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.