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---------------------- 2343 - The Story of a Moon Rock
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- A meteorite rock was found in Oman, Africa in September, 2002. The rock tells us that it came from the Moon and it even tells us which crater on the Moon it was ejected from.
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- By carefully analyzing the rock we learn that it came from the Imbrium Mare Basin, the crater Lalande on the Moon’s near side. With a small telescope you can pick out that very crater about a day after the First Quarter Moon.
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- The Oman rock was fist size and weighed 1.4 pounds. It is called impact breccia which means it is rock composed of angular fragments of older rock melded together. There are three types of rocks : igneous, sedimentary, and metamorphic.
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- Igneous rock originates as a molten mass and forms by cooling. The larger the crystals in the rock the slower the cooling process that made it. This particular rock experienced three lunar impacts while on the Moon before a forth meteor impact ejected it and sent it towards Earth.
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- The rocks composition was found to be richer in the radioactive element thorium than any other lunar rock known. The decay time of radioactive thorium isotopes provides a reliable elapsed-time clock.
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- Thorium is a common rock that contributes to heating the Earth with its radioactive decay. This decaying leads to the formation of the radon gas that may dangerously accumulate in building basements.
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- Thorium can be transmuted to Uranium 233, it melts at 3,182 degrees Fahrenheit. The date for the rocks first gigantic impact was 3,909,000,000 years ago. The impact crater is what formed the Imbruim Mare Basin on the Moon.
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- The second impact occurred 2,800,000,000 years ago and this is what created the Lalande Crater.
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- The third impact was 200,000,000 years ago creating another crater close to Lalande. The 200,000,000 date did not come from radioactive dating, it was determined by the amount of cosmic ray absorption occurring in the regolith. Regolith is the loose, disintegrated rock particles and mineral grains formed from the weathering of bedrock.
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- The forth impact came less than 340,000 years ago. It ejected the rock off the Moon’s surface and into outer space. This big of impact had to create a crater a few kilometers in diameter and in the neighborhood of Lalande. The rock drifted through the solar system until it was captured by Earth’s gravity and smashed into the ground in Sayh al Uhaymir region of Oman, Africa 9,700 years ago.
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- The 340,000 year age for the last impact was determined by dating the rock’s time on the ground plus the travel time from the Moon to Earth.
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- The location on the Moon comes from the fact that there exists distinct chemical differentiation of the Moon’s surface as it formed its core, mantle , and crust. The last stage of crustal formation left a chemically unique region rich in incompatible elements at the place the crust was struck by the Imbrium meteor.
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- Around the Mare Imbrium Basin on the northwest side of the Moon lies an unusual region of rocks containing highly enriched potassium, rare-earth elements, and phosphorous along with thorium and uranium. The crater Lalande is 25 kilometers (15 miles) across and lies right in the middle of this region.
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- A more recent find was a rock in Dchira, Western Sahara that is 2,865,000,000 years old. It was also an impact breccia rock made up of fragments of the lunar surface welded together by the shock and heat of a meteorite impact. -
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- It had a large piece of olivine gabbro rock inside it that was dated at 2,865,000,000 years old making it the youngest lunar rock ever found. Gabbro is a rock similar to basalt lava rock except it is cooled within the interior instead of on the surface. It is rather low in silica for igneous rock, about 50% is silica.
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- This Sahara rock probably came form the same region on the Moon as the Oman rock but their histories appear to be unrelated. When the Moon’s magma ocean cooled various minerals began crystallizing in sequence. Olivine was first to crystallize.
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- It is a magnesium iron silicate, (MgFe)2SiO4, that is olive green in color. Pyroxene was next to crystallize out of the molten lava. It belongs to another group of silicates that are unique with a characteristic cleavage angled at 90 degrees.
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- Next to crystallize was plagioclase feldspar. Feldspar is a common mineral that is a combination of aluminum, silicon, and oxygen together with a metal of sodium or calcium, Na(AlS3O8).
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- Plagioclase is the type of feldspar with a triclinic crystal shape. Triclinic means the crystal has a unique three axes of unequal length all inclined at some angle. When feldspar weathers into small particles it turns into clay. Feldspar is lighter than the other rock so it floated on top of the magma to form the crust.
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- These particular rocks contain high concentrations of radioactive elements, thorium and uranium. The radioactive elements produce the heat which is the energy source to create the relatively young magna. And therefore, this relative young rock, only 2,865,000,000 years old.
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It is so amazing to me that science can tell so much just carefully looking at a rock and knowing what they are seeing.
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- The rock has quite a story to tell if you can only read the writing. I hope mine is easier.
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- April 25, 2019 74
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-------------------------- Thursday, April 25, 2019 --------------------------
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