Earth is a violent planet, and always has been. In fact it is much calmer today than in the past. As the planet continues to cool – 4.5 billion years after it formed – what was once likely a lava world has become a temperate planet that's two-thirds covered by water and hospitable to life.
But recent events and new research show that the geologic havoc is far from over.
A cosmic impact ended the age of dinosaurs, scientists now confirm. Such blasts and explosions have been pivotal throughout Earth's history, since its very beginning, with a colossal collision that apparently created the moon and reshaped Earth just as it was developing. Researchers say we'll eventually be struck by another devastating comet or asteroid, though the odds go down with time as the solar system's formation debris gradually becomes scooped up by our world and others.
Since continents first formed, they've constantly shifted, with volcanoes and earthquakes shaping and reshaping the surface, islands and mountains emerging and being wiped out over the eons.
The fact that the recent earthquake in Chile was powerful enough to shift Earth's axis by 3 inches just goes to show the planet remains a violent, dynamic place to live.
Cosmic impacts
Currently the leading idea behind the origin of the moon has it born in violence "in a great collision between the Earth and an object the size of Mars roughly 4.5 billion years ago, just 30 to 50 million years after the Earth formed," said planetary scientist Jay Melosh at Purdue University.
The evidence for this idea comes from moon rocks the Apollo missions returned. While the moon differs from the Earth in having fewer volatile materials, such as water, sodium and potassium, it remained eerily similar to our planet when it came to its oxygen, silicon and magnesium content.
"It's clear the moon is a close relative," Melosh said. "The composition of the moon could be explained if you took material from the Earth's mantle, put it in [a] vacuum, vaporized it and recondensed it, which can pretty well be explained by a giant impact."
Giant craters on most of the planets in the inner solar system hint that it was a shooting gallery between 4.1 billion and 3.9 billion years ago, a period scientists dub the Late Heavy Bombardment.
"This huge pulse of impacts might have helped the continents on Earth form," Melosh said. These collisions might have re-melted our young planet's solidifying crust, with light minerals eventually cooking out to form the surface of shifting continents we have on the surface today.
"Giant impacts have punctuated the history of Earth from its beginnings, and even before that, with the Earth's core probably formed by impacts of large protoplanets colliding together," Melosh said. "Our planet was born in violence, and even though the violence has subsided a bit, moderate-sized events still happen now and again."
A giant impact also might have ended the age of dinosaurs. A panel of scientists, reporting this week in the journal Science, say they have come to a consensus that a large asteroid impact 65 million years ago in modern-day Mexico is the major cause of the mass extinction of dinosaurs and other forms of life.
"Once the impact occurred, almost overnight, the land was wiped clean of large animals, with nothing larger than 15 kilograms [33 pounds] surviving, and oceans that once were dominated by large predators, such as mosasaurs and plesiosaurs, saw them disappear, as well as nautilus-like ammonites that had lived for 300 million years and even very complex single-celled organisms such as radiolarians."
With the global devastation the impact wrought, little food would have been available to support large animals.
Still, impacts not only might have claimed lives on Earth, but also helped start it. A rain of cosmic impacts might have delivered water and carbon-based molecules from space.
"Comets and asteroids could have seeded the early Earth with the building blocks of life," Melosh said.
Volcanic eruptions
Earth has seen massive volcanic eruptions that dwarf anything noted in recorded history. For instance, a series of colossal volcanic eruptions between 63 million to 67 million years ago may have spread lava over as much as 580,000 square miles (1.5 million square kilometers) — more than twice the area of Texas — creating the gigantic Deccan Traps lava beds in India.
"We call these events 'flood basalts,' and they would have lasted thousands and thousands of years," said volcanologist Scott Rowland at the University of Hawaii.
Supervolcanoes have also generated eruptions up to thousands of times larger than anything modern times have ever seen. At Yellowstone, scientists have seen evidence for three supervolcano eruptions about 2 million years ago, 1.2 million years ago and 600,000 years ago that explosively sent ash over hundreds of square miles.
Eventually, researchers predict, the Yellowstone supervolcano will blow again, covering half the United States in ash up to 3 feet deep.
The eruptions that humanity has witnessed may have dramatically shaped the course of our history. In 1815, Mount Tambora in Indonesia exploded with the force of roughly 1,000 megatons of TNT, the largest volcanic eruption in recorded history. The blast hurled out roughly 140 billion tons of magma and not only helped kill more than 71,000 people on the island of Sumbawa and nearby Lombok, but the ash it released created global climate anomalies. The following year, 1816, became known as the Year Without a Summer, with snow falling in June in Albany, N.Y., river ice seen in July in Pennsylvania, and hundreds of thousands of people dying of famine worldwide.
"There are stories of explosive eruptions throughout history that may have cause a society to fail or a tyranny to come to power," Rowland said.
Still, as destructive as volcanoes can be, "the gases in the atmosphere were released by volcanic eruptions, so one could argue that without volcanoes we would not have an atmosphere, and probably not us as well," Rowland said.
The same movement of Earth's crustal plates that builds mountains and fuels volcanoes also triggers the most unpredictable of Mother Nature’s deadly arsenal: earthquakes. Scientists say the recent earthquake in Chile, though at magnitude 8.8 is one of the largest in recorded history, is not the biggest we can expect.
Though earthquakes can't be predicted with any timely precision, seismologists are confident that similar or larger quakes are possible, within our lifetimes, in several earthquake hotspots around the globe.
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