Chapter Fourteen — The Fire Below
A Short History of Nearly Everything – by Bill Bryson
Chapter 14 — The Fire Below
- It turns out that under the western United States there is a huge cauldron of magma, a colossal volcanic hot spot, which erupts cataclysmically ever 600,000 years or so. The last such eruption was just over 600,000 years ago. The hot spot is still there. These days we call it Yellowstone National Park.
- The distance from the surface of the earth to the center of the Earth is 3,959 miles, which isn’t so very far. It has been calculated that if you sunk a well to the center of the Earth and dropped a brick into it, it would take only forty-five minutes for the brick to hit the bottom of the well (though at that point it would be weightless, since all the Earth’s gravity would be above and around it rather than below it).
- Most mines on Earth go no more than about a quarter of a mile beneath the surface. If the planet was an apple, we wouldn’t yet have broken through the skin. Indeed, we haven’t even come close.
- The Richter scale, by which earthquakes are measured, is more an idea than an object. It is an arbitrary measure of the Earth’s tremblings based on surface measurements. The scale rises exponentially, so that a 7.3 quake is fifty times powerful than a 6.3 earthquake, and 2,500 times more powerful than a 5.3 earthquake.
- The largest earthquake since the invention of the Richter scale was either one centered on Prince William Sound in Alaska in March 1964, which measured 9.2 on the Richter scale, or one in the Pacific Ocean off the coast of Chile in 1960, which was initially logged at 8.5 magnitude but later revised upward to 9.5.
- For pure, focused devastation, the most intense earthquake in recorded history was one that struck (and essentially shook to pieces) the city of Lisbon, Portugal on November 1, 1755. Just before ten in the morning, the city was hit by a sudden sideways lurch now estimated at magnitude 9.0 and shaken ferociously for 7 full minutes. The convulsive force was so great that the water rushed out of the city’s harbor and returned in a wave 50 feet high, adding to the destruction. Two destructive aftershocks followed. At the end of it all, 60,000 people were dead and virtually every building for miles around was reduced to rubble.
- Earthquakes are fairly common. Every day, on average, somewhere in the world there are two earthquakes of magnitude 2.0 or greater.
- Tokyo has already suffered one of the most devastating earthquakes in modern times. On September 1, 1923, the city was hit by what is known as the Great Kanto quake. This event was more than 10 times as powerful than the Kobe earthquake. Two hundred thousand people were killed.
- What little is known about the mantle of the Earth is due to structures known as kimberlite pipes, where diamonds are formed. These pipes are produced when explosions deep in the Earth fire a cannonball of magma to the surface at supersonic speeds. These explosions are totally random events. A kimberlite pipe could explode almost anywhere on the surface of Earth. These pipes come from great depths (up to 120 miles) and bring up all kinds of things not normally found on or near the surface of Earth (peridotite, olivine, and diamonds).
- Scientists are generally agreed that the world beneath us is composed of four layers — a rocky outer crust, a mantle of hot viscous rock, a liquid outer core, and a solid inner core. The surface is dominated by silicates. We know that to generate our magnetic field somewhere in the interior there must be a concentrated belt of metallic elements in a liquid state.
- The pure mantle of the Earth accounts for 82 percent of the Earth’s volume and 65 percent of its mass.
- The pressures at the center of Earth are three million times those found at the surface.
- Since Earth was formed over 4 billion years ago, the temperature at the core has fallen by no more than 200 degrees Fahrenheit. It is estimated that the temperature at the core is between 7,000 and 13,000 degrees Fahrenheit, or about as hot as the surface of the Sun.
- We know that the Earth’s magnetic field changes in power from time to time. During the dinosaur ages, it was up to three times stronger than presently. We also know that the magnetic field reverses itself every 500,000 years on average. The last reversal was about 750,000 years ago. Scientists do not understand why the field reverses itself. This has been called ‘the greatest unanswered question in the geological sciences’.
- Earth’s magnetic field protects us from dangerous cosmic rays that, in the absence of magnetic protection, would tear through our bodies, leaving much of our DNA in useless tatters. The magnetic field herds these rays safely away from the Earth’s surface.
- The eruption of Mount St. Helen’s in 1980 sent an enormous avalanche of dirt and rock rushing down the mountain slope at 150 mph. It was the biggest landslide in human history and carried enough material to bury the whole of Manhattan to a depth of 400 feet. The mountain exploded with the force of 500 Hiroshima-sized atomic bombs, shooting out a murderous hot cloud at up to 650 mph. The mountain lost 1300 feet of elevation and 230 square miles of forest were devastated. The resulting damage was estimated at $2.7 billion. A giant column of smoke and ash rose to a height of 60,000 feet in less than 10 minutes.
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