) most accessible and studied (Fig. 1.7)•2 elements (the big 2) make up 75% of total •In order: •Oxygen (0) and silicon (Si) •These two with six other elements (the big 8) make up 99% of the crust.
•These are: aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), sodium (Na). •Contrast this with the composition of the whole earth. Most abundant in order: iron, oxygen, silicon, magnesium. Most of Fe in the core. Most of Mg in the mantle. As we go into earth’s interior gets hotter. Rate is called geothermal gradient. T at earth’s center ~5,000 degree C. Consider heat (transfer). Heat is one form of energy – involves motion of atoms of matter. Rapid motion – high temperature heat always flows from place with high temp to one of low temp. How? Three methods.•1) Conduction – atoms pass their motion directly to neighbors. Good for metals (solids) but not for rocks. •2) Convection (Fig. 1.11) – atoms travel from place to place carrying heat. Good for liquids and gases. Also for rocks close to their melting point. Convection (through the asthenosphere) is an efficient form of heat loss from earth’s (hot) interior. •3) Radiation – heat transfer by electromagnetic waves – can travel through vacuum. Sun’s heat reaches earth’s surface by this process, but not important process inside the earth. Outer cool, rigid shell (lithosphere) moves around on underlying “plastic” or ductile layer (asthenosphere.) This moves continents and creates and destroys (intervening) oceans – leads to plate tectonic theory (Ch. 2) Lithosphere is broken up into smaller pieces called plates. Each of these is ~100 Km (thick), varies in surface size and each acts (moves) as an entity (Fig. 2.5). Some of these have only oceans at the surface; others have continents and oceans at the surface (Fig. 2.5) Look at places where plates interact (meet) – plate boundaries, these can be of 3 different types (Fig. 2.5)a)Divergent plate boundaries – two plates move away from each other (Fig. 2.5) many examples seen at earth’s surface. Look to the middle of the Atlantic Ocean – mid-Atlantic
ridge separates South America and African plates (Fig 2.6a) another… separates Nazca and Pacific plates. Divergent boundaries create oceans (e.g. Atlantic). b)Convergent plate boundaries- two plates move towards each other (Fig. 2.5) one of these bends and goes back into the earth’s interior – process called subduction. (The other plate “rides” on top). Denser plate (usually oceanic) subducts, lighter plate (continental) stays up. Examples:1)