Inside Our Earth
— Exercise Solutions
Answer the following questions
The earth is made up of three concentric layers — the crust (the thin, outermost layer), the mantle (which extends up to a depth of 2900 km below the crust), and the core (the innermost layer, made up mainly of nickel and iron).
Any natural mass of mineral matter that makes up the earth’s crust is called a rock. Rocks can differ in colour, size, and texture.
The three major types of rocks are igneous rocks, sedimentary rocks, and metamorphic rocks.
Extrusive igneous rocks are formed when molten lava erupts onto the earth’s surface and cools rapidly, giving these rocks a fine-grained structure (e.g. basalt). Intrusive igneous rocks are formed when molten magma cools slowly deep inside the earth’s crust, which allows large grains to form (e.g. granite).
The rock cycle is the continuous, cyclic process by which one type of rock is transformed into another. Cooled magma solidifies into igneous rock; igneous rock breaks down into sediments that form sedimentary rock; heat and pressure turn igneous and sedimentary rocks into metamorphic rock; and metamorphic rock, under still greater heat and pressure, melts into magma again — restarting the cycle.
Hard rocks are used for making roads, houses, and buildings. Rocks are also used in traditional games (such as seven stones/pitthoo, hopscotch/stapu, and five stones/gitti), and rocks like granite are shaped into grinding stones used to prepare spice and grain pastes.
Metamorphic rocks are rocks formed when existing igneous or sedimentary rocks are changed in form under great heat and pressure — for example, clay changes into slate, and limestone changes into marble.
Tick the correct answer
Match the following
| Term | Matches with | |
|---|---|---|
| i. Core | → | (e) Innermost layer |
| ii. Minerals | → | (d) Has definite chemical composition |
| iii. Rocks | → | (b) Used for roads and buildings |
| iv. Clay | → | (f) Changes into slate |
| v. Sial | → | (c) Made of silicon and alumina |
Options (a) “Earth’s surface” and (g) “Process of transformation of the rock” are extra choices in the original list and are not used in the final matches.
Give reasons
We cannot go to the centre of the earth.
The earth’s radius is about 6371 km, but temperature and pressure rise enormously with depth — even the deepest mines and oil wells reach only about 4–6 km. To dig all the way to the centre, a hole nearly 6000 km deep would be needed, and no material or machine could survive the extreme heat and pressure encountered along the way.
Sedimentary rocks are formed from sediments.
Rocks at the surface roll, crack, and collide with one another, breaking down into small fragments called sediments. These sediments are carried away and deposited by wind, water, and other agents. Over time, the loose, layered sediments get compressed and hardened, and it is this compaction that turns them into sedimentary rock — which is why the rock is named after the process that forms it.
Limestone is changed into marble.
When limestone, a sedimentary rock, is subjected to great heat and pressure inside the earth, its mineral structure is altered without it melting completely. This process of metamorphism recrystallises the limestone into marble, a denser and harder metamorphic rock.
For fun — minerals in everyday objects
ii. A few more everyday objects made of minerals: coins (copper/nickel alloys), window glass and mirrors (silica), pencil “lead” (graphite), cement and chalk (limestone), and electrical wiring (copper).