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Unit 4
10–15% of examThis unit covers the physical Earth that every ecosystem sits on: moving tectonic plates, soil, the atmosphere, wind and water. You'll see how uneven heating by the sun drives winds, seasons and climate, and how changing ocean temperatures in the Pacific can shift weather around the world.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Earth's outer shell is broken into plates that slowly move. Where plates collide (convergent boundaries) you get mountains, island arcs, volcanoes and earthquakes; where they pull apart (divergent boundaries) you get seafloor spreading and rift valleys; and where they slide past each other (transform boundaries) you get earthquakes.
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Soil forms when parent material (the underlying rock) is weathered and broken down, moved, and mixed with organic matter from living things. It builds up in layers called horizons, and it can be eroded by wind and water, which also hurts water quality because healthy soil filters water.
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A soil's mix of sand, silt and clay controls how much water it holds and how fast water drains through it. You can identify a soil type with a soil texture triangle; loam, a balanced mix, supports many crops, and soil quality also depends on nutrients, organic matter and pH.
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The atmosphere is mostly nitrogen (about 78%) and oxygen (about 21%), with small amounts of argon, carbon dioxide and water vapor. It's divided into layers by how temperature changes with height: the troposphere, where weather happens, then the stratosphere, which holds the ozone layer, then the mesosphere, thermosphere and exosphere.
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Global winds start because sunlight is most intense at the equator. Warm, moist air rises there, expands and cools, drops its rain, then flows toward the poles high up and sinks around 30° north and south, a loop called a Hadley cell. Earth's rotation bends the path of moving air (the Coriolis effect), producing the trade winds, the westerlies and the polar easterlies.
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A watershed is all the land that drains into a particular river, lake or other body of water. Its size, slope, soil and plants affect how water flows across it, and what people do anywhere in a watershed, like farming or building, can change the water quality downstream.
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Incoming solar radiation (insolation) is strongest near the equator, where sunlight hits the ground most directly, and weaker toward the poles, where the same light is spread over more area. Seasons happen because Earth's axis is tilted, which changes the angle of sunlight and the number of daylight hours during the year.
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Climate depends on more than sunlight: mountains and oceans matter too. Air pushed up over a mountain cools and drops its rain on the windward side, leaving a dry rain shadow on the leeward side, and ocean currents carry heat that keeps some coastal areas milder.
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El Niño and La Niña are swings in Pacific Ocean surface temperatures and winds. During El Niño the trade winds weaken and warm water spreads east toward South America, cutting off the cold, nutrient-rich upwelling that feeds fisheries there and changing weather far away; La Niña is roughly the opposite, with stronger trade winds and cooler water in the eastern Pacific.
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