A playa is a dry lake. The water that used to fill Racetrack Playa evaporated (disappeared into the air) many years ago and left a three-mile-long (4.8 km) and two-mile-wide (3.2 km) layer of thick, yellowish-brown mud.
Racetrack Playa lies in between two mountain ranges (a group of mountains in a line) in Death Valley – the hottest, driest, and lowest area in the U.S. Death Valley is in the Mojave Desert in eastern California, about 140 miles (~225 km) west of Las Vegas.
Nearly 100 years ago, visitors noticed that the rocks on the playa – some larger than a man – moved from time to time. One year they would be in one place and the next year in another. And when they moved, they left tracks, or trails, in the soft mud.
The rocks’ movement was rarely the same. Sometimes they moved a few inches (1 inch = 2.54 cm), other times much farther. Sometimes the tracks were straight, other times they curved or even zigzagged (moved like a “Z”) across the playa.
Many scientists have tried to explain why the rocks move the way they do. But no one has succeeded, that is, until recently.
The mystery was solved one day last December by two scientists, Richard and James Norris, according to an article in the Los Angeles Times. When they arrived at the playa, Richard said that it was covered with ice. They also noticed new rock trails near piles of broken ice along the shoreline (edge of the lake).
The next day, the two cousins were sitting nearby when they heard loud cracking (breaking) sounds from the playa. “It’s happening,” Richard yelled.
The sun had begun to melt the ice, and when the wind began to blow, the ice began to break into floes (areas of floating ice). The wind blew the floes across the lake and into the rocks. As the Norrises watched, the large, thin floes pushed the rocks so they began to slide across the slippery (wet, smooth) mud of the normally-dry lakebed (bottom of the lake).
So, what happened? What made it possible for the Norrises to see what no one had ever seen before? The answer is that they were there at the right time, when the all the conditions (things that must happen before something else can happen) were just right. What were these conditions?
First, there was water in the playa from one of the infrequent (rare; not happening often) rains or runoff (water from melted snow) from the nearby mountains. The water makes the lakebed soft and slippery. And it was deep enough for ice to float on top of it, but not deep enough to cover the rocks.
Second, the water froze enough to form what they call “windowpane” ice – ice that is thin enough to move freely (easily) on top of the water but thick enough that it doesn’t easily break.
When the ice began to melt, it broke into floes that a light (not strong) wind was able to blow across the shallow lake. When the floes moved, they pushed the rocks in front of them, and the rocks left their telltale (a sign that shows that something has happened) trails in the soft, slippery mud.
Mystery solved (to find the right explanation for something that is difficult to understand)!
~ Warren Ediger – ESL coach/tutor and creator of the Successful English web site, where you’ll find clear explanations and practical suggestions for better English.
Photo credit: www.onlinefreecomputers.com.