Abstract

The McCartys flow is approximately 30 miles long, in places several miles wide, approximately 1.7 cubic miles in volume, and it has an area of about 119 square miles. The flow is in the main unweathered, uneroded, and barren. Pueblo I potsherds have been found in a valley fill which is probably buried by the flow, indicating that extrusion occurred in the last 1200 years.

Minor features such as grooved lava, squeeze-ups, ropes, spatter cones, tree molds, cracks, banded lava, and cavities are present. The extrusion of the flow impeded the Rio San Jose and diverted drainage from it into the Rio Salado and Little Colorado. The flow moved by the flow-unit and single-unit mechanisms, and its velocity varied greatly from place to place.

Near the terminus of the flow are about 100 collapse depressions. Analysis of the probable strength of the roofs of lava tunnels indicates that they owe their origin not to the collapse of one large tunnel but to the partial collapse of several small tunnels. In some cases collapse occurred while liquid lava was still present and when the under side of the crust was still soft and plastic.

On the last mile of the flow pressure ridges are very common. They are as much as 1200 feet long, 25 feet high, and 100 feet wide. In transverse section they resemble the gable of a house. In general they parallel the flow and are close to its margin. They formed when liquid lava was beneath the crust. It is suggested that the collapse of a dome between 27 and 38 feet high, due to withdrawal of lava from beneath, resulted in compressional forces which buckled the crust and produced the pressure ridges.

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