Static Quiz 18 July 2022 (Geography)

Solution: A
Justification: Azoic Age was used to describe the age of rocks formed before the appearance of life in the geologic sequence. Also known as Pre-Cambrian rocks these are the oldest rocks of the earth’s crust. The Archean period covers over 85% of the total geological history time of earth and therefore is very significant. This period marks the development of the first photosynthesis, the life support atmosphere. The Archean or Purana rock system in India is found in Aravallis mountains, 2/3rd of the Deccan peninsula and some parts of north east. These rocks have abundant metallic and non-metallic minerals such as iron, copper, manganese, bauxite, lead, zinc, gold, silver, tin, tungsten, mica, asbestos, graphite, etc. They serve as the basement complex or fundamental gneisses.

Solution: A
Justification: The key difference between intrusive and extrusive rocks is that the intrusive rocks are formed from magma whereas the extrusive rocks are formed from lava. Rest of the differences follow from this basic structure. Intrusive rocks: With no air to cool the magma, these rocks are formed very slowly. Composition of these rocks reflects presence of large crystals. These crystals interlock to form the rock. These rocks take a very large amount of time to solidify and they remain buried deep inside the surface of the earth being surrounded by country rocks that have been there already. Very slow cooling means that these rocks remain coarse-grained. Some of the perfect examples of intrusive rocks are the diorite, gabbro, and granite. Much of the core of various mountain ranges around the world is made up of these intrusive rocks. Extrusive Rocks: Sometimes, molten rocks find a way to come out of the surface of the earth through cracks and openings. This magma flows in the form of lava and cools down quickly as it comes into contact with air. Igneous rocks that are formed from the magma that pours out of the surface of the earth are called extrusive rocks. As these rocks cool down and solidify very quickly, they do not get sufficient time to form large crystals. Thus, they have small crystals and boast a fine texture.

Solution: C
Justification: Enrichment of many low-grade ores occurs when the metal-bearing solutions of these valuable metals drain downward along the vein or other deposits and then are re-precipitated into concentrated, bonanza grade deposits at shallow depths. Some of these enriched deposits are quite large, being the product of long, continued forces of weathering with the descending mineral salts acting to concentrate the metal values from a great thickness of vein matter and rocks that were once located above the current outcrop, but now have been removed by erosion. This process of natural concentration is how this action of weathering can produce such amazingly rich ore bodies.

Solution: D
Justification: Statement 1: If a rock contains copper, it looks blue in color. For e.g. Azurite contains copper, which gives its blue color and a specific gravity of 3.7 to 3.9, which is exceptionally high for a non-metallic mineral. Statement 2: This is not necessary. Rocks contain exfoliation or layers depending on their terrain and geographical environment around.

Solution: A
Justification: Rocks in Western Australia date from 4,300 million years ago, only 300 million years after the earth was formed. These are Zircon stones. Confirmation of the zircon age holds enormous implications for models of early Earth. Trace elements in the oldest zircons from Australia’s Jack Hills range suggest they came from water-rich, granite-like rocks such as granodiorite or tonalite, other studies have reported. That means Earth cooled quickly enough for surface water and continental-type rocks just 100 million years after the moon impact, the massive collision that formed the Earth-moon system.