1. Magnetism Bar magnets have distinct N & S poles –Break a bar magnet in two, & get two smaller magnets Like poles repel; unlike poles attract –Attraction & repulsion cause magnets to align themselves with external magnetic fields –Use magnetic field lines to denote magnetic field about a magnetic source –Field lines emerge from N poles, & enter S poles of magnets Magnetic field generated within Earth, called the geomagnetic field, surrounds us

2. Geomagnetic field Geomagnetic field lines are rarely horizontal and rarely aligned perfectly N-S Angle between horizontal plane & magnetic field line = inclination –Inclination is ~ 0° at equator & ~ 90° at poles Angle between direction toward true north and magnetic field line = declination –Declinations vary from point to point on earth at any one time, & vary from time to time at any one point on earth Intensity, magnitude, or strength of field varies with position on earth and with time –Strength is greatest at poles & weakest at equator

3. Properties of magnets Moving electric charges generate magnetic fields –In permanent magnets (ferromagnetic materials), magnetic fields due to electrons moving about nuclei & those due spin of electrons align & amplify each other When heated, ferromagnetic materials loose their magnetism at a temperature called the Curie temperature –Curie temperatures for ferromagnetic minerals are 200-700°C; minerals & rocks melt at 1000°-1400°C When cooled back below their Curie temperatures, ferromagnetic materials will reacquire their magnetism parallel to the ambient magnetic field –Thermal remanent magnetism = reacquired magnetism parallel to the ambient magnetic field

4. Geomagnetism Earth’s magnetic field is dominated by axially symmetric dipolar component –Other components exist, but are less important Dipolar axis is not exactly parallel to rotation axis Curie temperatures and geothermal gradient indicate that there is no bar magnet inside Earth TRM & DRM provide data on Earth’s magnetic field at different times in the past –1500 years ago, field strength = 1.5 x present magnitude –5500 years ago, field strength = 0.5 x present magnitude – Field has always been dominated by dipolar component, with dipole axis parallel to rotation axis on average –The field has had reversed polarity at different times in the past

5. Geomagnetic polarity reversals Recognize reversals in places where we have extensive history of volcanism - many rocks that have cooled through their Curie temperatures Roughly half of rocks have normal polarity; half have reversed polarity Reversals are worldwide & restricted to specific times in the past Pattern of normal & reversed polarity is sufficiently distinctive that we can assign ages to rocks in which we can recognize their TRM –Pattern is a song with two notes of different duration; distinctive rhythms in a well-known song

6. Sea floor magnetic patterns In ocean basins, recognize distinctive pattern of linear magnetic anomalies Positive & negative anomalies occur where field strength is greater than or less than expected value Regions of positive & negative anomaly occur in linear regions aligned parallel to MOR axial valleys Can identify identical anomalies on opposite sides of MOR Anomaly patterns truncated & offset by oceanic fracture zones

7. Geomagnetism & plate tectonics Recognize geomagnetic reversal pattern in linear magnetic anomalies, with partiular anomalies corresponding to rocks of specific ages When sea floor volcanic rocks cool through their Curie temperatures, they acquire TRM - that leads to anomalies Linear magnetic anomaly patterns –Confirm that rock farther from MOR is older –Corroborate earthquake first-motion studies that show plates to be diverging across MOR –Give plate velocities of 1-10 cm/yr

8. Hot spots PT predicts that volcanism on earth is either associated with divergent (MOR or rift) or with convergent (DOT or mountains) plate boundaries There are about 20 locations where volcanism does not match this pattern; volcanism occurs within a plate or with unusual character for plate boundary setting These so-called hot spots correlate with regions of mantle upwelling; plumes of warm mantle rise, melt, and cause intra-plate volcanism Mantle plumes are relatively fixed, i.e. they move slowly relative to each other & we can use them to fix the ‘absolute’ motion of plates

9. States of matter Matter commonly occurs in three forms Gases -expand to fill the container in which we place them Liquids -conform to container's shape, even if they cannot fill the container Solids -have fixed shapes and do not flow freely H 2 O is unique because it occurs naturally in all three phases (i.e. physically separable entities) on Earth's surface

10. Gases Lowest density, highest energy form Individual molecules or atoms in a gas are disordered Molecules or atoms collide with others or the walls of container to produce a measurable pressure On condensing, gas shrinks by a factor of about 1000 Volume of liquid condensate is ~1/1000th volume of gas Condensing gases give off heat (∆H of condensation), even though the temperature remains constant Liquids have lower internal energy than the corresponding gas

11. Gases Lowest density, highest energy form Individual molecules or atoms in a gas are disordered, i.e. they fly about in "empty" space, independent of each other –So independent are the individual molecules in gases that highly compressed gases still behave like gases Molecules or atoms collide with other atoms or molecules or the walls of our container, producing a measurable pressure –Raising the temperature (i.e. adding energy to gas) causes the pressure to increase Condensing vapors shrink by a factor of about 1000; i.e. the volume of the liquid is ~ 1/1000th the volume of the gas Condensing vapors give off heat (∆H of condensation), even though the temperature remains constant Condensed phase, a liquid, has lower internal energy than the corresponding gas

12. Liquids Higher density, lower energy form Atoms or molecules in liquids to cohere, but they are not bound to specific neighboring atoms or molecules, so liquids pour Most liquids are incompressible –Liquids have densities nearly equal to those of corresponding solids, suggesting that atoms or molecules in liquids are closely packed –When liquids change to solids, their volumes usually decrease by 3 to 30% Liquids must give off heat to become solid; ∆H of crystallization is usually 3–30% ∆H of condensation

13. Liquids Higher density, lower energy form –Energy level is sufficiently high that substances diffuse through liquids easily and rapidly Attractive forces between molecules or atoms enable liquids to cohere, but atoms or molecules are not bound to specific neighboring atoms or molecules –Liquids pour Most liquids are incompressible –Liquids have densities nearly equal to those of corresponding solids –Suggests that atoms or molecules in liquids are closely packed –When liquids change to solids, their volumes usually decrease by 3 to 30% Materials give off heat during the liquid to solid transformation; ∆H of crystallization is usually 3–30% ∆H of condensation

14. Solids Highest density, lowest energy form Solids only slightly more dense than liquids; atoms or molecules not really in closer proximity than in liquid Solids do not deform readily; atoms or molecules have ‘fixed’ positions Crystallinity, the ordered internal arrangement of atoms or molecules, is characteristic feature of solids A single compound in different crystal structures solids will exhibit different physical properties –Diamond & graphite –Ice I, ice II, etc. to Ice X

15. Solids Highest density, lowest energy form Densities of solids only slightly greater than densities of corresponding liquids; atoms or molecules not really in closer proximity than in liquid Solids do not deform readily; atoms or molecules are in relatively fixed positions The attractive forces between molecules or atoms are significant, but crystallinity, the ordered internal arrangement of atoms or molecules, is characteristic feature of solids Crystalline materials have the possibility of exhibiting a range of physical properties –diamond and graphite –Sillimanite, andalusite, and kyanite –Olivine and spinel (Ringwoodite) –Ice I, ice II, etc. to Ice X