Interpretation of an aeromagnetic survey of the Amchitka Island area, Alaska

......................................................................................... 1 Lava flows and thick sills Continued Introduction ........................................... ________________________________________ 1 Previous magnetic surveys.......................................................... 2 Aeromagnetic survey ... ·-·······-··················-·······-······-···-·······-····-··3 Reduction of data .............................. ---···---·-----··------------------3 Ground magnetic surveys............................................................ 3 Magnetic properties of rock samples......................................... 4 Measuring magnetic susceptibilitY-------········-········-·········-4 Measuring remanent magnetization.................................. 4 Geologic setting............................................................................. 5 Volcanic breccia .......................... !................................................. 6 Magnetic properties·-··········-·····-----·--···--·-···-·····-··-··-·-···----·-·· 6 Aeromagnetic anomalies...................................................... 8 Ground survey anomalies..................................................... 8 Volcanic sandstone, siltstone, and tuff breccia......................... 9 Dikes and small sills..................................................................... 10 Intrusive rocks .......... ·--·-········-··-···················-··········-···················· 11 Magnetic properties.............................................................. 11 Aeromagnetic anomalies...................................................... 12 Lava flows and thick sills............................................................ 12 Magnetic properties.............................................................. 14 Aeromagnetic anomalies...................................................... 16 Ground survey anomalies.................................................... 16 Analysis of magnetic anomalies................................................. 17 Depth estimates..................................................................... 17 Dipole and sheetlike models................................................ 19 Interpretation of anomalies........................................................ 19 East Cape anomaly ............................................................... 20 White House Cove and shelf-break anomalies................. 20 Pillow Point and Rifle Range Point anomalies................ 22 St. Makarius Point anomaly ............................................... 22 Mex Island anomaly ............................................................. 23 Bird Rock, Windy Island, Chitka Point-Constantine Point anomalies ................................................................. 23 Site B anomaly ...................................................................... 23 -780 anomaly ....................................................................... 23 Site F anomaly ...................................................................... 23 Relation of submarine structure south of Amchitka to aeromagnetic anomalies .............................................. 24 References cited ............................................................................ 24