San Diego Association of Geologists

San Diego Association of Geologists home

SDAG Monthly Meeting
Wednesday - March 22

Location: Casa Guadalajara
4105 Taylor St.
San Diego, CA 92110
619-29-55111

Directions:

6:00pm -
Social hour

SDAG Monthly Meeting

6:00pm - Happy Hour
6:45pm - Dinner
7:45pm - Announcements
8:00pm - Presentation

6:45pm

Menu: MEXICAN BUFFET (Includes Vegetarian options)
if pre-registered by the deadline, $5 extra if you did not make a reservation. Click the SDAG member checkbox on the reservation form if you are a member.

Cost: FREE: $50.00 for non-members, $45.00 for members, $30.00 for students
Reservations: Make your reservation online by clicking the button below no later than noon, Monday, March 20
RESERVATIONS CANNOT BE ACCEPTED AFTER Monday at 12:00pm
Late reservations/cancellations are preferred over walk-ins or no-shows. Fees payable at the meeting or pre-pay with PayPal.
There also will be a phone credit card reader at the meeting.

IF YOU DO NOT MAKE A RESERVATION, WE CANNOT GUARANTEE YOU A MEAL.

7:45pm - Announcements
8:00pm - Program

" THE EARTH'S MAGNETIC FIELD - NOW AND, THANKS TO PALEOMAGNETISM, IN THE PAST "

Speaker: MONTE MARSHALL, Professor, Emeritus, of Geology and Geophysics - San Diego State University

Of the three forces we encounter in everyday life - gravity, electricity, and magnetism, magnetism is probably the most complicated. A single particle will have a gravity field, whose lines of force extend radially outward. A single charged particle will have an electric field with the same shape. But if the charge is moving, or even spinning, it will generate a magnetic field whose shape/force lines are much more complicated. If a charged particle spins or moves in a circle it causes the simplest magnetic field, a dipole, with north and south poles. And the force lines arc between the two poles. There is no such thing as a magnetic monopole.

In the first part of this talk we will examine the current dipolar shape of the geomagnetic field and the interaction of electric currents with the convection cells of the liquid iron in the outer core that cause it. We will see that the current geomagnetic field isn't a perfect dipole, that its axis doesn't coincide with the earth's rotation axis, and that compass directions change on almost a yearly basis.

The second part of the talk will deal with the nuts and bolts of paleomagnetism. As the name suggests, this is the branch of geophysics that attempts to learn the shape, intensity, and changes of the geomagnetic field long before the first compass was invented. Like most everything in geology, the evidence/record lies in the rocks! The direction and intensity of ancient geomagnetic fields can be recorded by certain magnetic minerals in a rock, especially magnetite and hematite. But the magnetic record in a rock can have an overprint or be completely erased, e.,g., if a magnetite crystal rusts or the rock is near a lightning stroke. And the process of erasing overprints isn't simple and sometimes the primary magnetism can't be separated from a later remagnetization. The last part will describe the most spectacular and consequential features of the geomagnetic field—its polarity reversals. The north and south magnetic poles switch places. If that happened now, compasses would point south. This change takes only about 10,000 years, and during the switch the field strength decreases to about one tenth of its normal value, and they occur at random. This last feature of the reversals proved to be critical for proving seafloor spreading and for their current use in dating rocks

During the Tertiary the average interval between reversals was about 200,000 years, but some polarity intervals lasted for only a few tens of thousands years, while other intervals lasted for a million years. The last geomagnetic reversal was about 800,000 years ago. During the sixties a combination of paleomagnetic measurements and K-Ar dating established the time intervals when the field was normal, like now, and when it was reversed (when compasses would have pointed south). The geomagnetic reversals and their timing were at first only of interest to the handful of scientists studying the earth's core. But in 1969 they were essential in proving that continental drift/seafloor spreading was real, and that ushered in the era of plate tectonics! The proof of seafloor spreading was the exact match between the geomagnetic reversal time scale and the pattern of unusual linear magnetic anomalies in the world's oceans. A few oceanographers had been mapping them for some years, but, like the reversal time scale, they were initially considered to be of minor importance! Oceanic spreading centers, like the mid-Atlantic Ridge and the east Pacific rise, are like two-headed tape recorders. As the basaltic lava that erupts in their central valley solidifies, it becomes magnetized in the direction of the ambient geomagnetic field, whether normal or reverse. As the two sides of the ridge or rise move away from one another, the pattern of normal and reversed oceanic crust on one side is the mirror image of that on the other side. Because the geomagnetic reversals are random, the pattern of the linear oceanic magnetic anomalies is exactly like a bar-code.

When ship-towed magnetometers are over normally magnetized crust, the magnetic field of the crust reinforces the geomagnetic field and causes a positive magnetic anomaly, and vice-versa.

The last contribution of paleomagnetism to be discussed is its use in determining the latitude and azimuthal orientation of the place where the sampled rocks formed/got magnetized

I'm a fourth generation San Diegan and was born in Mercy Hospital. I attended Villanova University, in the suburbs of Philadelphia, where I majored in philosophy and minored in astronomy and got my bachelor's degree in 1961. Upon returning to San Diego, I enrolled at San Diego State as an astronomy major, but was lured into geology by a charismatic geology professor. I received my second bachelor's degree in geology and geophysics in 1966. I did my senior thesis under a professor at Scripps on the paleomagnetism of some sandstones in the Transverse Ranges. I started my PhD studies at Stanford in geology and geophysics in 1966 and, after two years of course work, I did my research on the magnetic properties of seafloor basalts. Allan Cox, who was one of the 'fathers' of paleomagnetism, was my advisor. I am probably one of the few people still alive who sat in a large auditorium at an AGU convention on a December day in 1969 and watched a series of talks that conclusively proved that seafloor spreading/plate tectonics was real!

We looked at one another and knew that we had just experienced a revolution in earth science! After graduating in 1971, I continued my paleomagnetic research at the USGS paleomag lab in Menlo Park. I really wanted to teach and so, after 6 months of biking around western Europe, I joined the SDSU geology department in 1975. My main courses were geophysics, structural and petroleum geology, and paleomagnetism and plate tectonics.

With my students I conducted paleomagnetic studies in southern California and gravity studies of the faults in metropolitan San Diego. I had three sabbatical years - in France, Russia, and the Czech Republic. I retired in 2004.

My academic life has continued by giving geology talks (the locally most memorable was a series of "what neat geology did Monte see on his last international trip?" ) I teach the Natural History Museum's docents and hiking guides geology every year. And, I have written many papers for our field trip guidebooks. I am just finishing a paper on the geology of the White-Inyo Mountains for the guidebook on our trip to the Owens Valley - our last trip before the covid shutdown.

Upcoming SDAG meetings - 2023

April 19

Meetings are usually scheduled for the 3rd Wednesday evening of the month. Meeting information on this website is normally updated the second week of the month.

Recordings of past meetings

1/18/2023: Mike Hart "A (VERY) SHORT COURSE ON THE IDENTIFICATION OF PALEO SLIDES"
https://us02web.zoom.us/rec/share/ipqVFMcCEQCGIE0DZpOSncE99bAUdTjqzUharpQZPhk-ML_mrIJAEz6NDH8EGywk.RgAsMTVXjubOIqZP
Passcode: 4#fT^CXz
[this video will be available until 3/15/23]

3/16/2022: Dr. David A. Novelo-Casanova & Dr. Gerardo Suárez "The Risk Atlas of Mexico City, Mexico: a tool for decision-making and disaster prevention"
https://us02web.zoom.us/rec/share/6cXhxheuNRLf951mBU-ehWXBSYm-7RrzQya-pTvG29-XrMIeo3OUPiuDsSjMhkVV.Du82etdkgOSB1C6b
Passcode: S5UVV8y$

2/16/2022: Dr. Isabelle Sacramento-Mcjilton "The Great Geological Perils of Portugal: Quiet now, big later..."
https://sdsu.zoom.us/rec/share/cFG-2q3UJ0_MArSODCZDwYbSx6MIWxNEJfYLvOGbax2c5Tqx9VBEQuE_6flSs0IO.C5a4zKF1_fLocf10

1/19/2022: Dr. Norrie Robbins "Extending the Busch-Miller (2016) Hypothesis: Acid Rain from K-Pg Bolide Impact Chemically Altered Exposed Igneous Rocks in San Diego County"
https://us02web.zoom.us/rec/share/g1owcE-POmcFiwun3utUNutGFjND072PQiSyg_YHAWz1zcwLXFyKRGBp6K8DPo3r.X15QHo14H8xGU60G
Passcode: K8^!F=ZE

Recordings of other past meetings are temporarily unavailable