The Manicouagan Impact Crater EarthCache
Hidden : 3/6/2007
Difficulty:
2 out of 5
Terrain:
3.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


Impact craters are geologic structures formed when a large meteoroid (asteroid or comet) smashes into a planet or a satellite. All the inner bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The surfaces of the Moon, Mars, and Mercury, where other geologic processes stopped millions of years ago, record this bombardment clearly. On the Earth, however, which has been even more heavily impacted than the Moon, craters are continually erased by erosion and redeposition as well as by volcanic resurfacing and tectonic activity. Thus only about 120 terrestrial impact craters have been recognized, the majority in geologically stable cratons of North America, Europe, and Australia where most exploration has taken place. The Manicouagan Impact Crater in Quebec, Canada is one such crater.

The Manicouagan Impact Crater is located in Manicouagan Reservoir (also Lake Manicouagan) which is an annular lake in northern Quebec, Canada, once known as part of Panagaea. It is the remnant of an impact crater made approximately 212 million years ago, towards the end of the Triassic period. Recent research has shown that the impact melt within the crater has an age of 214±1 Ma. As this is 12±2 Ma before the end of the Triassic, it implies that it was not the cause of the Triassic-Jurassic extinction event.
The crater was created by the impact of a 5 km diameter asteroid which excavated a crater originally about 100 km wide it is one of the largest known although sediments and erosion have since reduced its diameter to about 72 km. It caused a fireball as far as present-day New York, a melting pot boiling the local bedrock over 50 km extent and 9 km depth. The subsequent glaciations Ice Age scraped off a kilometer of overlying rock including the outer crater wall. They also scooped up and excavated a wide ring of impact-fractured bedrock around the harder central melt, producing a dug-out moat around what is now a lonesome Triassic hump in the Precambrian Canadian shield.melt, producing a dug-out moat around what is now a lonesome Triassic hump in the Precambrian Canadian shield.
The Manicouagan impact structure is one of the largest impact craters still preserved on the surface of the Earth. This shuttle oblique view looking south shows the prominent 70-km-diameter, ice-covered annular lake that fills a ring where impact-brecciated rock has been eroded by glaciation. The lake surrounds the more erosion-resistant melt sheet created by impact into metamorphic and igneous rock types. Shock metamorphic effects are abundant in the target rocks of the crater floor. Although the original rim has been removed, the distribution of shock metamorphic effects and morphological comparisons with other impact structures indicates an original rim diameter of approximately 100 km.
The moderately eroded, central part of the structure (the plateau surrounded by the lake) is partly covered by impact melts and contains shattered rocks and several uplifted peaks about 5 km north of the center. The vast quantity of data obtained on the melt sheet and the underlying target rocks make Manicouagan the most intensively studied large complex impact structure in the world, and it is the primary source of ground-truth data for understanding the cratering process and the substructural configuration of large complex craters on other planets.
An image taken from the Space Shuttle easily shows the Manicouagan Impact Crater. The diameter of the rim is approximately 70 km (43 miles), and it's estimated to be about 212 million years old. It's perhaps the largest and oldest of the preserved impact craters visible from space. The light colored band defining the crater rim is an annular lake that formed when impact-brecciated rock was eroded by glaciation and subsequently filled with water when the ice melted. In wintertime view, the annular lake is ice covered. Also visible in space images are the radial drainage pattern of streams draining into the crater. Nearly all of Earth's existing large impact craters (more than a km in diameter) were produced from asteroids or planetesimals that smashed into the Earth more than 40,000 years ago.

Impact craters are divided into two groups based on morphology: simple craters and complex craters. Simple craters are relatively small with depth-to-diameter ratios of about 1:5 to 1:7 and a smooth bowl shape. In larger craters, however, gravity causes the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20). The diameter at which craters become complex forms depends on the surface gravity of the planet: The greater the gravity, the smaller the diameter that will produce a complex structure. On Earth, this transition diameter is 2 to 4 km (depending on target rock properties); on the Moon, at 1/6 Earth’s gravity, the transition diameter is 15 to 20 km. On Earth, this transition diameter is 2 to 4 km (depending on target rock properties); on the Moon, at 1/6 Earth’s gravity, the transition diameter is 15 to 20 km.
In recent years numerous studies have uncovered a physical marker of impact structures, shock metamorphism. Certain shock metamorphic effects have been shown to be uniquely and unambiguously associated with meteorite impact craters; no other earthly mechanism, including volcanism, produces the extremely high pressures that cause them. They include shatter cones , multiple sets of microscopic planar features in quartz and feldspar grains, diaplectic glass , and high-pressure mineral phases such as stishovite . All known terrestrial impact structures exhibit some or all of these shock effects.

Pour enregistrer cette Earthcache vous devez inclure une photo d’au moins une partie de vous-même tenant votre récepteur GPS. A l’arrière-plan de la photo, veuillez inclure soit la partie sud du barrage, une partie du lac, l’île ou le poste d’essence. Vous devez également me faire parvenir par courriel la reponse à 3 des 5 questions suivantes:

1) Estimez la grandeur du cratère d’impact Manicouagan.

2) Estimez le volume d’eau contenu par le cratère.

3) Que pensez-vous être la hauteur des pics qui entourent le cratère?

4) A votre avis, quel est la caractéristique la plus remarquable du cratère d'impact de Manicouagan ?

5) Si l'impact de Manicouagan se produisait aujourd'hui, le considériez-vous un événement catastrophique? Pourquoi ou pourquoi pas ?

To log this Earthcache you must post a picture of at least a part of yourself holding your GPS with either the dam at the south end, part of the lake, island or the service station the background. You must also email me an answer to three of the following questions. (1) How large do you estimate the crater to be? (2)The Manicouagan Impact Crater in your estimation contains approximately how much volume of water? (3) How high do you estimate the peaks surrounding the crater to be? (4) What do you believe to be the most outstanding feature of The Manicouagan Impact Crater? (5) If The Manicouagan Impact happened today would you consider it a catastrophic event and why or why not?
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