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Okay, so how do geologist know when then rock was formed and how do they find out?


So there are three types of rocks


Igneous rock are the original rocks when the Earth cooled down from gaseous to molten, and finally solid state. Further cooling and other natural processes resulted in formation of water and the pouring rains. Erosion of igneous rocks followed, and the eroded material was deposited in depressions as sedimentary rocks. The piling up of sediments buried the older sedimentary rocks deeper and deeper where the higher temperature changed the sedimentary rocks into the metamorphic rocks. Further deepening melted the rocks, which when cooled down became the igneous rocks again. And the cycle is being repeated ever since.

Now the age of the igneuos rocks can be ascertaind through radioactive minerals - the half life of a radioactive material.

The life proliferates in water. The big aquatic animals when die, their remains get destroyed. However, macro- and micro-fossils get burried with the sediments and get petrified to their exact shapes and form. Now these fossils have been dated with different methods, and a geological age range has been established for different species. The age of the sedimentary rocks is then correlated with the established age of fossils found in them. Some fossils had a long range and some had a short time span, and then disappeared completely (extinct); such fossils are called 'Index fossil'. For example any rock having Trilibite fossil is of Cambrian age. Sometimes a sedimentary rock may not have any preserved fossils. Its age is then established with repect to its stratigraphic position. The age of the overlying rocks and the underlying rocks help in establishing the age of the rock sandwiched in between.

Hope you will find it useful.


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Nusrat Kamal Siddiqui (Mr)


I am a petroleum exploration geologist with about 40+ years of diversified experience. I can answer questions on stratigraphy, log interpretation, field studies based on set of logs and core results, general geology etc. Can provide guidance for career planning in earth sciences. Can not help in questions about hard rocks, igneous petrology & mineralogy, sequence stratigraphy and seismic.


Petroleum Exploration - field geology, stratigraphy, prospect generation, interpretation of satellite imagery (visual), log interpretation.

Retired in May, 2006 as Senior Manager Exploration from Pakistan Petroleum Limited after serving 26 years. Since then continuing in active service with petroleum exploration companies.A cumulative experience of about 40 years covers 30 years in petroleum exploration, 5 years hydrogeology experience in Libya, and about 5 years of dam geology in Pakistan.

-AAPG Bulletin (Vol.88, July, 2004); paper on Sui Main Limestone (Eocene), a prolific gas reservoir in Pakistan. -Have presented papers on diversified topics ranging from spirituality, flood control, remote sensing and off course geology in France, Nepal, Bangladesh, Sri Lanka, UAE, Kashmir, India and Pakistan that have been published in respective Special Proceedings. -ONGC Bulletin(India) Vol. 44, Number 2, December, 2009; Chapter 19.A & 19.B on basin architecture and stratigraphy of Pakistan, p.402-474.

BSc Honours in Geology and Master of Science in Petroleum and Structural Geology. PGD from ITC, The Netherlands.

Awards and Honors
-Remained Active Member of AAPG from 1990-2006; founding member of PAPG (a local affiliate of AAPG). -Life member of SEGMITE & AGID, and Senior Editor of SEGMITE magazine. -Member of the Board of Studies, i. Department of Petroleum & Gas Engineering (BUITMS, Quetta), and ii. Geology Department, Sindh University, Jamshoro. -Member of Stratigraphic Committee of Pakistan. -Represented Mining Journal, UK in Pakistan till 2007. -Chairman of Annual Technical Conference (ATC)& Oil Show, an SPE/PAPG annual event, in 2003. -Chairman Technical Committee (ATC, 2002).

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