Earth Science
Earth Science is the study of our planet and the processes that shape it. It covers what is happening deep beneath your feet right now, how the continents got to where they are, how rocks form and change over millions of years, and what drives the weather outside your window. This guide walks through the major topics you will encounter in an earth science course.
Earth's Layers
Earth is not a solid ball of the same material throughout. It has distinct layers, each with different composition, temperature, and physical properties.
CROSS-SECTION OF EARTH (not to scale)
___________
/ \
/ CRUST \ 0 - 70 km
| ___________ |
| / \ |
| / UPPER \ |
|| MANTLE || 70 - 660 km
|| __________ ||
|| / \ ||
||| LOWER | ||| 660 - 2,890 km
||| MANTLE | |||
||| _________ | |||
|||| OUTER || ||| 2,890 - 5,150 km
|||| CORE || ||| (liquid iron/nickel)
|||| ______ || |||
||||/INNER \ || ||| 5,150 - 6,371 km
||||\ CORE / || ||| (solid iron/nickel)
|||| \____/ || |||
| Layer | Depth (approx.) | State | Composition | Temperature |
|---|---|---|---|---|
| Crust | 0–5 km (ocean) / 0–70 km (continent) | Solid | Silicate rocks (granite, basalt) | 0–900°C |
| Mantle | 70–2,890 km | Mostly solid (plastic/ductile) | Silicate rocks rich in iron and magnesium | 900–3,700°C |
| Outer Core | 2,890–5,150 km | Liquid | Iron and nickel | ~3,700–4,300°C |
| Inner Core | 5,150–6,371 km | Solid (despite high temp) | Iron and nickel | ~5,000–6,000°C |
Plate Tectonics
Earth's crust and the uppermost part of the mantle together form a rigid layer called the lithosphere. This lithosphere is broken into about 15 major pieces called tectonic plates. These plates float on the partially molten rock of the upper mantle (the asthenosphere) and move slowly — about 2 to 10 centimeters per year.
Why Do Plates Move?
The driving force behind plate movement is convection currents in the mantle. Hot rock deep in the mantle rises toward the surface, cools, spreads sideways, then sinks back down as it becomes denser. This circular movement drags the plates along the surface, similar to the way foam pieces floating on boiling water move when the water circulates beneath them.
Types of Plate Boundaries
The most geologically active events on Earth (earthquakes, volcanoes, mountain building) occur at the borders between plates, called plate boundaries.
| Boundary Type | Plate Movement | Features Formed | Real-World Example |
|---|---|---|---|
| Convergent | Plates move toward each other | Mountains, ocean trenches, volcanic arcs; one plate subducts under the other | Himalayas (India + Eurasia); Andes (South American + Nazca plates) |
| Divergent | Plates move apart | Mid-ocean ridges, rift valleys; new ocean floor created | Mid-Atlantic Ridge; East African Rift Valley |
| Transform | Plates slide horizontally past each other | Fault lines; frequent earthquakes | San Andreas Fault (California) |
The Rock Cycle
Rocks do not stay the same forever. They are constantly being created, broken down, and transformed. The rock cycle describes the processes that change one type of rock into another over millions of years.
The Three Rock Types
Igneous Rock
Igneous rock forms when magma (molten rock underground) or lava (magma that has reached the surface) cools and solidifies. Intrusive igneous rock (like granite) forms slowly underground and has large crystals. Extrusive igneous rock (like basalt or obsidian) forms quickly at the surface and has small crystals or a glassy texture.
Sedimentary Rock
Sedimentary rock forms when particles (sediment) like sand, silt, shells, or organic matter are deposited in layers, buried, and compressed over time in a process called lithification. This rock type often contains fossils. Examples include sandstone, limestone, and shale.
Metamorphic Rock
Metamorphic rock forms when existing rock (of any type) is subjected to intense heat and pressure that change its mineral structure without melting it. Examples: marble (metamorphosed limestone), slate (metamorphosed shale), quartzite (metamorphosed sandstone).
THE ROCK CYCLE
Magma --> (cooling) --> Igneous Rock
|
(weathering & erosion)
|
v
Sediments
|
(burial & compaction)
|
v
Sedimentary Rock
|
(heat & pressure)
|
v
Metamorphic Rock
|
(melting)
|
v
Magma (cycle continues)
Any rock type can also be melted into magma or compressed
into metamorphic rock, making the cycle non-linear.
The Water Cycle
The water cycle (also called the hydrological cycle) describes the continuous movement of water through Earth's atmosphere, land, and bodies of water. No new water is created; the same water molecules have been cycling through this system for billions of years.
Key Stages
| Stage | Description |
|---|---|
| Evaporation | Heat from the sun causes liquid water in oceans, lakes, and rivers to become water vapor and rise into the atmosphere. Transpiration from plants also adds water vapor (together called evapotranspiration). |
| Condensation | As water vapor rises, it cools. Cool air cannot hold as much vapor, so the vapor condenses into tiny water droplets that form clouds and fog. |
| Precipitation | When water droplets in clouds combine and grow heavy enough, they fall as rain, snow, sleet, or hail. |
| Collection / Runoff | Precipitation collects in oceans, rivers, and lakes, or soaks into the ground (groundwater). Runoff flows over the land back to bodies of water, completing the cycle. |
Weather vs. Climate
These two terms are often confused but refer to different things:
- Weather is the state of the atmosphere at a specific place and time. It changes from day to day: sunny today, rainy tomorrow, cold next week.
- Climate is the long-term average pattern of weather in a region over at least 30 years. A desert has a dry climate even if it rains occasionally. A rainforest has a wet climate even if there are dry spells.
Types of Clouds
Clouds are classified by their shape and altitude. The two main shape categories are cumulus (heaped/puffy) and stratus (layered/flat). The prefix cirro- indicates high altitude clouds (above 6,000 m), alto- indicates mid-level clouds (2,000–6,000 m), and no prefix indicates low clouds (below 2,000 m). Nimbo- or -nimbus means rain-producing.
| Cloud Type | Altitude | Description | Associated Weather |
|---|---|---|---|
| Cirrus | High (>6,000 m) | Thin, wispy, white streaks made of ice crystals | Fair weather; may indicate change coming |
| Cirrostratus | High | Thin, sheetlike, often creates halo around sun/moon | Rain or snow likely within 24 hours |
| Altostratus | Mid (∼2,000–6,000 m) | Gray or blue-gray sheet covering sky; sun appears watery | Continuous rain or snow approaching |
| Altocumulus | Mid | White or gray puffy patches in rows or waves | Generally fair; possible afternoon thunderstorms |
| Stratus | Low (<2,000 m) | Gray, uniform layer like fog not reaching ground | Overcast, drizzle possible |
| Nimbostratus | Low to mid | Dark, thick, uniform layer blocking sunlight | Steady rain or snow; no thunder |
| Cumulus | Low to mid | Puffy, white, flat-bottomed clouds with clear sky between | Fair weather |
| Cumulonimbus | Low to very high (can reach >12,000 m) | Massive, towering anvil-shaped cloud | Thunderstorms, heavy rain, hail, tornadoes |
Frequently Asked Questions
How do scientists know what is inside the Earth if no one has ever drilled that deep?
Scientists study Earth's interior primarily through seismic waves — vibrations produced by earthquakes. Different types of seismic waves (P-waves and S-waves) travel at different speeds and behave differently depending on the material they pass through. P-waves (compressional waves) can travel through solids and liquids. S-waves (shear waves) can only travel through solids. By studying how waves created by an earthquake change speed and direction as they travel through Earth and emerge at stations around the globe, geologists can map the boundaries between layers and determine whether each layer is solid or liquid. The absence of S-waves on the far side of Earth from an earthquake revealed that the outer core is liquid.
What is the difference between magma and lava?
Magma and lava are both molten rock, but the term used depends on location. Magma is molten rock that is still underground, within or below Earth's crust. When magma reaches Earth's surface through a volcanic eruption, it is called lava. Once lava cools and solidifies on the surface, it becomes extrusive igneous rock. The distinction is purely about where the molten rock is: underground = magma, on the surface = lava.
Can plates move fast enough for us to notice in a human lifetime?
Tectonic plates move about 2 to 10 centimeters per year — roughly the same speed at which your fingernails grow. Over a human lifetime (say, 80 years), a fast-moving plate could shift up to 8 meters. We cannot see this directly, but scientists can measure it precisely using GPS satellites. In geologic time, these small movements add up enormously: 2.5 cm/year over 100 million years moves a plate 2,500 km, which is how continents that were once joined have drifted to opposite sides of the globe.
What causes earthquakes and where do most of them happen?
Earthquakes happen when stress that has built up along fault lines (fractures in Earth's crust) is suddenly released, sending out seismic waves in all directions. Most earthquakes occur at or near plate boundaries, because that is where plates are grinding past each other, colliding, or pulling apart. The "Ring of Fire" — a horseshoe-shaped zone around the Pacific Ocean — is where about 90% of the world's earthquakes and 75% of its volcanoes occur, because it marks the boundaries of multiple large plates. Earthquakes can also happen far from plate boundaries along ancient faults, though these are usually less frequent and often smaller in magnitude.
Quick Quiz
Check your understanding. Click an answer to see if you got it right.