Using LIDAR, geologists have created a remarkably detailed new map of Crater Lake National Park that reveals gentle ripples in the ground left by the eruption that formed the caldera 7,700 years ago.
The LIDAR technique uses a range finder that shoots lasers, or little pulses of light, to measure the distance between a plane and features on the landscape. Many of the laser pulses bounce off of the tree canopy, but others reach the bare ground, providing geologists with a detailed picture of the topography.
“The density that we work with for getting to the bare ground is 4 or 5 laser returns per square meter,” says Joel Robinson with the U.S. Geologic Survey.
Robinson says that underneath the dense forests in the Castle Creek and Annie Creek watersheds, the LIDAR data showed long ripples, just a few feet high, in the forest floor.
The subtle ripples were created by plumes of hot ash and gas — called pyroclastic flows — that billowed across the ground during the eruption. Robinson says the undulations in the forest floor, which are about 30 feet long, were impossible to see without the help of LIDAR.
Careful observers at Crater Lake can spot many other subtle geographic features created by Mount Mazama’s eruption, including charred tree trunks that have been preserved in deposits of pumice and ash for 7,000 years.