1. Describe the distribution of terrestrial craters through time. Why do you thi

Question

1. Describe the distribution of terrestrial craters through time. Why do you think are there so few craters older than 600 million years?2. When did the biggest meteoroids strike the Earth and why do you suppose we don't see more of them?

3. Briefly describe the distribution of impact crater diameters. What is a typical diameter and how much variation is there?

4. Briefly describe the distribution of impact crater ages. What is the distribution pattern for craters less than 500 million years old?

Age (Million Years) DIAMETER (km) 590 55 470 16 0.1 0.45 345 12.6 3 0.39 244.4 40 95 12 120 2 0.049 1.186 600 60 40 2.4 40 8 5 8 65.17 24 1.07 10.5 0.054 0.17 396 3.8 450 8.5 0.004 0.05 115 39 342 54 35.5 90 64.98 170 46 5.5 70 6 290 26 290 36 60 9 430 8 35 8.5 320 7 0.27 0.024 300 6 99 13 89 19 280 8 290 4.5 65 10 395 8 3.5 18 35 10 545 6 360 3.8 500 5 430 4 500 8 50 5.1 142.5 22 250 5 1400 3 470 3 49 3 345 14 23 24 0.001 0.015 0.0042 0.157 550 2.35 300 4 0.002 0.08 378 8.5 1000 3 700 14 0.004 0.11 1.8 0.64 380 15 49 25 70.3 65 5 52 455 7 1.88 1.5 5 10 550 10 97 13 250 6 400 8 73.3 23 515 18 150 1.6 455 7.5 40 20 42.3 15 0.052 1.83 1000 9 0.007 0.3 214 100 73.8 35 75 6 58 12.7 300 6 121 9 300 4 36.4 28 500 5 142 40 50.5 45 1 0.46 0.01 0.1 145 70 110 4 470 8 1.4 3.44 500 3.2 400 12.5 120 18 169 20 0.05 0.168 70 3.5 1800 10 360 7 445 6 35.7 100 500 24 167 80 46 9 200 9 200 4.5 15.1 24 0.1 0.75 214 23 505 6 3.7 2.5 120 2.7 560 6 600 1.5 220 40 320 8 300 12 1630 30 45 2.8 100 13 0.000055 0.027 368 52 450 30 0.001 0.053 600 5.5 570 13 91 25 15 3.8 646 25 2400 16 1850 250 1000 4 150 1.3 3 1.75 0.0214 1.9 280 11 70 6 128 55 0.22 1.13 455 2 170 10 70 12 1 0.08 160 8 190 2.5 2023 300 0.00014 0.116 37.2 7.5 200 12 100 2.44 81 6.5 0.3 0.875 364 40 2000 30 165 3.2 80 2.5 0.9 14

Explanation / Answer

1.Terrestrial craters are the roughly circular depressions formed on the surface of earth by the impact of the high velocity impact of the smaller objects like meteorites. These craters can be identified by the shock metamorphic effects like melted rocks,crystal deformation and shatter cones.

There were about 190 craters identified at the surface of earth.One of the most notable terrestrial impact craters which were formed at K- T boundary was Chixulub crater in Mexico. The asteriod body was supposed to be 10 km in diameter which caused about 200 km diameter crater in the area and this impact crater was responsible for large mass extinction at K-T boundary. Barringer crater was the first meteorite crater found in Arizona.

There are so few craters older than 600 million years because they have been obliterated or their history have been eroded by the active geological process operating on the surface of earth like erosion, tectonic movement etc.

2.There was a time about 3.9 billion years ago when there was intense early bombardment in the inner solar system. At that time earth averages about 1-3 impact craters which were large enough to produce 20 km diameter crater on the surface of earth once in every million years. We dont see more of them because the rate of impact cratering much different or rapid as compared to that of present rate.

3. There are two types of impact craters simple and complex. Simple craters have a diameter to depth ratio between 1:5-1:7 and are comparitively small with smooth bowl shape. Complex craters are those in which gravity causes the steep wall of the impact craters to collapse inward and downward which forms a complex peak or peak ring. These complex craters have a shallow depth in comparison to diameter i.e. 1:10-1:20. The diameter at which simple craters transforms into complex craters depends upon the gravity of the planet. The greater the gravity, smaller diameter will be required to produce a complex structure. The transition diameter for earth is 2-4 km.

4. During the time of formation of solar system before 3.9 billion years ago also known as intense solar bombardment, there were lots of solar debris which were striking the planet surface. So there were more number of impact craters and also these impact craters were larger in size to that to younger craters. There were few large craters found on the surface of earth whose age is more than 500 million years.

Young craters( 10 ka-1Ma): the diameter of impact craters is 0.1 km or more whose age is less than 10,00 years old. there are about only 7-8 such craters. Largest crater of this period is Rio Carter crater in Argentina with a diameter of about 4.5 km

Large craters(10 ka-1Ma): these craters have usually diameters of about 1km or more. eg. 1 km Meteor ( Barringer crater) , 14 km Zhamanshin crater in Kazakhstan.

Larger crater( 1Ma-10Ma): these craters have a diameter of 5 km or more. eg. 52 km Karakul crater( age less than 5 Ma) or Eltanin crater(2.5 Ma)

Largest crater(10 Ma or more): these craters have diameter of 20 km or more. There are about more than 40 such crater. eg. Chixulub crater at K-T boundary extinction and Popigai impact crater for Eocene oligocene extinction event

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