2.1a Nearly all the atmosphere is confined to a thin shell surrounding Earth. The atmos-

phere is a mixture of gases, including nitrogen and oxygen with small amounts of water

vapor, carbon dioxide, and other trace gases. The atmosphere is stratified into layers,

each having distinct properties. Nearly all weather occurs in the lowest layer of the


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


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


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.