Tuesday

Rocket draws a big circle on the Sky Deck chalkboard. "Melt, cool, break, settle, squeeze, melt again," he says. "A loop!"

Raven studies the circle. "What do you notice about the arrows? They do not all go one way."

"Oh." Rocket looks again. "A sedimentary rock could get pushed up and worn down without ever being squeezed."

Nova projects a small glowing diagram above the board. "Two engines run this cycle," she says. "One is inside Earth. One is in the sky."

"Heat inside," Raven says. "And the Sun, which moves the water and the wind."

Rocket taps the board. "So energy flows and rock goes round." Nova hums. "Now you have the big idea."

The open textbook Physical Geology says the rock cycle is driven by two forces.

The first is Earth's internal heat engine, which moves material in the core and mantle and slowly changes the crust.

The second is the hydrological cycle, the movement of water, ice and air at the surface. The Sun powers it.

The Next Generation Science Standards say the same thing. All Earth processes are the result of energy flowing and matter cycling. That energy comes from the Sun and Earth's hot interior.

Magma is rock hot enough to be entirely melted, between about 800 and 1300 degrees Celsius.

Magma that cools slowly inside the crust, over centuries to millions of years, forms intrusive igneous rock. Magma that erupts and cools in seconds to years forms extrusive igneous rock.

Mountain building lifts rocks to the surface. There they are weathered, broken by frost and roots and changed by chemistry, and the bits are carried off as sediment.

Buried hundreds of meters deep, sediments are compressed and cemented into sedimentary rock.

Buried deeper still, rocks are heated and squeezed into metamorphic rock. Heated enough, they melt, and the loop begins again.

Strong thinking. Tomorrow is Sky Deck Lab: a rock walk, a sorting table and a rock cycle model built from cards.