Dozens of Yellowstone-Sized Volcanoes Once Drowned Nevada in Thousands of Feet of Lava

Parts of western U.S. were once covered in thousands of feet of lava from dozens of Yellowstone-size supervolcanoes that erupted between 40 and 20 million years ago, scientists have said.

These eruptions pushed the Great Basin upwards, leading it to become the highest point in North America for about 10 million years, before continental shifts pulled it apart, forming the Sierra Nevada we see today.

Elizabeth Miller, a geologist from Stanford's School of Earth, Energy and Environmental Sciences, was mapping a fault system in California's Death Valley when she performed analysis on some rocks believed to have been formed in the region. Her findings showed that they were actually from central Nevada.

How the Sierra Nevada mountain range formed has long been the subject of scientific research. As new evidence is unearthed, the geological history of the region has changed multiple times. Some of the oldest rocks can be traced back over 450 million years, while the granite dates to around 220 million years.

Around 100 million years ago there was a chain of volcanoes, some of which reached 20,000 feet. This is what Miller and colleagues refer to as the ancestral Sierra range.

The team has now published two chapters in a Geological Society of America Special Paper that looks at the formation of the modern-day Sierra Nevada, saying the ancestral range and the mountains we see today were born at different times.

The researchers suggest this chain of volcanoes was dwarfed when an extreme period of volcanism began in southern Idaho, Nevada and Utah about 40 million years ago.

They say there were dozens of supervolcanoes with calderas comparable to Yellowstone, as well as hundreds of smaller volcanoes. Activity in this region lasted for around 20 million years, and saw parts of western U.S. covered in thousands of feet of lava.

These events saw the Great Basin rise up, making it the highest point in the U.S. for a period. The Sierra Nevada mountain range acted as a "ramp" to this high country in Nevada.

This upheaval led to a new system of rivers that carried sediment southwards and deposited it in Death Valley for Miller and her team to analyze millions of years later.

"The material from those volcanoes made it all the way out to the Pacific side of the Sierra Nevada—that's how we know the region in central Nevada where the eruptions occurred was higher than everything else," Miller said in a statement.

The land remained at a high elevation, they say, until around 10 million years ago. At this time, the western U.S. was pulled apart by basin and range faulting. This broke up the Nevada plateau and Sierra Nevada became the mountain range we see today.

"You need to know when things happened and how long it took things to happen to truly understand them in the geologic context," Miller said. "It's an evolving story, and as we pick up more pieces, the story begins to get tighter and tighter."