The outermost part of the Earth is known as __________.

Correct — C. The Earth is read in three shells from the outside in — crust, mantle, core — and the outermost of them is the crust, the thin solid skin we live on.

It is by far the thinnest layer: roughly 35 km thick under the continents and only about 5 km under the ocean floors, against a mantle that runs down to about 2,900 km.

Everything below the crust is out of reach of direct observation and is read instead from earthquake waves.

Why the others are wrong

• (a)
  the core — The core is the innermost shell, from about 2,900 km down to the centre. It is the opposite end of the section from the outermost layer the stem asks for.
• (b)
  the mantle — The mantle lies between crust and core. It is the thickest of the three shells, but it sits beneath the crust, so it is not the outermost part.
• (d)
  volcano — A volcano is a landform — a vent through which magma reaches the surface. It is a feature built on the crust, not one of the layers of the Earth.

The Earth's interior is described in two schemes, and SSC uses both.

By composition there are three layers: the crust, the mantle down to roughly 2,900 km, and the core below it, largely iron and nickel.

By mechanical behaviour the top is divided differently: the rigid lithosphere is the crust plus the uppermost mantle, and it rides on the weaker asthenosphere.

So 'outermost part' takes the compositional answer, the crust, while a stem asking for the rigid upper shell takes the lithosphere.

Key facts

• The Earth's compositional layers, outward to inward, are crust, mantle and core.
• The crust is about 35 km thick under the continents and about 5 km under the ocean floors.
• The mantle extends to a depth of about 2,900 km.
• The lithosphere is the crust together with the uppermost part of the mantle.

Common traps

• Lithosphere and crust are not the same thing, and SSC keys them to different wordings.
• The mantle is the thickest layer, which makes it feel like the significant answer to a stem that actually asks for the outermost.

No directly related past PYQ was found.