Interior structure of the earth

Correct answer: B. Magnesium

The direct answer is B: in SIMA, MA represents magnesium. The term is formed from Si, meaning silica or silicon-related silica content, and Ma, meaning magnesium. In school geography, SIMA is used for the denser, darker layer associated mainly with the oceanic crust, in contrast with SIAL, which refers to silica and aluminium. Option B is correct because the letters MA stand for magnesium. Option A is wrong because manganese is not the meaning of MA in this term. Option C is wrong because methane is a gas and is not part of the abbreviation. Option D is wrong because mercury is a different element and is not represented here. Exam cue: SIMA = Si + Mg; SIAL = Si + Al. Note that this is a simplified textbook classification.

Correct answer: B. Outer core and inner core boundary

Correct answer: B, the boundary between the outer core and the inner core. To understand this, remember that Earth is arranged in layers: crust, mantle, outer core and inner core. A discontinuity is a marked change in material or wave behaviour between two layers. The Lehmann discontinuity was identified from the behaviour of earthquake waves and indicates the transition from the liquid outer core to the solid inner core. A is incorrect because the crust–mantle boundary is the Mohorovičić or Moho discontinuity. C is not an important internal Earth boundary; the lithosphere and hydrosphere are different Earth systems. D concerns the atmosphere and is unrelated to the metallic core. Memory cue: Lehmann means the inner-core boundary; remember “Lehmann reaches the heart of Earth.”

Correct answer: A. Inner Core

The inner core lies at the centre of the Earth and is considered the hottest internal layer. It is mainly made of iron and nickel and remains solid because of immense pressure. The outer core is also very hot, but it is a liquid layer surrounding the inner core. Exam tip: Remember the order of layers—crust, mantle, outer core, inner core.