Lecture 2 The Earth. I. The Interior. Earth vital statistics. Planet size & mass determination. Orbital semi-major axis x 10 8 km (1.

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1 Lecture 2 The Earth. I. The Interior Vital statistics Size & mass of a planet Gravity & magnetic mapping Seismology Structure of Earth s interior Plate tectonics Reminder: lectures at Earth vital statistics Orbital semi-major axis x 10 8 km (1.0 AU) Orbital period days Rotational period hours (sidereal day) Inclination of rotation axis º Eccentricity of orbit Diameter (at equator) km Mass x kg Mean density (observed) 5520 kg m -3 Planet size & mass determination Size Measure distance and angular size to get actual size. Baseline = Parallax Diameter = ang. diameter 2π x distance 360 o 2π x distance 360 o Size of Earth - Eratosthenes (~200 B.C.) measured size using angular displacement of the Sun at same time at 2 locations. He used Syene and Alexandria 5000 stadia (780 km) apart. Using his value of 7.2 degrees get radius =? 6200 km (cf km actual!) 1

2 Determine the Astronomical Unit Mass Use radar and apply Newton/Kepler s laws: Bounce radar off Venus. 1 AU = 149,597,870.7 km Now defined by IAU Newton s laws of motion: Bodies of masses M and m in an elliptical orbit: M+m = 4π 2 a 3 /GP 2 and M/m = r m /r M Where P = period, a = semi-major axis and r m and r M = distance of M and m from the centre of mass. If m << M then a 3 P 2 Kepler s third law. Back to observed density of Earth (5520 kg m -3 ) Uncompressed densities of some important compounds Substance Formula Density (kg m -3 ) Liquid water H 2 O 998 Hydrated minerals (X is rock, n 1) X(H 2 O) n <2000 Diopside (a pyrozene) CaMgSi 2 O Forsterite (an olivine) Mg 2 SiO Alkali feldspars (Na, K)AlSi 3 O Iron-nickel Fe + 6% Ni 7925 Australia: Red = stronger g Blue = weaker g Geoid is the surface of equal g Gravity Mapping Small variation in gravitational acceleration. Can also be used to map Ocean/Ice levels. 2

3 Australia: Red = stronger Blue = weaker Magnetic Mapping Earthquakes and Seismology Boxing Day 2004 earthquake, magnitude 9.1, Indian Ocean largest earthquake for 40 years, lasted ~10 minutes Presume planets with strong magnetic field have an internal dynamo converting the kinetic energy of a conducting, moving fluid into magnetic energy. Strong evidence for molten material inside Earth Seismic recording from two islands in the Indian Ocean: Cocos Island and Diego Garcia (~1000 km apart) Two main types of seismic waves Primary (P) are longitudinal pressure waves Secondary (S) are transverse sheer waves Waves speed varies due to differences in density (faster at lower density) and temperature (refraction). Observed seismic waves P waves travel at 5-6 km s -1. Go through solids, liquids and gases. S waves travel at 3-4 km s -1. They cannot pass through liquid. Shadow zone receives neither P nor S waves (refraction/liquid). Faint P waves seen in the shadow zone must have been reflected off an inner solid core. 3

4 Earth the present Most geological activity occurs at plate boundaries. Most boundaries are compression boundaries marked by mountain ranges where plates collide (e.g. Himalayas) and/or deep trenches, where old crust is subducted (e.g. Peru-Chile Trench). Cause the deepest and strongest earthquakes (~700 km) Earth sea-floor crustal age: Red = young (10 Myr); blue = old (200 Myr) Extension boundaries due to separation of plates on the ocean floor (e.g. mid-atlantic ridge). Form new crust from the mantle material. Determine age from magnetic striping and radioactivity. Shallow earthquakes (<25 km). Also have transform boundaries where plates slip past each other (e.g. San Andreas fault). Shallow, violent earthquakes. Tectonic activity can resurface most of the Earth in ~500 Myr 4

5 Volcanoes Terrestrial Impact Craters Near plate boundaries magma comes up due to stress fracture in crust. Etna (left) is an example. Over hot-spot a buoyant mantle plume. Hawaiian island chain best example, e.g. Kilauea (right). Hot spot moves creating chain. Highest most recent hence Big Island. Barringer Meteor crater, Arizona Radius 0.6 km, Age 0.05 Myr Chicxulub, Yucatan Peninsula Radius 85 km, Age 65 Myr Few craters (<200) known on Earth due to erosion (weathering), volcanic resurfacing and tectonic activity. Most < 500 Million years old. Oldest ~2 billion years old. If impact crater size > 1 km object impactor melts completely. Should we be worried? The End 5