MAGNETISM and THE SAN FRANCISCO BAY AREA


What is magnetism?"

The force a magnet exerts on an iron filing or the force the Earth's magnetic field exerts on the needle of a compass are the most common examples of magnetism. People have known about the phenomenon of magnetism for a long time. The ancient Egyptians described magnetic repulsion in about 600 B.C., and the Chinese have known about the magnetic properties of the mineral magnetite (or lodestone) since at least the 1st century AD. In the western world, magnetism was not described until Neckham did so in 1187. The Earth's magnetic field resembles that of a huge bar magnet (see figure below). Like all magnets, the Earth's magnetic field is dipolar; in other words, it has two poles, a north pole and a south pole. Like poles will repel; unlike poles attract ( see box 3.1). The Earth's magnetic field is most likely caused by movement of partially molten iron in the Earth's outer core, more than 2900 km below the surface of the Earth. One model to explain the Earth's magnetic field is the dynamo. In this view, the Earth's magnetic field is a dynamo powered by the Earth's rotation. This dynamo must be coupled in such a way that reversals in the direction of the magnetic field can take place since we observe these field reversals frozen in magnetic rocks ( see box 3.2).

Unlike gravity, a force that always points down towards the center of the Earth, the Earth's magnetic field points in different directions depending on location. The north end of a compass points north, roughly towards the geographic north pole of the Earth, but because opposite poles attract, the compass is actually pointing to the Earth's magnetic south pole (which is NOT in exactly the same place as the geographic north pole-see below).


The Earth's magnetic field

The angle between the compass needle and true geographic north (measured horizontally at the Earth's surface) is called the declination . Looking at map GP-1007, what is the magnetic declination in the Bay area? If the needle is free to orient itself in a up-down direction, it will point down in the northern hemisphere and up in the southern hemisphere. The angle between the freely oriented needle and horizontal is called the inclination. A freely oriented compass needle will point straight down at the magnetic north pole, straight up at the magnetic south pole, and horizontally at the magnetic equator (see above). In the Bay area, the inclination is about 62o.

Geophysicists measure the Earth's magnetic field intensity or strength in units called nanoteslas (nT) in honor of Nicola Tesla, a pioneering investigator of magnetic phenomena. A nanotesla is one-billionth of a tesla. The Earth's magnetic field strength varies from 25,000 nT at the magnetic equator to 70,000 nT at the magnetic poles. The magnetic field strength and direction also change slowly with time, and occasionally rapidly during a magnetic storm associated with sunspot activity. Magnetometers can measure the strength of the magnetic field to an accuracy of 0.1 nT. Just as with gravity measurements, magnetic measurements can also be corrected for known variations of the Earth's magnetic field, so that the resulting variations in the local magnetic field (magnetic anomalies ) depend only on variations in magnetization of nearby rocks. The magnetic anomalies give us a tool for "seeing" these rocks even when they are hidden below the surface. The force of the Earth's magnetic field is not very strong, but is large enough to magnetize certain kinds of rocks which contain iron and nickel minerals ( see box 3.2).

Box 3.3 Bio-magnetism

Looking at GP-1007: An aeromagnetic map of the Bay area

For map GP-1007, the magnetic measurements were made by magnetometers mounted on airplanes (thus the term "aeromagnetic" used in the map title). A total of 8 different surveys were flown at different times, at different elevations, and along different flight directions. The measurements have been mathematically adjusted so that it appears that the whole area was flown at the same height above the ground. Although you can measure the magnetic field intensity by walking on the ground, you can cover large areas more quickly if you mount a magnetometer onto an airplane (see figure 2). Airplanes are also farther away from manmade objects, such as powerlines and metal pipes, that are very magnetic and may completely otherwise overwhelm the magnetic anomaly caused by the rocks ( see box 3.4).

Because the magnetic field in the Bay area is inclined and does not point straight down, a magnetic rock creates both a magnetic high and a magnetic low. As a rule of thumb for this map, the location of the magnetic high is over the southern edge of the body and the magnetic low sits over the northern edge of the body, if the edges of the body are near vertical (see Fig. 2 on GP-1007).

Measurements of many rock samples indicate that sedimentary rocks are generally not magnetic, whereas igneous rocks rich in iron and magnesium (mafic or ultramafic ) tend to be very magnetic. After comparing GP-1007 with the simplified geologic map, what kinds of rocks correspond to the magnetic anomalies? Look at the region just west of Lake Berryessa (use a road map or atlas to help locate geographic features). Most of the anomalies in this area are caused by serpentinite (unit Mzsp on the geologic map). Serpentinite is a type of metamorphic rock caused by low-temperature alteration of very mafic (ultramafic) oceanic rocks. Serpentinite is a greasy-feeling, easily deformed rock that tends to occur along fault zones in California and is indeed magnetic. This pale-green rock is also the official state rock of California. Look along the Hayward fault in the vicinity of San Leandro; again, there is a close correspondence between the large, elongated magnetic high and outcrops (exposures) of serpentinite. However, the serpentinite does not crop out at the surface to the extent that its associated magnetic anomaly would suggest. Look at the large magnetic anomaly over Mt. Diablo. Serpentinite is only exposed near the center of the anomaly, but the large magnetic high allows the geophysicist to infer that serpentinite is present below the surficial exposures of the Franciscan Complex (unit KJf) and sedimentary rocks (units Ks, Ts, Qs), rock types that are generally weakly magnetic.

Two other rock types in the Bay area are capable of producing magnetic anomalies. Look at the magnetic anomalies north of San Pablo Bay and near the southeast corner of the map east of Hollister. Describe the magnetic anomalies in terms of their amplitude and shape. Volcanic rocks, which occur in these areas (map unit fv), tend to produce spotty anomalies, in contrast to the serpentinites' elongated magnetic highs. The other rock type capable of producing magnetic anomalies is gabbro (a mafic igneous rock; part of unit ci) which is responsible for the magnetic high along the San Andreas fault southeast of Santa Cruz (4 on Fig. 3 of GP-1007).

Not all of the large, broad magnetic anomalies on GP-1007 correspond with surface outcrops of serpentinite, volcanic rocks, or gabbro. The magnetic high in the Santa Cruz Mountains occurs over young sedimentary rocks (unit Ts) rather than serpentinite (unit sp) or volcanic rock (unit fv). Does this mean that the sedimentary rocks are unusually magnetic? Measurements of its magnetic properties indicate that the sedimentary rocks are only slightly magnetic, not strong enough to cause this anomaly. Thus, geophysicists would conclude that this magnetic high indicates buried mafic or ultramafic rocks in the heart of the Santa Cruz mountains. Some geophysicists infer that the mafic rocks under the Santa Cruz Mountains may be the same gabbro that is exposed along the San Andreas fault southwest of Bakersfield, 300 km away to the southeast. According to this interpretation, the exposure of gabbro would be a sliver comprised of the mafic basement rocks that was transported many hundreds of kilometers northward along the San Andreas fault. Do you see any other large, broad magnetic anomalies that lie over young sedimentary materials? Look along the eastern edge of the color magnetic map. There you'll see the western flank of a broad magnetic high called the Great Valley magnetic high. This magnetic high also suggests that mafic rocks underlie the young sedimentary rocks exposed in the valley. Geologists believe that these mafic rocks are part of an ophiolite sequence and are a piece of old oceanic crust, now part of the North American continent.

Are the earthquakes concentrated along elongated magnetic highs? In the areas where earthquakes occurred along elongated magnetic highs, the serpentinite is concentrated along fault zones that were seismically active between 1972 and 1989. What about linear magnetic highs that do not have yellow clusters (earthquake epicenters) along them? Are the serpentinites which cause these anomalies marking old, long-dead faults or, more ominously, are these faults just dormant?

To consider the importance of the time interval of the seismicity on the map, let's consider the pattern of seismicity along the San Andreas fault. Many people look at the trace of the San Andreas fault just south of San Francisco on this poster and feel comforted that there are few earthquakes, when in fact, this area has one of the higher probabilities for a large earthquake. The presence of so many small earthquakes south of this section suggests that the stress is high along the San Andreas fault. Scientists know that the Pacific plate is sliding to the northwest relative to the North American plate at an average rate of 5 centimeters per year. The faults in this area are merely the surface expression of the edges of these two tectonic plates grinding past each other. The movement along different segments of these faults varies from place to place. Some segments experience relatively smooth, continuous slippage, but other segments undergo jerky movement because they do not slip continuously but instead are locked over tens to hundreds of years, during which time the rocks of the moving plates are deformed elastically near the fault and store energy like a stretched spring. The motion of the plates builds up strain (as stored elastic energy along these faults) until the stress becomes too much, the fault unlocks, and the built-up pressure is released as an earthquake. Geologists know that the San Francisco area has been rocked with large earthquakes fairly regularly throughout geologic time. The seismicity gap (lack of seismic or earthquake activity) south of San Francisco indicates that pressure is building up in this area. Thus, the lack of observed seismicity along the long, linear magnetic anomalies may mean that the faults are very old and dead or, if still alive, that the last earthquake along these faults occurred before 1972, a very short time indeed on the scale of geologic time. Considering that several belts of magnetic anomalies point towards San Francisco and San Jose, information on when these faults last moved is needed in order to assess their earthquake potential.

As with gravity anomalies, magnetic anomalies can indicate whether the contact between magnetic and non-magnetic rocks is vertical or not. Find where the offshore part of the San Gregorio-Hosgri fault lies on GP-1007. Note that the seismicity along this fault is fairly scattered, but essentially parallels the magnetic contours. Where do the earthquake epicenters and the mapped location of the fault lie with respect to the magnetic high and low (compare GP-1007 with the color magnetic map)? If the fault were vertical, where should the earthquakes fall with respect to the magnetic low? The configuration of the magnetic anomaly along the San Gregorio-Hosgri fault suggests that the fault is dipping to the east.

The broad magnetic high in the Santa Cruz Mountains hints at an asperity along the San Andreas fault. Are there many earthquakes within the magnetic high? Describe what happens to the trend of the San Andreas fault in this area. Take a ruler and try to fit one straight line along the San Andreas fault between San Francisco and Hollister on the color magnetic map. Is it easier to fit the trace of the fault if you draw two straight-line segments? The magnetic high lies near the intersection of the two straight-line segments. Why would the San Andreas fault bend in this area? Gabbro, the inferred source of this magnetic high, can be very strong and resist deformation. It may be easier to deform around this gabbro than to break through the harder, more resistant material. As a result of this bend in the fault, the Santa Cruz Mountains are being formed (figure 3; see exercise E ).


Plan View of fault


Magnetism and GP-1007 Map Exercises

Table of Contents Conclusion