Sand Spikes: Century-Old Mystery Solved and a New Tool to Identify Regions Prone to Major Earthquakes

Miocene-age sand spikes from Ochsenhausen (southern Germany) with several intergrown individuals, almost identical in appearance to those found at Mount Signal, California. The spikes are on display at the Braith-Mali-Museum Biberach an der Riss. Credit: Volker J. Sach.

Sand spikes, which are pin-shaped, carbonate-cemented sandstone bodies sometimes more than a foot long, were discovered at Mount Signal in southern California around a century ago. However, until recently, the formation of sand spikes has been enigmatic. Scientists have been puzzled by the eye-catching bulbous heads and protruding tails of these spikes and have commonly interpreted them as concretions that grew slowly below the groundwater table. Alternatively, it has been suggested that they are petrified mushrooms or animal burrows.

In a recent paper, Elmar Buchner (HNU Neu-Ulm University of Applied Sciences) and colleagues present a novel, dynamic formation model for sand spikes. The authors investigated several sand spike occurrences and, based on structural and stratigraphic features of these occurrences, suggested that sand spikes are a previously unrecognized type of seismite — a rock modified by the passage of high-energy seismic waves. They pointed out that sand spike tails at Mount Signal consistently point away from the nearby San Andreas Fault, and sand spikes in Miocene-age sands in the North Alpine Foreland Basin of southern Germany systematically point away from the 15-mile-wide Nördlinger Ries impact crater located some tens of miles to the north. The latter is confined to the Ries seismite, up to a 15-meter-thick layer of intensely deformed, sand-dominated sediments that were located near the paleo-land surface at the time of the Ries impact, which caused a magnitude 8.5 earthquake 14.81 million years ago.

Although sand spikes are rare geologic phenomena, there may be additional places on Earth where they may occur. According to Buchner and colleagues, sand spikes are preferably produced in partially water-saturated sands during strong earthquakes of magnitude 7 and greater. Sand spikes are, therefore, a promising new tool for identifying strong tectonic or impact-induced palaeo-earthquakes when assessing the natural hazard potential of seismically active regions. READ MORE