Laboratory studies of clay-mineral samples have been made from two mud volcanoes on the island of Trinidad--Moruga Bouffe and Palo Seco. The mineral content and physical properties have been compared with clay samples from the lower Cruse and Nariva Formations, through which the throats of the mud volcanoes are believed to have penetrated.

The clay minerals are mixed-layer illite-montmorillonite and kaolinite. In the formational samples, illite forms the larger part of the mixed-layer clay. Montmorillonite is preponderant in the clay from the mud volcanoes. The kaolinite in the formational clay is more ordered in crystal structure than that of the mud volcanoes. These slight differences produce interesting variations in the behavior of these clays. The formation samples contain 55-69 percent < 2µ material, but the mud-volcano samples, possibly because of mixing with silt and fine sand during extrusion, contain only 33-44 percent. The shear strength of the mud-volcano samples at 35 percent water content ranges from 0.2 to 0.4 tsf, whereas the formational samples with the same water content range from 0.4 to 0.7 t f. At greater water contents the shear strength drops rapidly. Viscosity measurements show that all the clay samples examined, when water saturated, become thixotropic with the capacity to flow as a fluid.

Higgins and Saunders suggested that the mud-volcanoes have formed either as a result of tectonic movement, accumulated gas pressure, or a combination of the two mechanisms. It is suggested herein that the thixotropic character of the clay may have greatly accelerated either mechanism. Thixotropic clay saturated with water becomes a highly mobile fluid with great transporting power. Where such a mobile mass accumulates along a partly broken anticlinal structure, any release of pressure will occur upward. A gas pocket, tectonic squeeze, or both will provide pressure to maintain upward motion. In this way the clay would be extruded to form a mud volcano. Although moving vertically and supported by gas and tectonic pressure, the clay flowage has characteristics comparable to horizontal cl y movement in landslides where motion is maintained by gravity.