File:Re-exhumed sub-fossil trees (died late 1700s) (Sandy River, Oxbow Park, Oregon, USA) 51 (27020702579).jpg

These remarkable, still-standing, prehistoric trees are exposed along the Sandy River in northwestern Oregon.

The Sandy River has a deeply incised, meandering river valley. The river itself is about 300 feet below the surrounding land surface (= a Middle Pleistocene-aged surface). In 2008, flooding in the Sandy River resulted in erosion that exposed once-buried trees - a ghost forest. Shown here is the site in fall 2009.

The trees - apparently conifers - died in the late 1700s. They were part of a forest that was buried starting in 1781. It probably took a decade to bury them and kill them. The 1781 event was deposition of a lahar - a volcanic mud flow - from Mt. Hood, a subduction zone stratovolcano in northwestern Oregon. Mt. Hood is part of the north-south trending Cascade Range of volcanoes. Other famous Cascade volcanoes include Mt. St. Helens, Mt. Rainier, Mt. Shasta, and Mt. Mazama (now Crater Lake Caldera).

Mt. Hood area volcanism started in the Middle Miocene (8 to 10 million years ago), just after Columbia River Flood Basalt volcanism. Miocene and Pliocene andesites and basalts cap topographic ridges in the Mt. Hood area. During the Late Pliocene (sensu traditio), at 3 to 1.3 million years ago, the Sandy Glacier Volcano occupied the site of the current Mt. Hood. It has been mostly buried by the modern Mt. Hood volcanic cone. Mt. Hood itself is less than 730,000 years old - all of its rocks have modern magnetic signatures. Lavas and other eruptive materials are high-silica andesites and low-silica dacites. 70% of the Mt. Hood cone consists of lava flows, while the remaining 30% is volcaniclcastic deposits. The modern cone is less than 300,000 years old. Near the summit of Mt. Hood are <200,000 year old lava flows.

Unlike nearby Mt. St. Helens, Mt. Hood is much older and has had little explosive activity. Most of Mt. Hood's volcanism has consisted of andesite lava flows and dome-building. Few explosive events have occurred through time here - few tephra deposits have a Mt. Hood source.

Mt. Hood rocks are often porphyritic two-pyroxene andesites, plus a little olivine. There's been little chemical variation in Mt. Hood lavas through time. Because of this, individual lava flows are difficult to date based on lithology - they're all the same. Much of Mt. Hood itself is hydrothermally-altered rocks.

Names are assigned to the various eruptive phases in Mt. Hood's history. The Polallie eruptive phase occurred from 12 to 25 thousand years ago. The Timberline eruptive phase occurred ~1500 years ago. The Old Maid eruptive phase occurred over 200 years ago, often dated to 1780-1801 A.D. Dome building occurred from 1781 to 1793. During that time, periodic, relatively small eruptions occurred.

Because Mt. Hood is a snow-clad volcano, activity results in melting of snow and mobilization of loose materials. Rainstorms could also mobilize loose debris in the area. Mt. Hood lahars have probably formed by both mechanisms.

Looking back at the photo, a 1781 lahar deposit is exposed near the present river level. This site is 70 kilometers downstream from Mt. Hood. All of the overlying sediments (= most of the riverbank slope here) are fluvially-redeposited volcaniclastic sediments, deposited during the decade after 1781. These sediments are mostly cross-bedded, medium-grained to coarse-grained sands with some pebbles - they are all fluvial and loosely compacted.

In 1793, the Sandy River started to incise through the sediment fill. Most of the 1800s was needed for the river to erode through Old Maid-aged deposits to the present level of the river.

The trees shown here are not fossils. A fossil is any evidence of ancient life - "ancient" meaning Pleistocene or older. This ghost forest is late Holocene in age (late 1700s A.D.) and the trees, once buried and now exposed, are "sub-fossils". The wood is unaltered - they are not "fossilized". There is a difference between being a fossil and being "fossilized". The former refers to the age, while the latter refers to the preservation style involving altered parts (altered hard parts or altered soft parts or both).