Burial of trees by eruptions of Mount St Helens, Washington:implications for the interpretation of fossil forests

Lahars and fluvial sediments which buried trees following the 18 May 1980 eruption of Mount St Helens were investigated during August of 1982. Trees buried in older mudflows, dated at a.d. 1885, a.d. 1450–1550, and 36000 years b.p., also were examined. Although many logs clearly were transported, large numbers of trees were buried in growth position. Burial by lahars generally resulted in the death of trees, whereas some trees survived burial by fluvial sediments. Scanning electron microscope studies show that trees buried in lahars are well preserved. Pre-1885 buried woods show incipient silicification, and woods buried 36000 years b.p. show silica impregnation of cell walls.Features of in situ and allochthonous burial very similar to those seen in southern Washington are found also in Eocene silicified forests in Yellowstone National Park, Wyoming, U.S.A., and in Cretaceous fossil forests in southeastern Alexander Island, Antarctica. Observations of Recent wood from volcaniclastic deposits of Mount St Helens provide valuable insights into processes of burial and silicification of fossil forests of various ages around the world.