Chapter 4 Earth’s Structure & Motions

Formation of Earth Big Bang ~ 13.7 Billion Year ago ~4.6 BYA rotating gases & dust compressed into our solar system members.

Earth’s appearance Oblate spheroid 29% land mass 71% water Demonstrated by measuring gravitational forces Larger around equator. 29% land mass 71% water

Inside Earth Nebular hypothesis believes original planet looked much like current moon. Inner core: solid Fe, Ni Outer core: liquid Fe, Ni Mantle: thick layer of Fe, Si, Mg. Crust: thin, rigid outer layer

Subdivisions of upper layers Properties subdivide upper regions of the mantle: Lithosphere: upper portion of mantle and lower crust. always moving, but very slowly broken into huge sections called tectonic plates extreme heat from the mantle makes it easier for the plates to move

Asthenosphere: malleable layer of the mantle white hot and can be deformed. not liquid but contains small amount of melted rock more solid than liquid

Note the depth of layers and temp changes.

It’s Hot Down There! Sources of internal heat Meteorite impacts Weight of overlying materials (gravitational contraction) Radioactive decay Earth is slowly losing heat due to variations in: rock types crust thickness

Magnetic Field Origins Hypothesis: field caused by movement of the outer core. Generates electrical field as it flows across solid inner core.

Earth Goes Round and Round 24 hour rotation/day actual time = 24 h, 56 m, 4 sec Proof of Rotation: Foucault pendulum proved rotation Coriolis effect shows deflection of wind rather than straight line path (turns right relative to surface in N.Hem)

Tilted Tilted at 23.5° from vertical. Pointed toward Polaris (N. Star) Parallelism: always pointing toward same object in sky. Precession: gradual shift in the orientation of Earth's axis of rotation a (26,000 yr. cycle)

Rotation Average rate = 15°/hr. Distance traveled varies by latitude. Rotation = degrees/time unit Speed of rotation = distance/time unit In the typical time it takes to read this sentence (about 15 seconds), the Earth will have moved through space by about 450 km, or about the width of the state of Ohio. Distance traveled varies by latitude. Ex: equatorial speed = 1690 km/hr. polar speed = ~0 km/hr

Rotational Effects Solar noon: sun’s maximum altitude overhead. Sun’s position appears to move 1°/4 minutes due to rotation. Standard Time Zones created at 15° longitude creating 24 time zones. Time meridian: center of any time zone and is divisible by 15. Prime meridian: Greenwich, England (0° longitude) point for adjusting time forward or backward. International Date Line: mid-Pacific (180 ° longitude)

Revolution Proof of revolution around the Sun. Parallax: apparent shift of stars in sky. Rate of Revolution: direction of revolution is the same as direction of rotation (clockwise when viewed looking down over N. Pole). Elliptical orbit: perihelion: nearest the sun aphelion: farthest from sun

Rotation makes sun appear to move across the sky while revolution around the sun causes the apparent path to change over the course of the year. Zenith: point directly above observer’s head. Thus when sun is at zenith = 90° sun on horizon = 0°

Axial tilt of 23.5° = the distance from equator to Tropic of Cancer (23.5°N) and Tropic of Capricorn (23.5°S). Named lines of latitude: Sun directly over these lines during the solstices! Tilt determines the seasons.

Neither hemisphere tilts toward sun: Vernal equinox: Mar 21 Autumnal equinox: Sept 23 Longest day in N. Hemisphere : Summer solstice: June 22 (Tropic of Cancer 23.5°N) Shortest day in N. Hemisphere: Winter Solstice: Dec 21 (Tropic of Capricorn 23.5°S) (Dates are approximate as they do shift)