MAP I-1148
Miscellaneous Investigations Series Map
U.S. Geological Survey

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Map showing landslides and areas of susceptibility to landsliding in Puerto Rico

Author: Watson H. Monroe

ABSTRACT

Landslides in Puerto Rico have caused considerable damage to property and some loss of life. The industrial development of the island since 1950 has necessitated excavation of slopes that has produced many potentially danger¬ous areas. Landslides are relatively common in a variety of rock types. The largest slides, several hundred cubic meters in volume, happen where lime-stone rests on clay that becomes plastic when wet; mudflows and rockfalls, on the other hand, are common in areas of deeply weatherd volcanic and intrusive rock.

Puerto Rico is a tropical island, elongate east-west and lying between the Caribbean Sea and the Atlantic Ocean. A central mountainous upland is composed of extrusive and intrusive igneous rocks and minor amounts of limestone, sandstone, and conglomerate of Early Cretaceous to Eocene age. The upland separates belts of limestone of middle Tertiary age cropping out near the southern and northern coasts. Along the coasts, relatively narrow coastal plains (fig. 1) are made up of beaches, swamps, sand dunes, alluvial plains, and alluvial fans; these features consist of sand, gravel, and clay.

Different rock types and unconsolidated materials have different weather¬ing characteristics. The alluvium, in general, is completely weathered to sand and clay. Limestone is dissolved in acid rainwater, and the impurities in the rock remain as a residuum of clay. Commonly the residual cover developed on limestone is washed away during heavy showers, leaving bare rock ex-posed. This exposed limestone is soon case hardened (Monroe, 1976, p. 17, 44), and further solution and weathering is retarded. The intrusive rocks, largely granodiorite and quartz diorite, weather very rapidly into ferruginous sandy clay, which is subject to rapid gullying; where the natural slope has been disturbed, small landslides occur. Residual boulders and corestones on much of the quartz diorite terrane are likely to move downhill during rainy periods. Corestones also form on some thicker lava and some tuffaceous sandstone. Most of the other rocks, such as lava, volcanic breccia, tuffaceous sandstone, claystone, siltstone, and conglomerate made up of volcanic debris, weather into tough clay that resists gullying. In the central mountainous portion of the island, the weathered material has been eroded into deep canyons; the walls of the canyons slump in massive landslides.

The citation for this map, in USGS format, is as follows:

Monroe, W.H., 1979, Map showing landslides and areas of susceptibility to landsliding in Puerto Rico: U.S. Geological Survey Miscellaneous Investigations Series Map I-1148, 1 sheet.