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Earthflows are born of plate tectonics. Roughly 60-140 million years ago much of the landscape that forms the California coast ranges was born out of two of the earth’s plates colliding in deep oceanic settings. At this convergent boundary, a denser oceanic plate grinds underneath the continental plate in a process known as subduction. Here, in the subduction zone, materials pass through the great tectonic blender. Oceanic sediments are scraped, folded, sheared and faulted perhaps many times over. Gradually, as material accumulates at the subduction zone interface and the plates continually buckle upwards, this material is uplifted as new land: our coast ranges are born. Pockets of the most pervasively sheared and deformed material are left scattered about the landscape.

These sheared and broken bodies of rock are inherently weak. As water accumulates, the material takes on more fluid properties. Thrust upwards from the ocean deeps, it’s as if they want to slowly find their way back to their birthplace in the ocean depths.

When and how earthflows activate and move is poorly understood. Shifts in the distribution of mass in the earthflow body likely play a key role. Similarly, abrupt changes in groundwater pathways may reactivate formerly quiescent areas. Stream erosion at the toe can undercut the earthflow and instigate movement. Excessively wet winters can enhance earthflow movement. Earthquakes, particularly during the wet season, are a factor. Finally, climatic fluctuations drive earthflow activity. With ongoing climate change, we are entering a new and unknown era for earthflow activity as rainfall and vegetation patterns shift.

Movement rates for earthflows vary enormously and discussing an “average” movement rate can be misleading. For example, large swaths of an earthflow might be dormant while one active lobe moves over 20 meters (60 feet) in a single year. In other cases, the movement might only occur during excessively wet winters. In slower moving earthflows, mature forests may hide evidence of movement.

Over a period of decades to centuries or longer, the source material is gradually depleted and the earthflow becomes dormant. But this may be short lived, geologically speaking. A period of rejuvenation may ensue as steep, unstable gullies propagate through the stagnant earthflow mass, further destabilizing the feature and breathing new life into the landform.

If we step back, large swaths of the landscape we see is a complex mosaic of active and dormant earthflows. Some stretch quietly under old forests, having exhausted themselves after a long, vigorous life. Others are perched, awaiting the arrival of the next wet winter to come to life.

(Sam Flanagan is a geologist who works with the Bureau of Land Management out of the Arcata Field Office)

This article is part of a series about natural life on the Lost Coast, sponsored by the Lost Coast Interpretive Association, which may be contacted at lostcoastintassoc@gmail.com. For photos, educational information and news about the Lost Coast, please visit the Lost Coast Interpretive Association’s facebook page. _https://www.facebook.com/pages/Lost-Coast-Interpretive-Association/241253955966296.

photo caption:

Recent work by researchers analyzed the movement of earthflows along the mainstem Eel River approximately 19 km (12 mi) upstream of Alderpoint. Individual earthflows are outlined and line segments show movement of trees, shrubs and rocks over a period of 62 years. The Boulder Creek earthflow in the center of the photo extends approximately three miles, pushing the mainstem Eel River to the west. Figure courtesy of Dr. Ben Mackey, University of Canterbury, New Zealand.