Creep, slump, and slide
A hiker's-eye view of Bay Area earth movements
Hiking in winter through the Bay Area hills, you may notice movements in the landscape. You may notice not just the movement of the trees in the winter wind, or of the over-wintering birds flitting here and there, but some of the much slower movements of the landscape itself - the landslides and slumps seasonally activated by our winter rains. The region's steep topography adds to its natural beauty, but also leads to gravity-caused movement of the soil and rocks. This movement, which I refer to here informally as "landslides", varies from imperceptibly slow, as in soil creep, earth flows, and debris slides, to extremely fast and dangerous, as in mud flows and rock falls.
Some fresh landslides are very obvious, exposing bare rock facies at their top and piles of loose debris at their base. Old and more subtle landslides and their results often can be detected by observing objects within the scenery. Signs of landslide activity include slopes with curving tree trunks growing from them, or with tilted trees or telephone poles, and hillsides with cracks or hummocks. In contrast, rapid-moving mud and debris flows cut small gullies or ravines in the soils on steep slopes, and often deposit a small, upside-down, cone-shaped pile of debris at the base of the slope.
Landslides result from the pull of gravity on rocks and soil. Active uplift and tectonism in the Bay Area has produced many steep slopes, a key element in producing landslides. Other factors include bedrock and soil type, amount of ground cover, and climate. Fires can make a slope more susceptible to sliding, particularly to fast-moving debris flows, by removing binding and absorbent groundcover. Landslides of all types are often triggered by heavy winter rainfall, particularly if it lasts for several days. As rainwater infiltrates the soil and rock, it adds to the weight of the material, increases its hydrostatic pore pressure (pushing the particles apart and making it easier for them to slide downhill), and lubricates the slope material, all adding to the potential for slope failure. During extreme rainfall periods, as in El Niño years, hundreds of landslides may occur within days, and ancient landslides can be reactivated. California's earthquakes can also trigger landslides, particularly when hills are already soaked.
Landslides are classified based on the type of the materials moved or deposited and the shape or form of the slide. Rock and/or soil are the basic materials and may be further categorized by particle size moved, ranging from house-sized blocks down to fine mud and soil. Landslide forms include falls, slides, and flows among others. Common Bay Area landslides are rock slides and earth flows. Rock slides move a large more-or-less intact block of bedrock and produce your classic landslide form, with a steep scarp at the head, a less-steep, rotated slide mass below, and an elevated hummocky toe of slide material at the base. In contrast, earth flows are mainly composed of soil that slowly flows downslope as a viscous fluid, forming a hummocky slide area below a complexly cracked head scarp. Our area's other typical landslide is the debris flow, a slurry of mud, gravel, and organic-rich soil materials that flows rapidly down a steep slope in response to intense rainfall events. A debris flow leaves behind only a small, shallow depression where the initial slide forms, before cutting a small denuded gully or ravine with a small debris pile at its base. Because of their speed, debris flows can be life-threatening.
A landslide can happen on essentially any rock type if the slope is steep enough, but several Bay Area rock types are particularly susceptible. The Franciscan Complex, which formed in a tectonically active subduction zone and underlies much of the Bay Area, contains highly fractured rocks, but some are more fractured than others, making them particularly susceptible to sliding. One such rock is serpentinite, an altered mantle rock that has made its way to the surface by squeezing up along faults. In the process the serpentinite becomes highly sheared and fractured, making it very unstable when cropping out on slope. Other areas of the Franciscan Complex are composed of a broken-up mixture of many types of rocks; such areas of "mélange" can be almost entirely covered by landslides. The hills of the Diablo Range in some areas are composed of relatively soft mudstone and sandstone that is also prone to creeping, slumping, and sliding.
Because landslides are a risk to structures and sometimes lives, they are the subject of extensive geologic study and mapping. Both the U.S. and the California Geological Surveys have conducted landslide research in the Bay Area and published maps showing landslide distributions and risks. You can find more information on Bay Area landslides at their web sites.
On steep slopes, landslides are inevitable and building in those locations is always a risk. But in the natural areas where we love to hike, landslides can provide an interesting aspect of the scenery and a glimpse into the geological processes that form our landscape. Next time you hike in the hills, look around for that winter movement.