Nature Notes

Volume II No. 3 - September 1, 1929

Pumice And Scoria
By Dale Leslie, Ranger Naturalist

Tourists who are interested in rock collections usually find the display of vesicular lava in the museum at Crater Lake National Park fascinating. These forms of lava are known as pumice stone and volcanic scoria. In physical appearance both are closely related since each is very porous. However, the color of pumice is usually very light while that of scoria varies from red to black. Another relationship may be shown when it is considered that both of these vesicular forms are found on the surface of lava flows.

When hot lavas pour out upon the earth's surface they are known to contain an abundance of gases as evidenced in the clouds of vapor which arise from them. From the study of scoria and pumice, which are found on the surface of cooled lava flows, it is believed that there is not a complete liberation of all gases since these rocks very porous.

This porosity is probably created by the expansion of gases which are retained in the body of the lava, the expansion being permitted when the extrusive lavas are subject to a release of pressure. The top layers assume a foamy aspect. Upon cooling, the surface lavas maintain this puffed condition and their porosity is in this way explained.

It is interesting to note that the cooled surface lavas known as pumice arose from a very viscous flow, while that lava known as scoria arose from one which was very liquid. The greatest expansion of gases occurring in lava flows occurs in the more liquid types and for that reason scoria has a characteristic spongy appearance. Pumice on the other hand contains many small gas chambers being formed from a viscous rock. Consequently, the porosity is sufficient to allow pumice to float.

At Crater Lake National Park the greatest amount of scoria is found on Wizard Island and pumice is very abundant around the old Crater Rim.

By F. Lyle Wynd

Throughout the summer several eagles have been noticed flying about the crags near the southeast part of the Rim. While it is very difficult to positively identify birds flying so far away, it is believed that these were Golden Eagles.

In their immature stages Golden and Bald Eagles are very much alike. The mature birds are very easily distinguished if one is sufficiently close to see the white head and tail of the Bald Eagle. Both species are known to occur at Crater Lake. The only Bald Eagle so far reported this year was seen by Ranger Croghan in the early part of the season.

Several years ago, Golden Eagles nested in the vicinity of Castle Crest and a pair of Bald Eagles have been known to nest on Wizard Island. It is presumed that those seen over the lake are nesting somewhere about the Rim.

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