Animation showing the overall process occurring during the impact cratering process.
Animation showing the overall process occurring during the impact cratering process.

Impact events release an unparalleled amount of energy in a very short amount of time. This process is unlike any other geologic process and produces a set of geologically unique rocks that we call impactites. When we describe the impact crater process we divide the event into three stages: contact and compression, excavation stage, and modification stage.

Contact and Compression Stage

When a hypervelocity impact occurs, the impactor compresses sending out a shock wave into the surrounding rocks. The shock waves also pass through the impactor itself, and as the wave passes through it causes the impactor to undergo a significant amount of pressure. When the shock wave reaches the upper surface of the projectile, it is reflected back as a tensional wave or rarefaction wave. It is this reflect wave that releases or unloads the built up pressure from the shockwave. The release of pressure results in a rapid release of energy that melts or vapourizes the projectile.

A similar process occurs within the surrounding. As the shock wave passes through, it induces a huge amount of pressure onto the rocks. As the shock wave interacts with the surface, a rarefaction wave is reflected down into the ground. Like in the case of the projectile, it is the build-up of pressure by the shock wave, followed by the release of the pressure by the rarefaction wave that transforms the surrounding rocks. The point at which the projectile is unloaded is generally taken to be the end of the contact and compression stage.

Animation showing a wave passing through particles. Note that the areas of compression are followed by rarefaction where the compression is released.

Excavation Stage

Bacolor Crater (~20 km wide) on Mars and it’s magnificent ejecta blanket (JPL NASA).

In the near surface, the interaction of these waves creates an “interference zone” causing the rocks within this zone to move outward. This force is so great that within the upper area of the zone, it excavates the rock out from the crater throwing it beyond the crater. This forms what we call the ejecta blanket. The rock lower portions of this zone move within the crater but are not throw beyond the rim.

The shape and extent of the ejecta are strongly controlled by the characteristics of the surrounding rock. Water, in particular, is thought to have a large effect on the way the ejecta blanket flows on the surface once it lands. The excavation process also brings material at depth to the surface and allow us to study the subsurface of other planetary bodies without physically removing the rock.

By the end of the excavation stage, a mixture of melt and rock debris lines the crater, representing the material moved and mixed from the lower area of the interference zone. In simple craters, the crater rim at the end of the excavation stage can be approximated by the final crater size; however, for complex craters, the modification stage greatly alters the shape of the cavity and, as seen in the top animation, by the end a bowl-shaped crater is not preserved.

Modification Stage

The final morphology of the crater depends heavily on the size of the crater. Smaller, simple craters undergo little change and remain bowl-shaped; however, larger complex craters (>4 km on Earth) undergo significant change. Modification in complex craters begins when gravitational force cause instability in the transient cavity. Uplift begins in the centre of the cavity as the material moves inwards and upwards forming a central uplift. The steep walls of the rim begin to collapse inwards causing the crater to “flatten” in appearance.

The modification stage of the impact cratering process is thought to begin when the transient cavity is excavated; however, the modification stage has no end. Once the walls collapse and the uplifting ceases erosion begins. On Earth, erosion and plate tectonics continues to alter the final morphology of the crater.