Falling Space Objects
One thing that all of the planets have in common is that they have always been struck by space objects. Indeed, the planets formed by a process of accretion from the solar nebula that involved repeated collisions and those collisions continued well after the planets were formed. Right after its formation the impact rate on Earth was millions of times greater than it is today because as impacts take place there are fewer and fewer objects available to impact in the future. That's not to say that after the early period of heavy bombardment by space objects that impacts no longer had a profound effect on the planet. Just 65 million years ago the impact of a 10 km diameter object along the coast of what is now Mexico caused a mass extinction that eliminated more than 70% of all species on Earth at the time. The fact is that similar impacts could take place on Earth at any time and humans are currently putting a lot of effort into finding those objects (asteroids and comets) that pose the greatest risk so that we can take measures to avoid another great extinction.
The record of impacts with Earth is very poor compared with that of the other planets, moons and small bodies in our solar system. Impact craters are certainly the best evidence of an impact by an asteroid or comet. If you look at our moon with just binoculars you can see a world that is densely scarred by impact craters that have formed over the last 4.5 billion years or so. In contrast, at the time of writing (June, 2016) the Earth has only 175 confirmed impact craters and an additional 118 probable impact craters (they are likely impact craters but they have not been confirmed as being so). The map below shows the confirmed impact craters in red. This map is from the Earth Impact Database site which also has a wealth of information on all confirmed and other impact craters on Earth. Note that many of the craters marked on the map are not even visible on satellite imagery because they are buried beneath sedimentary rocks that were deposited subsequent to the impact event. Some are hugely distorted by tectonic processes so that it's very difficult to even confirm that they are impact craters.
Why are there so few impact craters preserved on Earth compared to our moon and other bodies in space? The answer to this question has many components. First, because the Earth has an atmosphere, including water, that causes weathering which involves the break down of crustal rocks over time to produce sediment and materials in solution that are transported by rivers to the worlds oceans. As a result the land surfaces area constantly undergoing weathering and erosion that erases the craters produced by impacts. Much of the sediment that is produced by erosion and weathering ends up being deposited in the oceans and that sediment covers the surface of the seafloor, burying impact structures that have been produced in submarine settings. In addition, plate tectonics causes the elimination of even the largest impact craters through subduction of sea floor and intense deformation of continents adjacent to convergent margins. So, the preservation potential of impact craters is small on Earth compared to many other bodies where the most common process that will alter existing craters is a subsequent impact in the same vicinity.
In this section we'll look at how impact craters form, what characterizes them and what they look like on Earth.
Formation of Impact Craters
Impact craters are normally more or less circular in plan view and their size depends in large part on the size of the impacting object. Generally speaking, craters are normally 20 to 30 times the diameter of the object that created them. Of course the size, and some features, of an impact crater will depend on the amount of energy that is released on impact and that is proportional to not only the size but also the square of the velocity at which the object is travelling at the time of impact. The velocity of asteroids are typically in the range of 15 to 25 km/sec (54,000-90,000 km/hr) and comets are even faster with velocities up to 70 km/sec (252,000 km/hr).
We can break down the formation of impact craters into three stages. The first two take place in rapid succession whereas the third stage begins immediately after the second stage and may continue for many years after the actual impact.
Stage 1. Contact and Compression
This stage (shown in the figure below) begins at the instant that the impacting object (we'll call it the impactor) makes contact with the impacted surface. This is a very brief stage (on the order of a fraction of a second) when the pressures and temperatures are extreme. At the initial point of impact the temperature is high enough to vaporize bedrock and rocks making up the impactor and a little further away from that point this same material is melted. At this time a seismic shock wave is initiated that propagates outward to generate an earthquake with a magnitude far greater than has been experienced in historic times on Earth. The figure shows the location of rocks that will be ejected in the next phase and material that will subsequently be displaced upwards (but not ejected) to contribute to the formation of the rim. Note that "spalled" material in the figure are rock fragments derived from the impactor that had not melted or vaporized. The crustal rocks beneath the incipient crater are compressed downward to form what is called the "transient crater" because its existence is short lived as the crust rebounds from compression after the impact (stage 3). Stage 1 also results in extensive fracturing of rocks beneath the impact site; such fracturing is a diagnostic characteristic of impact craters in general.
Stage 2. Excavation
This is the stage when most of the crater is actually formed. Material is ejected upwards and away from the impact site, the largest blocks of material landing nearest the crater rim, becoming finer as you move outward from the centre of impact. Very fine debris that is produced by the impact can reach very high altitudes in the atmosphere and remain there for decades. Some molten rock is ejected but much of the molten material remains within the crater. The rock beneath the crater continues to be compressed, maintaining the transient crater. This stage takes just a few minutes to complete.
Stage 3. Modification
This stage begins after excavation and may continue for years as the initial crater equilibrates to the local conditions. One of the first things to happen is that the over-steepened walls will collapse into the crater. Molten material may sink into the underlying debris. If the impactor is particularly large and there is a large volume of molten rock, a molten pool develops which cools to form a "melt sheet" that makes up the floor of the resulting crater. The transient crater rebounds as the compressive forces relax after impact and the floor of the crater rises upwards. In the case of large impactors (on Earth these are craters that are 4 km in diameter or larger) the rebound is quite spectacular and forms a "central uplift" (a more-or-less circular mound with a single central peak or high point) or "central peak ring" (a donut-shaped structure made up of a circular ring of peaks with a central depression) at the approximate centre of the crater. A central uplift is shown in the figure, below, that illustrates the outcome of Stage 3.
Impact craters are commonly circular in plan shape but elongate shapes develop if the impacting object intersects the Earth's surface at a large angle from the vertical. There are two general types of impact crater: simple craters and complex craters (see the figure below).
"Simple" craters are relatively small (less than 4 km) circular depressions that have an outer rim that rises above the surface that surrounds the crater. The rim is made of material that is displaced upwards due to the impact, some of the rim material is otherwise intact whereas some of the material is coarse debris that was ejected upon impact. All around the crater there is a blanket of ejected material covering the land surface, becoming finer in size as you move away from the crater. The material within the crater includes ejected material that has fallen back into the crater along with fragments of melted rock material that was created during stage 1 of the impact. Beneath the crater the bedrock is extensively fractured due to the compressive forces that were generated on impact.
The photograph that forms the background to the title for this section is a relatively young simple crater on Earth. The crater that is pictured is Meteor Crater (also called the Barringer Meteorite Crater) in Arizona. It formed about 49,000 years ago and is 1.2 kilometers in diameter and 170 meters deep.
The following image is of the Clearwater Lakes craters, two craters that formed at the same time about 290 million years ago in what is now northwestern Quebec, Canada. The larger of the two craters is about 36 km in diameter and contains a prominent central ring. While the crater is rather deeply eroded elements of the complex morphology persist.
