Big Idea 4. Earth is continuously changing


4.1    Earth’s geosphere changes through geological, hydrological, physical, chemical, and biological processes that are explained by universal laws. These changes can be small or large, continuous or sporadic, and gradual or catastrophic.

4.2    Earth, like other planets, is still cooling, though radioactive decay continuously generates internal heat. This heat flows through and out of Earth’s interior largely through convection, but also through conduction and radiation. The flow of Earth’s heat is like its lifeblood, driving its internal motions.

4.3    Earth’s interior is in constant motion through the process of convection, with important consequences for the surface. Convection in the iron-rich liquid outer core, along with Earth’s rotation around its axis, generates Earth’s magnetic field. By deflecting solar wind around the planet, the magnetic field prevents the solar wind from stripping away Earth’s atmosphere. Convection in the solid mantle drives the many processes of plate tectonics, including the formation and movements of the continents and oceanic crust.

4.4    Earth’s tectonic plates consist of the rocky crust and uppermost mantle, and move slowly with respect to one another. New oceanic plate continuously forms at mid-ocean ridges and other spreading centers, sinking back into the mantle at ocean trenches. Tectonic plates move steadily at rates of up to 10 centimeters per year.

4.5    Many active geologic processes occur at plate boundaries. Plate interactions change the shapes, sizes, and positions of continents and ocean basins, the locations of mountain ranges and basins, the patterns of ocean circulation and climate, the locations of earthquakes and volcanoes, and the distribution of resources and living organisms.

4.6    Earth materials take many different forms as they cycle through the geosphere. Rocks form from the cooling of magma, the accumulation and consolidation of sediments, and the alteration of older rocks by heat, pressure, and fluids. These three processes form igneous, sedimentary, and metamorphic rocks.

4.7    Landscapes result from the dynamic interplay between processes that form and uplift new crust and processes that destroy and depress the crust. This interplay is affected by gravity, density differences, plate tectonics, climate, water, the actions of living organisms, and the resistance of Earth materials to weathering and erosion.

4.8    Weathered and unstable rock materials erode from some parts of Earth’s surface and are deposited in others. Under the influence of gravity, rocks fall downhill. Water, ice, and air carry eroded sediments to lower elevations, and ultimately to the ocean.

4.9    Shorelines move back and forth across continents, depositing sediments that become the surface rocks of the land. Through dynamic processes of plate tectonics and glaciation, Earth’s sea level rises and falls by up to hundreds of meters. This fluctuation causes shorelines to advance and recede by hundreds of kilometers. The upper rock layers of most continents formed when rising sea levels repeatedly flooded the interiors of continents.