How an impact crater forms

An asteroid strikes Earth at around 15 to 25 kilometres per second on average, and the whole crater is dug in seconds to minutes. Geologists describe it in three stages: contact and compression, excavation, and modification. Whether the result is a simple bowl or a complex crater with a central peak depends mainly on its size and on the gravity of the world it hits.

1. Step 1 of 7

   Contact: a shock wave is born

   When the impactor touches the ground, it is moving many times faster than sound travels through rock. Both the impactor and the ground are squeezed so suddenly that a shock wave forms and races out from the point of contact: down and out into the ground, and back up through the impactor.

   Pressures right at the contact reach hundreds of gigapascals, millions of times the pressure of the atmosphere. This contact stage lasts only about as long as the impactor takes to travel its own width, a fraction of a second for a body a few kilometres across. When the shock wave has passed through it, the impactor is almost entirely melted and vaporised.

2. Step 2 of 7

   Excavation: the ground flows out

   Behind the shock wave comes a release wave, and the shocked rock is set moving. It flows outwards and upwards along curved paths, opening a bowl called the transient crater, roughly a third as deep as it is wide. Rock near the top is thrown out as ejecta, which flies up and out in an expanding cone called the ejecta curtain and lands around the crater as a blanket.

   Deeper rock is pushed down and outwards rather than thrown out, and the rim is lifted and even folded back over itself. Because the impactor's energy is spread through so much rock, the crater ends up roughly 10 to 20 times wider than the object that made it. A small crater is excavated in seconds; one tens of kilometres across takes a few minutes.

3. Step 3 of 7

   The fingerprints of shock

   The shock wave weakens as it spreads, so it leaves a series of zones in the rock. Closest to the impact point the rock is vaporised or melted outright. Further out it survives as solid rock, but with features that only shock can make, and these are how geologists prove that a structure is an impact crater.

   Quartz grains gain sets of microscopic parallel planes called planar deformation features, at pressures of roughly 10 to 35 gigapascals. High-pressure forms of silica, coesite and stishovite, may appear. Further out still, rocks break into nested cones with grooved surfaces, called shatter cones. No volcano or earthquake produces these pressures at such speed.

4. Step 4 of 7

   A simple crater: the bowl

   If the transient crater is small enough, it is stable and stays as a simple crater: a smooth bowl with a raised rim. The rim stands up partly because the rock layers there are pushed up and partly because they are folded back on themselves, so the order of beds in the rim is upside down.

   Broken rock slides back from the walls and falls back from the ejecta curtain to form a breccia lens on the floor, so the finished crater is shallower than the transient one, about a fifth as deep as it is wide. Meteor Crater in Arizona, about 1.2 kilometres across and some 50,000 years old, is a classic example. On Earth, craters stay simple up to about 2 kilometres across in layered sedimentary rock and about 4 kilometres in crystalline rock.

5. Step 5 of 7

   A complex crater: collapse and a central peak

   Bigger transient craters are too deep and steep to stand, and they collapse within minutes. The floor rebounds upwards, rather as water splashes up in the middle when a drop hits a pond, raising a central peak of rock from deep below. Meanwhile the steep walls slide inwards in great blocks, leaving terraces.

   The result is a complex crater: wide, shallow and flat-floored, with a central peak or uplift, terraced walls, and a sheet of impact melt and breccia over the floor. The size at which craters turn complex depends on gravity: about 2 to 4 kilometres on Earth, but around 15 to 20 kilometres on the Moon, where gravity is weaker and walls can stand taller.

6. Step 6 of 7

   Peak rings and giant basins

   In larger craters still, the central uplift rises so high that it cannot hold itself up and collapses outwards, leaving a circle of mountains called a peak ring instead of a single peak. Chicxulub in Mexico, the crater about 180 kilometres across left by the impact that ended the age of the dinosaurs, has a buried peak ring; drilling in 2016 found it is made of granite lifted from kilometres down.

   The biggest impact structures, hundreds of kilometres wide, are multi-ring basins, with several rings of cliffs. The Orientale basin on the Moon, with an outer ring about 930 kilometres across, is the best-preserved example.

7. Step 7 of 7

   After the impact

   Once formed, a crater begins to change. On Earth, rain, rivers, ice and plate movements wear craters down or bury them under sediment, which is why so few survive compared with the Moon. Many craters fill with lakes; others vanish beneath younger rock and are only found by drilling or by surveys of gravity and magnetism.

   Even when the crater's shape is gone, its roots remain. At Vredefort in South Africa, the largest known impact structure on Earth, about 2 billion years of erosion have removed the crater itself, but shocked quartz, shatter cones and melt rocks still record the impact. Those same fingerprints, together with traces of the impactor such as unusual amounts of iridium, are how around 200 impact structures have been confirmed on Earth.

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