07 Physical Properties

Pumice Deposits of the Klamath Indian Reservation, Klamath County, Oregon

Physical Properties

The textures of the pumice flow and fall deposits are distinct. The fall deposits consist of fine-grained, well-sorted, fragments of pumice; whereas the pumice flow is characterized by abundant pumice lumps of various sizes in an impure matrix of crushed and powdered dacite glass. Numerous mechanical analyses made by Moore (1937), Williams (1942), and others have exhibited these gross differences in texture, but minor textural differences are infinitely variable. The writer has selected a number of representative mechanical analyses and has included the data in tables 1 and 2.

Pumice Fall

The fresh pumice fall material is white or light gray, but where weathered it is pale buff or brownish yellow. Little or no stratification is evident in these deposits; they are neither compacted nor cemented. The individual pumice grains are angular to subrounded and nearly equi-dimensional. They contain phenocrysts of feldspar, hornblende, hypersthene, augite, and magnetite. Commonly the material is well sorted; the grains range from 1. 2 to 2. 4 millimeters in diameter, though small amounts of both larger and smaller fragments have been found. The smaller fragments are usually mineral grains. A few fragments of foreign rock material are found in the fall deposits.

Air cells in the pumice are of nearly uniform size and are evenly distributed. As a result pumice fragments are fairly strong and resistant to crushing. One sample tested by the Bureau of Reclamation absorbed 40 percent its initial weight after being immersed in water for 5 minutes.

Pumice Flows

The pumice flow material is pale gray to buff where fresh, but the weathered top 5 to 10 feet of these deposits is stained a pink or pinkish buff. The pumice debris in the flows is neither cemented nor highly compact and stratification is faint or absent. Poorly defined layers and lenses of scoria and cinders are interbedded with the pumice. The distribution of these layers is haphazard, and it is not possible to ascertain either their distribution or thickness from surface evidence. The scoria and cinders are gray, black, or red and consist of highly vesicular brown or gray glass, which locally contains abundant phenocrysts. All samples of the scoria that were tested were basaltic in composition. Locally, the pumice flows contain some charcoal logs and fairly abundant extraneous flow rock fragments. The pumice flows consist of lumps of pumice embedded in a matrix of finer material made up of both mineral grains and glass fragments.

Mechanical analyses show a greater range in-the size of the pumice-flow particles than in that of the pumice fall. A fairly large portion of grains and fragments in the matrix are less than 1 millimeter in diameter, whereas the lumps are large subrounded fragments. that are as much as 1 foot or more in diameter. Locally, the matrix comprises 50 to 60 percent of the total material.

The pumice is highly vesicular and frequently very frothy and fibrous dacite glass, which nearly always contains moderately abundant phenocrysts and crystal fragments of plagioclase (oligoclase to labradorite), hornblende, quartz, hypersthene, biotite, and magnetite. Small amounts of opal have also been found as fragments in the matrix and, locally, the walls of some vesicles are thinly coated with opal. The vesicles have a great range in size and because of this the pumice commonly has little structural strength and can be readily crushed, differing in this respect from grains of pumice fall. Laboratory tests by the Bureau of Reclamation on lumps of pumice indicate a high degree of interconnection of voids. Some lumps absorbed as much as 1-1/2 times their initial weight of water during only 5 minutes immersion.

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