SDAG Monthly Meeting
Wednesday - March 22
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Location:
Casa Guadalajara
4105 Taylor St.
San Diego, CA 92110
619-29-55111
Directions:
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6:00pm -
Social hour
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SDAG Monthly Meeting
6:00pm - Happy Hour
6:45pm - Dinner
7:45pm - Announcements
8:00pm - Presentation
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6:45pm
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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.
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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.
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7:45pm - Announcements
8:00pm - Program
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" 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
May 17
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
If you are a current SDAG member and are not getting e-mail announcements,
make sure the SDAG secretary has your correct e-mail address.
If you have any information, announcements, ads or suggestions for an upcoming newsletter, please submit it to
2023 SDAG Secretary.
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The submittal deadline for the next SDAG newsletter is the last Friday of the month.
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