propose that mantle upwelling, responsible for delivering melt 

 to the seafloor, is focused at discrete points or plumes at slow- 

 spreading ridges. Mantle flow at fast-spreading ridges, on the 

 other hand, is often associated with proposed two-dimensional 

 or sheel upwelling in the mantle. While gravity data inferred 

 from Geosat and ERS-1 data have been used to address this 

 problem in broad brush, sufficiently high-resolution data 

 covering a large extent of the mid-ocean ridge simply have not 

 been available. The release of the Navy's gravity data would 

 greatly facilitate this fundamental research into the genesis of 

 the bulk of the Earth's surface. 



ii) Mapping ofCruslal Thickness 



The composition of the oceanic crust and its density variation 

 with depth are remarkably constant throughout the oceans. 

 Given this generally simple behavior and the significant 

 differential in density between the crust and underlying mantle, 

 gravity data can be used to infer the thickness of the crust and. 

 hence, the depth of the mantle throughout the oceans. While 

 allowing for the eventuality that variations in crustal density do 

 occur, a detailed gravity data set can be used to discover regions 

 of major variations in crustal thickness. For example, the 

 availability of Geosat data south of 30° S clearly outlined an 

 anomalous region of oceanic iithosphere south of Australia 

 which is now called the Australia- Antarctic Discordance. The 

 availability of more detailed gravity data throughout the Northern 

 Hemisphere would provide an unprecedented opportunity for 

 understanding variations in crustal thickness or variations in 

 density that might masquerade as variations in thickness. 



Hi) The Structure of Fracture Zones 

 Transform faults are found at mid-ocean ridges where older and 

 young Iithosphere slide past each other and are frequently 

 accompanied by earthquakes. The disrupted topography is 

 known as a "fracture zone." The bottom topography and the 

 differential in age across fracnire zones in these regions lead to 

 the introduction of loading effects and consequent flexure of 

 the Uthosphere. Gravity data collected across such fracture 

 zones and parallel to the original direction of mid-ocean ridges 

 provide important information on the long-term evolution and 

 strength of the Uthosphere. Again, the data sets currently 

 available for studying this phenomenon are quite limited. The 

 gravity data would provide the data needed for detailed studies 

 of these processes in the Northern Hemisphere. 



d. Findings 



The findings relative to marine gravity are' 



• The release of the Navy's high-resolution gravity data would 

 greatly facilitate fundamental research into the genesis of the 

 bulk ofEarth' s surface because of their unparalleled coverage, 

 detail, and accuracy. 



• Availability of detailed gravity data throughout the Northern 

 Hemisphere would provide an opportunity for understanding 

 variatioDS in crustal thickness or variations in density that 

 masquerade as variations in thickness. 



• These gravity data would ako provide the information needed 

 for detailed studies of Uthosphere processes in the Northern 

 Hemisphere. .,,..>,-:,.- , 



2. GEOMAGNETICS 



a. Data Description 



Geomagnetics is the science that deals with Earth's magnetism. 

 Magnetic data that can be used to depict the geomagnetic field 

 for any time and location can be used to improve the navigational 

 safety of ships and aircraft of all nations. Because Earth's 

 magnetic field is constantly changing, knowledge of magnetic 

 compass corrections essential to steer a true course are necessary 

 for safe navigation. Early sailors made these corrections by 

 comparing the compass with the direction of the North Star. 

 Today, ships and aircraft are guided by magnetic charts and 

 improved geomagnetic data supplied by NAVOCEANO. 



In addition, and for our purposes more importantly, geomagnetic 

 data can be used in scientific studies of the evolution of the 

 Earth's crust. The Earth's magnetic field results from flow in 

 the planet's molten, iron core. The surface manifestation of this 

 magnetic field is a dipole field with both a North and a South 

 (magnetic) Pole. Furthermore, these poles have been, during 

 much of the Earth's history, closely aligned with the spin axis 

 of the planet; i.e.. the geographic North and South Poles. For 

 geologists and geophysicists the magnetic field is interesting 

 because the North and South magnetic poles have reversed 

 themselves aperiodically throughout Earth's history; 

 furthermore, rocks cooling from a molten to a solid state can 

 record this ancient field. By mapping small anomalies in 

 Earth's field, therefore, it is possible, by knowing the reversal 



