Earth's Systems: Plates, Rocks, Water and Air

Earth's layers and plate tectonics

In plain words: Earth's surface is broken into giant pieces that drift slowly, like crackers floating on thick soup.

Think of a peach: thin skin, thick fruit, a hard pit. Earth has a thin crust, a thick mantle, and a metal core. Heat from the core and mantle causes slow flowing motion (convection) in the mantle. The rigid lithosphere on top is cracked into about a dozen large plates that move 2–10 cm per year.

Earthquakes happen when rock along a fault suddenly slips and releases stored energy as seismic waves. P waves travel faster than S waves. So the gap between their arrival times tells how far away the epicenter is. It is like seeing lightning, then counting seconds until the thunder. Three stations are needed to pinpoint it (triangulation). On the magnitude scale, each whole number means 10 times more ground shaking and about 32 times more energy.

Volcanoes form where magma reaches the surface. This happens in three places: at divergent boundaries, above subducting plates, and over hot spots in the middle of plates (Hawaii).

Evidence for plate tectonics:

• The coastlines of South America and Africa fit together like puzzle pieces.
• The same fossils and rock types appear on continents now separated by oceans.
• Seafloor rock is youngest at mid-ocean ridges and older farther away.
• Earthquakes and volcanoes line up along plate edges.

The rock cycle

In plain words: rocks get recycled, just like cans and bottles, only much more slowly.

Rocks are recycled over millions of years. Any rock type can become any other.

• Igneous rock forms from cooled magma or lava. Slow cooling underground gives large crystals (granite). Fast cooling at the surface gives small crystals or glass (basalt, obsidian).
• Sedimentary rock forms through weathering → erosion → deposition → compaction → cementation. It often has visible layers and may contain fossils (sandstone, shale, limestone). In undisturbed layers, the oldest layer is at the bottom.
• Metamorphic rock forms when heat and pressure change a rock without melting it. This often squeezes minerals into bands. Examples: limestone → marble, shale → slate, sandstone → quartzite.
• Melting any rock produces magma, which starts the cycle again.

Example: A rock with rounded sand grains, layers, and a shell fossil is sedimentary. A rock with large, interlocking crystals and no layers is probably igneous (granite).

The water cycle

In plain words: the same water goes around and around — up as vapor, down as rain.

Think of a pot of soup with a lid. Steam rises, forms drops on the lid, and drips back down. Water moves constantly among oceans, air, land and living things. The Sun supplies the energy and gravity pulls water back down.

1. Evaporation: sunlight heats surface water, which becomes water vapor. Oceans supply most of it.
2. Transpiration: plants release vapor through their leaves.
3. Condensation: rising air cools, and vapor becomes liquid droplets that form clouds. Dew on grass is condensation too.
4. Precipitation: droplets combine and fall as rain, snow, sleet or hail.
5. Runoff and infiltration: water flows over land to rivers and the ocean. Or it soaks into the ground and becomes groundwater.

Evaporation absorbs heat and condensation releases heat. That released heat powers thunderstorms and hurricanes. About 97% of Earth's water is salty ocean water. Most fresh water is frozen in ice caps and glaciers or stored underground.

The atmosphere, weather and climate

The atmosphere is about 78% nitrogen, 21% oxygen and 1% other gases (argon, carbon dioxide, water vapor). It is divided into layers by how temperature changes with height:

Air pressure decreases steadily with height in every layer, because there is less air above pushing down.

Weather vs. climate. Weather is what the air is like at a certain time and place. It includes temperature, precipitation (rain or snow), wind, humidity and pressure. Weather is what you wear today; climate is what clothes you own. Climate is the long-term average (usually 30 years). "It snowed in Denver yesterday" is weather; "Denver averages roughly 50 to 60 inches of snow a year" is climate. A single cold day does not disprove a warming climate, because climate is about averages.

What controls climate:

• latitude — how far from the equator, which sets the sunlight angle;
• distance from the ocean — water heats and cools slowly, so coastal places have milder temperatures;
• elevation — higher is colder;
• ocean currents;
• mountains — the side facing away from moist winds is often dry. This is a rain shadow.

Ocean currents and heat

In plain words: oceans and winds act like a giant heating system that moves warmth around the planet.

The Sun heats the equator more than the poles. Ocean currents and winds spread this heat around the planet.

• Surface currents are driven by winds. Warm currents move from the tropics toward the poles (the Gulf Stream warms the east coast of the U.S. and western Europe). Cold currents move from polar regions toward the equator (the California Current cools the U.S. West Coast).
• Deep currents are driven by differences in density (how heavy water is for its size). Cold, salty water is denser. It sinks near the poles, then slowly flows along the ocean floor. This worldwide loop is sometimes called the global conveyor belt.
• Because air takes on the temperature of the water below it, coastal climates follow nearby currents. Two cities at the same latitude can have very different summers.