As references, keep in mind that the 20 kiloton device exploded at Hiroshima was equivalent to about 10¹⁴ J, the Mt. St. Helens volcanic eruption involved 6 x 10¹⁶ J, and the largest earthquakes release up to 10¹⁸ J. In this context, the impact that produced the Sudbury structure (215 km [130 mile] initial diameter) in Canada released about 10²³ J, roughly 100000 times greater than earthquakes of magnitude 9 on the Richter scale (Sudbury, then, could have generated an earthquake-like response on the order of magnitude 14). In common, both earthquakes and impacts are the fastest known large geologic phenomena, each causing ground disturbances that last only a few minutes at most after their initiation times.

The source of this tremendous impact energy is the direct consequence of great solid mass moving at high velocity. Remember from physics that Kinetic Energy (K.E.) = 1/2 mv². To gain a sense of the magnitude involved, consider this calculation. Let a 30 meter (100 foot) diameter iron body (in effect, a large meteorite) weighing about 200,000 metric tons (around 440 million pounds) strike the Earth at a typical in-space velocity of 30 kilometers (20 miles) per second (not hours! - 20 mps corresponds to 72000 mph). This impact would generate about 20 megatons (TNT-equivalent) of energy (~10¹⁷ joules) that would cut out a crater about a kilometer and a half (almost a mile) wide and 185 m (600 feet deep) (this is the size of Meteor [Barringer] Crater, which we will examine later). The ejection process would scatter most of the excavated rocks out to 10+ kilometers around.

Let us now follow second by second the formation of a large or complex crater (one greater than about 5 km [3 miles] wide that has a central peak and concentric slump walls). We will use a series of sideview panels created by Dr. Raymond Anderson of the Iowa Geological Survey Bureau (and used here with his permission) to sequence the steps involved in the development of the Manson structure. (The writer [NMS], during the years in the 1960s when he was working primarily on impact structures, is generally credited with "proving" the impact origin of this very large crater which, at one time, was thought to be the "smoking gun" that killed the dinosaurs until an older age was determined that disqualified it.) This 74 million year old, 35 km (22 miles) wide crater whose centerpoint is some 130 km (80 miles) northwest of Des Moines, IA. is largely intact but now buried under 30 m (100 ft) of glacial debris.

At the instant of impact (0.0 sec), the target consisted of an average of 90 m (290 ft) of Mesozoic sedimentary rocks (mainly Cretaceous in age) (in green) overlain by as much as 52 m (170 ft) of young glacial till and underlain by 495 m (1600+ ft) of Paleozoic sedimentary rocks (light blue). These lie unconformably on top of Proterozoic sandstones and other red clastics (yellow)whose thickness increased to nearly 3 km (2 miles) going southwestward. This entire section rests on top of Precambrian crystalline (granites and metamorphic) rocks (red) buried at depths down to almost 4600 m (15000 ft).

As the incoming impactor (or bolide) impressed onto this late Cretaceous surface, at 0.15 seconds, it was totally fragmented and vaporized. At it penetrates into the rock, it imparts its energy (~2 x 10²³ J) in the form of supersonic shock waves that generated compressive pressures ranging up to a megabar (1,000,000 atmospheres; such pressures are normally attained only at depths well into the Earth [100s of kilometers]). Rock just beyond the point of impact itself is vaporized. An initial curtain of ejecta consisting of gases and melted rock is hurled upwards in a steep cone within which is a momentary partial vacuum caused by the passage of the missile. The energy released also generates electromagnetic waves that extend out into the atmosphere.

At 0.6 sec the shock wave has progressed along an enlarging hemispherical front well into the target, forcing rocks to be severely transformed at pressures ranging to about 600 kilobars (kb) (or 60 Gigapascals [Ga], a fashionable new pressure unit) close to the line of penetration. A fraction of the target (up to 10% of the total that is eventually displaced) is melted; some of that melt carries downward along with the now compressed and mobilized rock undergoing fragmentation, some is pushed out of the crater to fall back nearby and some is literally squirted as tiny blebs that may carry hundreds of miles out of and back into the atmosphere to come to rest as tektites (glass "pebbles"). A fireball similar to that caused by atmospheric burning at surface detonations of chemical or nuclear explosions starts to form. Within a few seconds, the excavation phase of the bulk of the crater, involving mechanical disruption of rock first placed into compression with shock wave passage and then fragmenting tension as a trailing wave (known as a rarefaction wave) moves through, has commenced. As the waves spread out outward and down, decreasing in intensity, peak pressures drop to a few 10s of kilobars.

By 6.9 seconds, the initial or transient crater arising from vaporization, melting, and direct ejection and from centrifugal "shoving" of the target outward under compression has reached its maximum depth. At Manson, this rapidly growing crater front cut down through the Mesozoic, Paleozoic, and Proterozoic sedimentary overburden well into the Precambrian crystalline rocks. Most of this material was shocked to varying degrees and the effects of these pressure waves are permanently imposed on the rocks. These rocks continue to be decompressed by trailing tension (rarefaction) waves that break them into fragments ranging from microscopic in size to objects bigger than a house that will eventually come to rest as deposits called breccias.

By 11.0 sec, as excavation continued, the peak shock pressures at the wave front have now decayed to under 20 kb (2 Ga). As the pressure waves advance outward from ground zero (point of impact), they keep on breaking the rocks into fragments and blocks, placing these into motion along ballistic trajectories that start particles downward and then swing them up above the still growing crater along arcuate paths, causing most material to leave the site along low to moderate angles. Generally, particles deeper and farther out from the impact center will be ejected later and will likely fall on top of earlier removed, near surface particles; ejecta layers will tend to be deposited in reverse order of initial position, with ones lower in the target falling on top of upper ones (although some mixing occurs). Beyond the edges of the crater walls, rock units experience faulting and folding; especially along the upper walls, sedimentary (layered) rocks can be pealed back so that the layers may even be overturned.

For Manson, the crater reached its maximum excavation diameter around 25 sec, as the last voluminous ejecta emerge. Its upper walls are especially unstable and begin to fail along steep concentric fractures and faults. At the central bottom of the crater, the rock below begins to rebound upward.

At the transitional 26 second mark, the last ejecta are well into flight. Slices of rock just past the walls now begin an inward sliding along faults. The crater base has started an upward movement that soon leads to a central peak. This rock material probably behaves plastically as it almost flows upward (a good analogy is the inner blob of water that shoots up into a momentary "crater" forming by dropping a stone into a pond ). The collapse of the upper walls may aid in this effect by pushing downward toward the center.

By 35 seconds, the central peak has attained its topmost height (overshoots) and begins to founder in collapse.

About a minute after the impact starts, the central peak has begun to subside into its final position and the walls have slid and tumbled inward to form nested or terraced rings (see the Tycho image for an overhead view of these conditions). By this time, some of the material ejected at high angles directly above the crater begins to descend. The heavier, larger particles will settle out first.

Over the next 30 minutes or so, this fallout will pile up in a continuous blanket both within the crater and outside. Other materials expelled at lower angles will form a wider apron of ejecta that may then be covered during this later stage of deposition. Small particles and dust from the event can be carried hundreds of miles Manson material has been found in a thin layer at sites in South Dakota up to 500 km (300 miles) distant and the finest sizes are transported in the stratosphere probably well beyond (likely global in extent).

At time of impact 74 million years ago, the Manson area was almost certainly under water inasmuch as the region lay within a shallow sea. This impact should have produced a tsunami-like disturbance (steep fronted waves that travel at velocities >800 kph (500 mph); large body impacts into the open oceans will spawn huge waves whose initial heights may exceed 325 m (1000 feet). If this is so, the various ejecta deposits would not form in the usual sense, as they would enter disturbed waters and would be irregularly deposited or stirred up by waves moving back into the crater area. The seas retreated a few million years later, leaving the land to be eroded. About 10% of the crater's upper structures and deposits have since been removed. Now the crater (which has no surface expression at all) is protected by the glacial deposits of the Pleistocene. A cross-section (side view) of the Manson crater as it remains today is shown here (glacial cover a thin gray line):

The dashed yellow line marks the boundary of the final transient crater, modified upwards centrally by the rise of its surface along the central peak. The curved concentric black lines are fault planes bounding slides of bedrock that dropped downward to help create terraces. Their outer limits define the maximum (apparent) crater diameter.

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