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Why does material fly upward and outward during crater formation?

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four impact craters — Meteorite crater Four impact craters of the same size (30 km [20 miles] in diameter) imaged by spacecraft on different solid bodies of the solar system and reproduced at the same scale. They are (clockwise from upper left) Golubkhina crater on Venus, Kepler crater on the Moon, an unnamed crater on Jupiter's moon Ganymede, and an unnamed crater on Mars. The images are oriented such that the craters appear illuminated from the left; the Venusian crater is imaged in radar wavelengths, the others in visible light.

During the formation of a meteorite crater, material is projected upward and outward due to the intense pressure gradients generated within the planet's surface.1

When a high-speed collision occurs, it creates a shock wave that travels into the surface material. This is immediately followed by a release wave, or rarefaction wave, which is caused by the reflection of the initial shock wave from the planet's surface. The interaction and dissipation of these two waves create the large pressure gradients necessary to drive the subsurface flow that excavates the material, pushing it away from the impact point in an outward-slanted curtain.1