EARTH AND PLANETARY SCIENCES 101, Fall, 2001

THE WAY THE EARTH WORKS

I. Finish Seismology

A. “First motion” studies to determine what happens when rocks rupture and a fault forms (or breaks again)

B. Define a strike-slip fault—vertical, with rocks on both sides of the fault moving past each other horizontally.

C. Set up a horizontal plane grid, with four quadrants.

1. What happens to ground motion (as a function of time) in each quadrant, and why?

2. Recognize that different quadrants behave differently (compression vs. dilation).

D. Utility

1. In many cases, faults do not actually rupture the earth’s surface, or it is too difficult to get to the faults.

2. first motion data allow us to infer the orientation and sense of offset along faults.

II. Heat Flow

A. Just like it sounds, the flow of heat from the earth’s interior

B. How measured?

1. look at the temperature gradient in the earth’s interior, ahhhhh, but what is the deepest “hole in the ground” (about 13 km, which is rather trivial).

a. compare thermal gradients

b. below Kansas, probably about 25 C/km

c. below ABQ, probably about 40 to 50C/km

d. near Redondo Dome in the Jemez, up to 300 C/km!!!

e. below Yellowstone Lake, probably about 400 C/km, locally

2. measure the thermal conductivity of rocks—just like you would compare the thermal conductivity of silver (high) with that of porcelain (relatively low).

C. What are the sources of heat from the earth’s interior?

1. mantle/core system as a slowly cooling entity.

a. thermal energy is conducted upwards

b. thermal energy is convected upwards

2. radioactive decay

a. concentration of elements with radioactive (unstable) isotopes (like K, U, Rb, Th) in the earth’s crust, relative to the mantle

b. not to say that there are not radioactive isotopes in the mantle

c. when workers in the 1800’s tried to calculate the age of the earth on the basis of its cooling history, they did not have knowledge of the radioactive decay of unstable isotopes

D. So, how is Heat Flow Distributed?

1. Very high above mid-ocean ridges

2. very low above subduction zones

3. relatively high along active margins of continents and in continental rift zones

4. low in regions where the earth’s crust is very old, and has been relatively “quiet” for a long period of geologic time

III. Geomagnetism and Paleomagnetism; Fun Stuff

A. Known for centuries that Earth has a magnetic field.

B. The geomagnetic field is self-generating, and is due to the flow of electrical currents, as charged material, in the outer, liquid core of the planet.

1. the overall orientation of the field is dictated by the general flow pattern of core material in response to the rotation of the planet about its geographic (spin) axis.

2. subtle and not so subtle changes in the geomagnetic field appear to be due in large part to interactions in the outer liquid core at the core/mantle boundary.

C. Fundamentals of a Dipole Field

1. Overall, geocentric and quasi-parallel to the spin axis of the planet.

2. Two quantities describe the orientation of the field

a. declination—measured in the horizontal plane

b. inclination—measured as a “plunge” of the field vector with respect to the horizontal.

3. Inclination varies regularly as a function of latitude.

4. Declination is generally northward, but diverges significantly near the north and south poles.

5. As the name implies, a dipole field can have two “polarities”

6. At present, the dipole of Earth is oriented such that magnetic lines of force leave from near the south pole and return near the north pole.

a. this is called “normal” polarity

b. the opposite, or “reverse” polarity is when the field lines leave from the near the north pole and return at near the south pole. In this case, the inclination values are simply inverted.

D. Fossil Magnetism in rocks

1. Many geologic materials are capable of recording the geomagnetic field, by virtue of the process in which they form, or subsequent processes affecting the rocks.

2. Igneous rocks can acquire a magnetization, essentially parallel to the geomagnetic field, during cooling because the rocks contain small quantities of magnetite or similar magnetic oxides, and a permanent magnetic ordering occurs in these minerals, which is controlled by the external field, below a certain temperature, referred to as the Curie temperature. TRM

3. Detrital sedimentary rocks can acquire a magnetization, essentially parallel to the geomagnetic field during their formation as fine magnetic grains, as volumetrically small constituents of the detritus, fall through water and become re-oriented with their moments parallel to the field. DRM

4. A chemical remanent magnetization can be acquired when new magnetic minerals are precipitated during a chemical reaction. CRM.

E. Reversals of the Geomagnetic Field

1. Very young geologic materials have fossil magnetizations reflecting a normal polarity field.

2. However, when workers started to look at magnetizations in older geologic materials, they found that their magnetization either reverse or normal polarity.

3. Either the geomagnetic field was capable of reversing it polarity, or rocks were very fickle magnets, so to speak.

4. We now know that Earth is capable of reversing its magnetic field

5. The most recent complete and long-lived reversal occurred about 778,000 years ago!

6. Implications are enormous.