AND SPACE SCIENCE

The Structure of Earth

Earth is made up of three main layers, each with its own unique materials, properties, and conditions.

1.  The Crust: The crust is the outermost and thinnest layer of Earth. It includes dry land (continents) and the ocean floor.
2. Oceanic crust: Made mostly of basalt. Dense, dark, and finely textured.
3. Continental crust: Made mostly of granite. Lighter, less dense, with larger crystals. The crust makes up less than 1% of Earth’s total volume.

       2. The Mantle: Lies directly beneath the crust. Composed mainly of silicate rocks with iron, nickel, and magnesium. The uppermost mantle together with the crust forms the lithosphere, a rigid, solid layer. Below the lithosphere is the asthenosphere: Hotter and under greater pressure. Rock here is plastic-like and flows slowly, allowing tectonic plates to move.

     3. The Core: Two parts:

• Outer Core: Molten (liquid) metal. Mainly iron and nickel.
• Inner Core: Solid and extremely dense. Also composed of iron and nickel.

Together, the core is nearly the size of the Moon!

Note: The movement of the liquid outer core generates Earth’s magnetic field.

Tectonic Plates

Earth’s lithosphere is broken into tectonic plates. Think of them like giant puzzle pieces covering Earth’s surface. There are seven primary plates and many smaller plates. Plates are named after the regions they lie near (for example, the Pacific Plate, African Plate, etc.).

Plate Movement

Tectonic plates “float” on the soft, flowing asthenosphere. Convection currents in the mantle: Hot molten rock rises. Cooler rock sinks. This cycle moves the plates 1–2 centimeters per year.  Even though it sounds slow, over millions of years this movement dramatically reshapes Earth’s surface.

Effects of Plate Movement

The movement of tectonic plates explains many of Earth’s features:

•  Mountains – Form when plates collide and push land upward.
•  Ocean Basins – Vast low areas below sea level covering ~75% of Earth’s surface.
•  Continental Shelves – Shallow, gently sloping areas at the edge of continents.
• Deep-Sea Trenches – Extremely deep valleys in the ocean floor where plates converge.
• Mid-Ocean Ridges – Underwater mountain ranges where new crust forms as plates pull apart.

 Plate tectonics is the fundamental theory explaining Earth’s dynamic surface.

Crust – Earth’s outermost solid layer.

Mantle – Thick layer of hot rock below the crust.

Lithosphere – Rigid outer layer made of the crust and upper mantle.

Asthenosphere – Hot, plastic-like part of the mantle that flows.

Core – Center of Earth, with a liquid outer core and solid inner core.

Tectonic Plates – Large pieces of Earth’s lithosphere that move.

Convection Currents – Circular currents in the mantle driving plate motion.

Ocean Basin – Large, deep areas of the ocean floor.

Continental Shelf – Shallow extension of a continent underwater.

Deep-Sea Trench – Very deep underwater valley.

Mid-Ocean Ridge – Underwater mountain range where new crust forms.

Types of Plate Boundaries

Earth’s lithosphere is divided into tectonic plates that constantly move. Where these plates meet, they form plate boundaries. There are three main types of plate boundaries: transform, divergent, and convergent.

1.  Transform Boundaries: At a transform boundary, two plates slide past each other horizontally. The movement is side by side rather than toward or away from one another. This movement can lock the plates together temporarily. When they suddenly slip, energy is released as an earthquake. A famous example is the San Andreas Fault in California.
2.  Divergent Boundaries: At a divergent boundary, two tectonic plates move apart or pull away from each other. As the plates separate, magma from the mantle rises to fill the gap. When the magma cools, it forms new crust. This process often creates mid-ocean ridges, like the Mid-Atlantic Ridge, where new ocean floor is continuously formed. Divergent boundaries can also occur on land, forming rift valleys.
3. Convergent Boundaries: At a convergent boundary, two plates collide with each other. This collision can involve: Two oceanic plates; Two continental plates, One oceanic and one continental plate

 The relative density of the plates determines which plate is forced down.

Features Formed at Convergent Boundaries

•  Subduction Zones: When an oceanic plate collides with a continental plate, the denser oceanic plate is forced down into the mantle in a process called subduction. This forms a deep-sea trench and often leads to volcanoes forming on the overriding plate. Example: The Mariana Trench is the deepest part of the ocean, formed at a subduction zone.
• Mountains: When two continental plates collide, neither is easily forced down. Instead, the crust buckles and folds upward. This creates fold mountains. Example: The Himalayas, formed by the collision of the Indian and Eurasian plates.
• Volcanoes: Subducting plates melt as they descend into the mantle. This melted rock (called magma) rises through the crust to the surface. When magma reaches the surface, it is called lava. Volcanoes are weak spots in Earth’s crust where this molten rock escapes.

Earthquakes and Faults

Earthquakes happen when stress builds up as plates move, then suddenly releases. The break in Earth’s crust where earthquakes occur is called a fault. Faults are most common along plate boundaries, especially transform boundaries where plates slide past one another. When the rock along a fault slips, it causes vibrations that travel through Earth’s crust.

Formation of the Atmosphere

Earth formed around 4.6 billion years ago. Over time, gases from Earth’s core were expelled through volcanic eruptions. These gases collected around Earth, forming an envelope of gases called the atmosphere. The atmosphere is most dense at sea level and becomes thinner with increasing altitude.

Layers of the Atmosphere

Earth’s atmosphere is structured in layers, each with distinct properties. These layers include:

• Troposphere – closest to Earth’s surface, where weather occurs.
• Stratosphere – contains the ozone layer, protects life by absorbing UV radiation.
• Mesosphere – where meteors burn up.
• Thermosphere – very high temperatures, auroras occur here.
• Exosphere – outermost layer, merging into space.

Atmospheric Gases

The air we breathe (in the troposphere) is a mixture of gases: 78% nitrogen, 20% oxygen. Trace amounts of other gases (e.g., carbon dioxide, hydrogen) These gases trap heat from the sun and support life. The greenhouse effect describes how gases like water vapor, carbon dioxide, and methane trap solar energy, keeping Earth warm enough to sustain life.

Human Impact on the Atmosphere

• Greenhouse Gases: Human activities release extra greenhouse gases, increasing global temperatures. This warming is called global warming and can lead to climate change.
• Ozone Layer: The ozone layer (O₃) in the stratosphere acts as a protective shield, filtering out harmful ultraviolet (UV) radiation.
• Ozone Depletion: Chemicals called chlorofluorocarbons (CFCs) were used in refrigerators and sprays. CFCs rise to the stratosphere and break down ozone molecules, thinning the ozone layer and reducing its protective effect.

 Weathering and Erosion

Weathering: Weathering is the breakdown or dissolving of rocks and minerals on Earth’s surface. It can be:

• Chemical weathering – changes the chemical composition of rocks. Example: Carbon dioxide + water → carbonic acid → dissolves limestone. Rust (oxidation) also causes chemical weathering.
• Mechanical (Physical) weathering – breaks rocks into smaller pieces without changing their composition. Example: Freeze-thaw – water in cracks freezes and expands, breaking rock apart.

Erosion: Erosion is the movement of weathered materials by wind, water, ice, or gravity. It transports broken rock from one location to another. Wind erosion carries sand and dust, causing wind abrasion, which wears rocks down (like sandblasting). Water erosion includes rivers cutting valleys and rain-washing soil away.

Wind: Wind is the movement of air caused by uneven heating of Earth’s surface. The sun warms the equator more than the poles, creating low-pressure systems at the equator and high-pressure systems at the poles. Air moves from high to low pressure, generating wind. The Coriolis effect (due to Earth’s rotation) causes winds to curve: Counterclockwise in the Northern Hemisphere, Clockwise in the Southern Hemisphere

Prevailing winds blow from consistent directions in specific regions. Where they meet are convergence zones, leading to complex weather patterns.

The Oceans

Composition: More than 70% of Earth’s surface is covered by ocean water—a mixture of water and dissolved salts: Average salinity: ~3.5% (35 grams of salt per 1000 grams of seawater), Main dissolved ions: chloride, sodium, sulfate, magnesium, calcium, potassium, Dissolved gases include oxygen, nitrogen, carbon dioxide. Ocean water is denser than freshwater because of these salts.

Ocean Currents: Currents are streams of water moving through the ocean. They are driven by: Wind patterns, Earth’s rotation (Coriolis effect), Differences in water temperature and salinity

Surface Currents: Move warm water from the equator toward the poles. Example: Gulf Stream – a warm current influencing climate far from its origin.

Deep Currents: Cold, dense water sinks and moves along the ocean floor. Upwelling occurs when winds push surface water away, allowing nutrient-rich cold water to rise. Upwelling supports plankton growth, forming the base of the marine food chain.

El Niño

A climate phenomenon where warm surface currents in the Pacific Ocean disrupt normal patterns. Displaces the cold Humboldt Current off South America. Alters weather globally, causing floods, droughts, and storms.

Ocean Layers

• Surface Zone: Warmest, well-lit, extends to ~200 meters, ~5% of ocean depth.
• Twilight Zone: 200–1000 meters, cold and dim.
• Deep Zone: Dark, cold, and under high pressure.

Coral Reefs

Coral reefs are found in the warm, shallow, and sunny surface zones of the ocean. They are formed by colonies of tiny marine animals called corals, which secrete hard, protective skeletons of calcium carbonate around their soft bodies. Over time, these skeletons accumulate to create large reef structures.

Inside the coral tissue live microscopic algae called zooxanthellae in a symbiotic relationship. These algae use photosynthesis to produce food, which they share with the coral host. In return, the coral provides the algae with a protected environment and access to sunlight.

Importance of Coral Reefs

Coral reefs are among the most biodiverse ecosystems on Earth: Although they cover less than 1% of Earth’s surface, coral reefs support about 25% of all marine species, providing food, shelter, and breeding grounds for fish, mollusks, crustaceans, and more. Coral reefs also provide significant benefits to humans, such as: Seafood resources and protein for coastal communities Natural barriers that protect shorelines from waves and storm surges Medicinal compounds for treating diseases Tourism income and recreation opportunities

Threats to Coral Reefs

Despite their importance, coral reefs are under serious threat—mostly due to human activity:

• Pollution: Waste and chemicals from land can poison reef ecosystems.
• Destructive fishing: Practices like blast fishing and overfishing damage coral habitats.
• Ocean acidification: Caused by excess carbon dioxide, making it harder for corals to build skeletons.
• Climate change: Warmer ocean temperatures lead to coral bleaching, where corals expel their algae and turn white, often dying.
• Invasive species: Non-native organisms can outcompete or prey on reef species.