An Unexplored Realm in the Heartland of the Southern Gulf ... - Famsi

etween like poles of adjacent individual atoms, which create a magnetic dipole effect.
The dipole effect is significant because it results in both positive and negative
perturbations in the magnetic anomaly map associated with a single block of basalt. The
magnetic anomaly observed on a map due to magnetic susceptibility changes as a
function of the strength and orientation of the Earth’s magnetic field. In contrast, TRM is
acquired at the time the rock cools from lava. The magnetic anomaly due to TRM is
independent of the strength and orientation of the Earth’s magnetic field. In basalt rocks,
TRM creates larger magnetic anomalies than those associated with magnetic
susceptibility. Thus, for practical purposes, the magnetic anomalies mapped at El
Marquesillo are a result of TRM.
The TRM of basalt is many orders of magnitude greater than that measured for
sediments (Connor and Sanders 1994:7/7), even in sedimentary deposits containing a
sizeable volcaniclastic component (Tarling 1983). Individual blocks of basalt cooled
from a lava flow have a consistently large TRM. Therefore, large blocks of basalt have a
large magnetic contrast with the surrounding sediment and will produce large and
coherent anomalies on maps. Structures created from many basalt blocks, such as walls
or pavements, have more complex anomalies because each block adds its individual
magnetic signal to the map anomaly. For example, if the blocks are laid down in
orientations other than the orientation they were excavated from in the quarry, their
magnetic signal will change. Features such as fire pits also carry TRM, but these
anomalies have lower amplitude.
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