Carnegie Ridge and Cocos Ridge in the East Equatorial Pacific

Abstract

Recently obtained sounding lines have been combined with published Hydrographic Office soundings to show the generalized bathymetry in the east equatorial Pacific. Carnegie Ridge is a relatively shoal area west of Ecuador, of which approximately 10,000 square miles lie shoaler than 1,000 fathoms and the shoalest portion is within 750 fathoms of the surface. The shallow northern extension of the Peru Trench bathymetrically separates Carnegie Ridge from the continental terrace. Cocos Ridge is a submarine ridge which extends southwestward from Costa Rica to the northern part of the Galapagos Islands. A relatively shoal zone of irregular topography is present for at least 560 miles west of the islands. Between Costa Rica and the Galapagos Islands the general depth along the ridge is between 900 and 1,000 fathoms; but irregularities project to lesser depths, and one of these forms volcanic Cocos Island. Four echograms taken across the ridge failed to reveal truncated seamounts upon it. On the basis of the bathymetry of Carnegie Ridge and the geological history of Colombia and Ecuador, it is speculated that the easternmost portion of Carnegie Ridge may once have been part of the western borderland for the early Tertiary geosyncline which existed along the present coastal lowlands of western Ecuador. This would have shortened the sea route to the Galapagos Islands and possibly facilitated the rafting of plants and animals to the Islands. A recently proposed explanation for the origin of Galapagan life called for the emergence of Cocos Ridge to within a short distance of the Islands in Miocene time, but the apparent absence of truncated seamounts on Cocos Ridge is evidence against a former emergence. The portion of Cocos Ridge lying west of the Galapagos Islands resembles, in trend and topography, the four great submarine fracture zones which are found west of North America.