Positive magnetic anomalies (shown in red colors) are produced by normally
magnetized rocks; negative anomalies (shown in blue colors) are related to
rocks less magnetic than adjacent rocks or to reversely magnetized volcanic
rocks. Positive anomalies in the Coast Range reflect exposed and buried
Eocene basalts.
Negative anomalies in the Coast Range result
from several sources: 1. sedimentary basins, 2. reversely magnetized
Eocene basalts and 3. tectonically overturned normally magnetized basalts.
High frequency anomalies typical of young volcanic rocks occur over the
Cascade Range. Linear positive and negative magnetic anomalies extending
several hundred kilometers are associated with normal and reversed dikes
and folds in the Columbia Plateau. Interpreted buried plutons in the
Cascade Range and northeastern part of the state are associated with
positive magnetic anomalies. One of the highest ampitude positive
anomalies on the map is associated with the serpentinized pre-Tertiary
Ingalls ophiolite.
Several faults in western Washington are easily seen on the magnetic map:
1. the Seattle Fault; 2. the South Whidbey Island fault; 3. the Leach
River fault; 4. the San Juan Island fault. In northeastern Washington
linear anomalies are associated with the Toroda Creek and Republic grabens
and the Pasayten fault, which bounds the eastern edge of the Methow trough.
Many prominent anomlies are not associated with the mapped geology. The
aeromagnetic, combined with other geophysical data, can be used to
delineate the buried sources of these anomalies.
U.S. Department of the Interior | U.S. Geological Survey