The Earth is a ball of liquid rock and iron, with a thin solid crust
| The Earth's crust moves like | the cool crust on a hot lava lake |
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The Earth is liquid, with a thin solid crust floating on the surface. Inside is liquid rock called 'magma', thousands of km deep, down to a core of liquid iron . This crust is broken up into massive 'plates' of solidified rock, 10-40km thick, and thousands of km across, that form our Continents and Oceans.
Powerful currents in the magma jostle them around, so they move very slowly over millions of years.
The modern continents of America, Europe and Africa were once all joined together. But during the first age of the dinosaurs, the Jurassic Era, they began to drift apart, forming the Atlantic Ocean in between. The Atlantic Ocean continues to split down the middle, and the continents move 2cm further apart each year. The hot centre of The Earth is 6,000°C in the inner core. The heat comes from when the planet formed 4.5 billion years ago, and from nuclear reactions happening inside (radioactive decay, which is atoms splitting). The heat keeps the earth liquid and drives powerful convection currents that circulate the liquid around inside. | |
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0-400°C - The lightest rock floats on the surface like a lightweight scum, 10-40km thick, frozen into a solid rock crust floating on the surface.
500-3,000°C - Most of the planet is made of hot liquid rock (called 'magma'), in a layer 3000km deep. 4-5,000°C - Heavy materials (iron and metals) mostly sink to the middle, forming the Earth's iron core, 3000km to the centre. Most of the core is liquid iron. >5,500°C - The last 1000km of the core is The pressure in the core is so extreme that, 1000km from the centre, iron compresses from liquid into a solid ball, surrounded by 2000km of hot swirling liquid iron. |
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Exploding Volcanoes Lava is melted rock from deep inside the Earth's hot centre, but sometimes it can be 'fizzy' almost like a soda drink, due to steam dissolved in it. Lemonade is fizzy because of carbon dioxide (CO2) gas dissolved in it under pressure. Release the pressure, and the CO2 escapes as an explosion of bubbles. | |
| Fizzy Drink |
Fizzy Volcano |
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The same happens in a volcano. Extreme high pressures many kilometres below the ground dissolve steam into the lava. The lava rises to near the surface where the pressure is much lower, and KABOOM!!!! ... As the lava reaches the surface, there is no pressure keeping the steam in the lava, so it bubbles out, sometimes explosively like a soda bottle.
At the famous Mount Saint Helens eruption in 1980, lava forced its way up from the depths, slowly pushing the volcano's peak higher, until one side of the mountain collapsed. The collapse removed tonnes of pressure from the lava in seconds. The resulting fizz caused a blast to rip sideways from the mountain at 200mph, wiping out over 250 square miles of mountain forest land. | |
Where does the steam come from?
The deepest parts of our world are ocean trenches, up to 10 miles deep, where strange fish and other creatures live in complete darkness. The pressure here is enough to crush a submarine, due to the weight of water above you at these depths. It presses everything a thousand times harder than the pressure of the air humans breathe every day. (Imagine trying to lift a bucket of water 10 miles high... that's what the water pressure is here.) The bottom of the ocean is a wet place, but it's wet below the sea bed too. The sand, mud, sandstone, cracks between stones and rock... the Earth's rocks are soaked in water, especially below sea level.
Subduction zone
Something strange happens in our deep ocean trenches. This is where oceanic crust slides under continents, down into the magma, where it melts into the mantle. And it drags water down with it, which doesn't boil because the pressure is too high, but it does dissolve into the magma. And so the magma is saturated with water.
(More information about how water content of magma affects eruptions:
https://www.nature.com/articles/s41467-018-02987-6)
https://phys.org/news/2016-11-temperature-content-magma-reservoirs-linked.html
Gentle volcanoes
Explosive eruptions usually happen near subduction zones, because lava has to force its way up under pressure through the thick layer of the earth's crust. However, not all volcanoes are explosive. In other parts of the world, tectonic plates are moving apart, and the lava can flow up through splits as they appear. Here the lava is hotter and more liquid, so it can flow out more gently. It still contains water, so can still be 'fizzy', but because it flows more freely without a 'lid' blocking its way, it is less explosive.
What happens in Subduction zones?
The earth's crust is constantly moving
Subduction zone
The Earth's crust is made up of huge 'plates' of solidified rock, 10-40km thick, and thousands of miles across, floating on heavier liquid magma underneath.
These plates form our Continents and Oceans.
Powerful currents in the magma jostle them around, so they move very slowly over millions of years.
The modern continents of America, Europe and Africa were once all joined together. But during the first age of the dinosaurs, the Jurassic Era,
they began to drift apart, forming the Atlantic Ocean in between.
The Atlantic Ocean continues to split down the middle, and the continents move 2cm further apart each year.
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