Note what happens to the path of a P wave that just enters the outer liquid core. The P wave slows down (the rigidity G is 0.0) and the wave is deflected towards the center of the Earth. From 105 to 140 degrees from the epicenter no P waves are recorded. This creates a belt from 105 to 140 around the Earth called the P Wave Shadow Zone.
Although it is not illustrated think what would happen to an S wave that just entered the outer liquid core. S waves can not travel through a liquid so from 105 to 105 degrees on either side of the epicenter no S waves are recorded. This is the S Wave Shadow Zone.
When something pushes or pulls on a body a force is exerted. Compressed air exerts a force on the walls of its container. A tractor exerts a force on the wagon it is pulling. The force we are probably most aware of is the gravitational attraction exerted on every object by the Earth. This force is the weight of the object. Gravitational forces, electrical forces and magnetic forces can act through empty space without physical contact.
Forces can be compressive (push) or tensile (pulling). Normal forces act at 90 degrees to the cross section of the object. Shearing forces act at some angle other than 90 degrees (think about pushing the top of a deck of cards). Solids will resist a force which attempts to change its shape:
- rigidity - measured by the shear modulus
- volume - measured by the bulk modulus
Review the equations in the previous chapter for the velocities of P and S Earthquake waves.
Another effect of a force is to alter the state of motion (a continuous change of position) of an object. The velocity of an object is the distance divided by time - average velocity with units of length/time. The instantaneous velocity is the velocity at one instant of time. Acceleration is a measure of the rate of change of velocity with units of L/time squared.
A common example of motion with nearly constant acceleration is that of a body falling toward Earth. In the absence of air resistance it is found that all bodies, regardless of their size or weight, fall with the same acceleration at the same point on the surface. The acceleration of a freely falling body is the acceleration due to gravity - called "little g". At the surface of the Earth little g has a value of (about) 32 feet/second squared. The distance an object will fall in time "t" is given by 1/2(g times t squared).
When a large force is needed to speed up a body the mass of the body is large. The body has a large inertia. Inertia is a measure of the resistance to change in position. The mass of a body is the ratio of the force (F) on the body to its acceleration (A). F = MA or M=F/A. The weight (W) of a body means the gravitational force exerted on it by the Earth.
M = F/A = W/g or M = W/g or W = Mg
The units of mass are pounds (weight)/feet/time squared - a rather awkward appearing set of units called a slug.
I weigh about 250 pounds and in Houston the value of little g is 32 feet/seconds squared. My mass is (250/32) slugs = 7.8 slugs. My mass (unfortunately!) is a constant - anywhere in the universe that I go it would be the same (until I change it!). If I go to a place on the surface of the Earth where g = 31 feet/seconds squared, my weight, however, changes to 242 pounds.
The Earth is not a perfect sphere. If it were my weight would be the same as long as I stayed at the same elevation above sea level. The Earth bulges at the equator and is flattened at the poles. Little g is smaller at the equator (further away from the center) and gets larger as you get to the poles (closer to the center).
A gravimeter measures the local value of little g. An object with a precisely known mass is suspended on a delicate spring. The greater the attraction between the object and the center of the Earth the greater the length of the spring.
Imagine taking measurements in an elevator that stops at each floor of a 10 story building. On floor 8 you are further from the center of the Earth than you were on floor 1 and the gravitational attraction decreases with increasing elevation. A gravimeter can distinguish about 1 meter difference in elevation.
The value of little "g" is sensitive to variations in the density of the material(s) between the surface and the center of the Earth. For example, if there is a salt dome, little g is less than it would be if "normal crustal" material were present (the density of salt is about 2.1 g/cc). This would be a "negative gravity anomaly" - the value is less than expected. If the material were more dense (a basalt flow encased in sandstone, for example) than average a "positive anomaly" would be detected.
A set of locations (stations) were laid off on a map. Gravimeter measurements were made at each station. The blue line in the figure above is the resulting plot of the individual measurements. Note the presence of a negative anomaly. At some depth below the surface at this location is a body that has a lower density than the enclosing rock. The greater the contrast between this object and the enclosing rock the greater the magnitude of the anomaly.The further the "anomalous mass" is from the observer, the smaller the anomaly. The bigger the "anomalous mass" is the greater the anomaly.
The values on the y-axis are in milli gals -- note that they are negative. If this had been a positive gravity anomaly (where the anomalous mass had a density greater than the enclosing rock) the anomaly would be positive and stand above the x-axis.
Isostasy Mountains have a low density "root". That is, mountains are not simply thicker continental crust sitting on denser rocks. Just like ice floats on water with about 10% of its mass above water, mountains have a significant part of their mass below the surface of the Earth in the form of the low density root.
In the following animation you should move from left to right using the arrows at the top of the page. Surveyors use a "plumb bob" (essentially a weight on a string) as part of their instrumentation. In the "experiment" section, move the gravimeter from left to right. Move it a small distance and release. On the right side of the screen is the deviation of the plumb line from the vertical. If the effect of the mountain were negligible, the plumb line would point to the center of the Earth. Note that in this simulation, the gravitational pull of the mountain causes the plumb line to be deflected from the vertical.
Move to to the error arrow. Note that maximum deflection occurs when there is not root to the mountain. As the size of the root increases, the amount of deflection decreases. Thus, a low density root to a mountain will result in less deflection than if there were no root.
The Earth's Magnetic Field
A simple animation of the pattern of magnetic field lines seen when iron filings are sprinkled on a thin sheet of paper lying over a straight bar magnet. A circle marked on the paper helps students understand the pattern of magnetic inclination around a line of longitude on Earth.
Lines of force from this field affect magnetic materials. The field is "dipolar"; a N and a S magnetic pole. To a first approximation, the Earth's field is like a simple dipole magnet. However, this model rather quickly fails because all known magnetic substances loose their magnetic character at a temperature called the Curie Point (temperature) . For the mineral magnetite the Curie point is about 580 degrees centigrade. A free swinging compass needle will adjust its vertical position to be parallel to the magnetic field. Its magnetic inclination will be zero at the magnetic equator and 90 degrees at the magnetic pole.
The Earth's N magnetic pole does not coincide with the Earth's Rotational N pole (geographic North); the two poles are about 11 degrees apart. A compass needed points to the N magnetic pole. Therefore, a compass reading would have to be corrected in order to point to geographic N (magnetic declination). Only on a parallel of longitude that goes through the N magnetic pole and the N geographic pole will the compass needle point to geographic N. The magnetic declination changes with time and it takes about 3,000 years for the magnetic N pole to "move" about the rotational pole.
A magnetometer measures variations in the strength of the magnetic field. Areas underlain by rocks containing magnetite will exhibit a stronger field and produce positive magnetic anomalies. Rocks deficient in magnetic minerals (many sedimentary rocks) will show negative anomalies.
When a magnetite in a magma cools below its Curie point the iron bearing minerals will gain magnetization and the field will align with the Earth's field and the magnetite will inherit both the direction of the field (where magnetic N is located) and the intensity of the field. Thus, the rock carries information about the field.
Imagine a simple dipole magnet with a N and S end. You could rotate the magnet by 180 degrees so that the S end is up and the N end is down; this would be a magnetic reversal and the N end of a compass needle would point to the S rotational pole. The Earth's magnetic field has reversed itself numerous times during the past. Detection of previous reversals recorded in the basalts on the sea floor proved to be a critical observation for confirming plate tectonic theory.
During a period of normal polarity (where the North magnetic pole is in the northern hemisphere) the magnetic field of sea floor basalts is strong for sea floor crust in the northern Atlantic. During a period of reverse polarity (where the North magnetic pole is in the southern hemisphere) the magnetic field of sea floor basalts is weak for sea floor crust in the northern Atlantic. Thus, the magnetic stripes in oceanic crust can be interpreted as reflecting polarity reversals.
During periods of normal magnetism, north-seeking magnetic poles point towards geographic North. During reversals, they point south. As the mid-ocean ridge spreads, the magnetic field is
frozen into lava as it cools through the Curie point, preserving a record of magnetic field reversals going back millions of years. Magnetic stripes are irregular due to the rough shapes of intrusives and extrusives. Those shown here are based on real data from the Atlantic crust south of Iceland. View an animation of the development of new sea floor at a divergent margin.
No one has observed a polarity reversal and on one knows for sure how long it would take for the polarity to reverse. If it takes a finite length of time (say 1,000 years) then for that period of time there would be no magnetic field surrounding the Earth. Cosmic radiation would no longer be deflected from the surface of the Earth by the magnetic field and genetic damage to living organisms is highly likely.
Copyright by John C. Butler, July 29, 1995