AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
Expert: Dr Thomas Bell Date: 6/22/2008 Subject: magnetic/gravity anomalies
Question What type or combination of rocks/minerals might: a) Give a magnetic anomaly, but not a gravity anomaly? b) Give a gravity anomaly, but not a magnetic anomaly?
Answer Jarrad,
Rocks that contain ferromagnetic elements like iron or nickel are magnetic. These elements have relatively high atomic mass so rocks that contain them will often be relatively dense as well.
Anomalies are relative. The term loosely means that something is unusually relative to something else. In the case of magnetic or dense rocks it means that they are more or less magnetic or dense than their neighbors. If the local magnetic background is low, any rock with more iron will produce a positive anomaly. Similarly if a geologic terrain is composed of low density rocks, any rock with a higher density will produce a positive anomaly. In both cases, the same rock might not be anomalous if the background magnetic field were higher or the density of the surrounding rocks were higher.
Anomalies can be negative or positive. Rocks with low concentrations of ferromagnetic elements surrounded by rocks with high concentrations of these elements will produce a negative anomaly. Similarly, low density rocks surrounded by high density rocks produce negative gravity anomalies.
Since anomalies of any type are relative, there is no answer to your question unless you specify the characteristics of the geologic terrain.
