2.+Many+of+the+phenomena+that+we+observe+on+Earth+involve+interactions+among+the+elements

MAJOR UNDERSTANDINGS 2.1a Nearly all the atmosphere is confined to a thin shell surrounding Earth. 2.1b As altitude increases, air pressure decreases.  2.1c The rock at Earth's surface forms a nearly continuous shell around Earth called the lithosphere. 2.1d The majority of the lithosphere is covered by a relatively thin layer of water called the hydrosphere. 2.1e Rocks are composed of minerals. Only a few rock-forming minerals make up most of the rocks of Earth. Minerals are identified on the basis of physical properties such as streak, hardness, and reaction to acid. 2.1f Fossils are usually found in sedimentary rocks. Fossils can be used to study past climates and environments. 2.1g The dynamic processes that wear away Earth's surface include weathering and erosion. 2.1h The process of weathering breaks down rocks to form sediment. Soil consists of sediment, organic material, water, and air. 2.1i Erosion is the transport of sediment. Gravity is the driving force behind erosion. Gravity can act directly or through agents such as moving water, wind, and glaciers. 2.1j Water circulates through the atmosphere, lithosphere, and hydrosphere in what is known as the water cycle. 2.2a The interior of Earth is hot. Heat flow and movement of material within Earth cause sections of Earth's crust to move. This may result in earthquakes, volcanic eruption, and the creation of mountains and ocean basins. 2.2b Analysis of earthquake wave data (vibrational disturbances) leads to the conclusion that there are layers within Earth. These layers--the crust, mantle, outer core, and inner core--have distinct properties. 2.2c Folded, tilted, faulted, and displaced rock layers suggest past crustal movement. 2.2d Continents fitting together like puzzle parts and fossil correlations provided initial evidence that continents were once together. 2.2e The Theory of Plate Tectonics explains how the 'solid' lithosphere consists of a series of plates that 'float' on the partially molten section of the mantle. 2.2f Plates may collide, move apart, or slide past one another.  2.2g Rocks are classified according to their method of formation. The three classes of rocks are sedimentary, metamorphic, and igneous.  2.2h The rock cycle model shows how types of rock or rock material may be transformed from one type of rock to another. 2.2i Weather describes the conditions of the atmosphere at a given location for a short period of time. 2.2j Climate is the characteristic weather that prevails from season to season and year to year. 2.2k The uneven heating of Earth's surface is the cause of weather. 2.2l Air masses form when air remains nearly stationary over a large section of Earth's surface and takes on the conditions of temperature and humidity from that location. 2.2m Most local weather condition changes are caused by movement of air masses. 2.2n The movement of air masses is determined by prevailing winds and upper air currents. 2.2o Fronts are boundaries between air masses. Precipitation is likely to occur at these boundaries. 2.2p High-pressure systems generally bring fair weather. Low-pressure systems usually bring cloudy, unstable conditions. The general movement of highs and lows is from west to east across the United States. 2.2q Hazardous weather conditions include thunderstorms, tornadoes, hurricanes, ice storms, and blizzards. Humans can prepare for and respond to these conditions if given sufficient warning. 2.2r Substances enter the atmosphere naturally and from human activity. Some of these substances include dust from volcanic eruptions and greenhouse gases such as carbon dioxide, methane, and water vapor. These substances can affect weather, climate, and living things.