MEASUREMENT OF CORAL THICKNESS AT MIDWAY LAGOON.
Abstract
A seismic survey of Midway Lagoon shows that the volcanic rocks beneath the coral had two terraces cut on them before they were covered by the coral atoll. One terrace, under the north half of Midway Lagoon, lies at a depth between 0.63 and 0.71 km; the second, beneath Sand Island and the southern portion of the lagoon, lies at a depth of 0.26 km. A ridge or peak of the volcanic rock comes within 0.2 km of the surface under Eastern Island, This morphology suggests that the original volcano was partially eroded on the north side and then protected from further cutting by growing reef material while the volcano sank into the sea floor toward isostatic adjustment. At some stage in the sinking, growth of croal was slowed to an extent that permitted further terrace cutting before renewed coral growth again protected the volcanic rocks from erosion. The thickness of coral at Midway is no greater than at Gardner Pinnacles, 1000 km to the southeast, despite the presumed greater age of Midway. Observed seismic velocities are: 2.3 to 2.8 km/sec in the coral; a linear increase of velocity with depth in the volcanics approximately in the form 4.00 + 0.8z. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 01, 1964
- Accession Number
- AD0455540
Entities
People
- George G. Shor Jr.
- Helen K. Kirk
- Richard P. Phillips
Organizations
- Scripps Institution of Oceanography