QUIZ 21a. Types of Rock and the Rock Cycle

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   core      crust      density      deposited      different      erosion      faulted      forces      fossils      high      igneous      iron      layered      liquid      magma      mantle      metamorphic      more      nickel      older      pressure      ripple      sedimentary      solid      tectonic      temperatures      unstable      upside      younger   
(a) The Earth is nearly spherical and has a structure consisting of three layers:
(i) a thin ; which is the least dense layer.
(ii) a liquid extending almost halfway to the Earth's centre which has all the properties of a solid except that it can flow very slowly like a 'plastic liquid'.
The third layer (iii), is the , which is about half of the Earth's radius, made of weakly magnetic and mainly strongly magnetic . It outer part is and the inner part is .

(b) The overall of the Earth is much greater than the mean densities of the rocks which form the crust. This indicates that the inner layers of the Earth are made of and dense material than that of the crust.

(c) At the surface of the Earth rocks usually lie on top of older rocks. Sediments contain evidence for how they were (e.g. layers formed by discontinuous deposition, marks formed by currents or waves) and tell us what plants and animals were alive at the time.

(d) Sedimentary rock layers are often found tilted, folded, (fractured) and sometimes even turned down. This shows that the Earth's crust is and has been subjected to very large . Large scale movements of the Earth's crust can cause mountain ranges to form very slowly over millions of years. These replace mountain ranges worn down by weathering and .

(e) Most rocks are associated with the Earth movements ( activity) which created present-day and ancient mountain belts. They are evidence of them being heated to and squeezed at high when created in these mountain-building processes.

(f) Rocks formed from cooled solidifying from the very hot mantle are called rocks.