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The Earth Strip in the Hallway and the Magma of Erciyes
Defne and Kerem, sixth graders in Kayseri, lay out Earth's layers as a scale strip along the school hallway. When the strip shows just how thin the crust is and how thick and solid the mantle is, they discover for themselves where magma forms and why it rises through ice and warm water experiments.
Page 1
From Defne's classroom window, the majestic peak of Mount Erciyes was visible. In science class, they were studying the layers of the Earth, and during recess, her deskmate Kerem excitedly came up with a theory. "I think there's a fire-filled pipe beneath Erciyes that goes all the way down to the center of the Earth," he said. Defne had never thought about how long such a pipe would have to be, and she furrowed her brow with curiosity. The next morning, she arrived at school with a huge roll of paper tucked under her arm and a tape measure.
Page 2
Defne knew that Earth's radius was about six thousand four hundred kilometers. She decided that on the paper, each centimeter would represent ten kilometers, meaning the strip would be six hundred forty centimeters long. Kerem unrolled the paper along the hallway and measured its length with the tape measure. Defne then marked the layers with colorful paints. The mantle took up two hundred ninety centimeters, the liquid outer core two hundred twenty-five, and the solid inner core one hundred twenty-two centimeters.
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The crust, however, was merely a thin line of three and a half centimeters at the very start of the strip, because beneath continents, the crust was roughly thirty-five kilometers thick. Kerem walked from the beginning of the strip all the way to the center, taking more than ten steps. Meanwhile, Defne looked at the foam globe in their classroom: most of the red pins marking volcanoes lined up along the fracture lines where tectonic plates meet. What's more, the mantle was not molten, but solid rock flowing only a few centimeters each year. "Your pipe would have to pass through two hundred ninety centimeters of solid mantle rock," Defne said.
Page 4
So how did solid rock melt at plate boundaries? Defne had read that when an oceanic plate dives beneath another plate, it carries seawater deep down along with it. Maybe water caused the hot rock to melt at a lower temperature. Bringing real mantle rock into the classroom was impossible, so Defne designed an analogy experiment. From the cafeteria, she asked for two ice cubes from the very same tray, two small plates, and a pinch of salt.
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Defne placed the ice cubes on two separate plates and sprinkled salt only on the one to the left. In this experiment, the ice represented mantle rock, and the salt represented the water carried by the sinking plate. Ten minutes later, a wide puddle of water had formed around the salted cube, while the other cube was practically dry. "Salt lowered the melting point of the ice," Kerem said with excitement. "Water must lower the melting point of hot rock in a similar way!"
Page 6
The next question was why magma travels upward through kilometers of rock. Kerem filled a large glass jar to the brim with room-temperature water. Defne filled a small bottle with cold water from the refrigerator, added drops of red food coloring to it, and lowered the open bottle to the bottom of the jar using a string. Instead of rising, the red water spread across the bottom like a carpet and stayed there. "Cold water is denser, so it waits at the bottom," Defne said, pursing her lips.
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Defne emptied the bottle and this time filled it with hot water from the tap, adding two drops of food coloring. As soon as the bottle touched the bottom, a red plume curled upward and spread across the surface of the water. When water is heated, it expands slightly, meaning it takes up more space than the same amount of cold water. Because of this, it is less dense and rises. Magma, too, is less dense than the solid rock surrounding it, so it slowly floats upward.
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The first twenty centimeters of the strip represented the uppermost two hundred kilometers of the Earth, which was far too small to draw details. For that reason, they decided to draw that section as a close-up on a large piece of poster board. Defne painted an oceanic plate diving beneath a continent at an angle. At a depth of about one hundred kilometers, right above the sinking plate, she placed a red pocket of magma. Kerem then drew a fracture extending upward from there, along with a magma chamber.
Page 9
Defne knew that steep mountains like Erciyes did not grow from a single eruption, but from layers of lava and ash piling up over hundreds of thousands of years. On the poster board, she alternated thin layers of gray clay with strips of red clay to build a pointed cone. Kerem added an ocean floor to the other end of the poster board, where two oceanic plates were slowly pulling apart. There, mantle rock welling up from the depths was partially melting because the pressure upon it was decreasing.
Page 10
On presentation day, their classmates walked along the strip in the hallway, starting from the thin crust line all the way to the golden section at the center. One student placed two fingers over the crust and laughed in astonishment upon seeing that this layer was only two finger-widths wide. Kerem lowered the bottle of hot red water to the bottom of the jar once more. Defne first pointed to the rising red plume. Then, she slowly slid her finger from the magma pocket on the poster board toward the majestic peak of Erciyes glowing through the window.
