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WalesGeologyQuick questions
F4: Earth Structure and Global Tectonics
Quick questions on Earth internal structure - WJEC A-Level Geology
5short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What are the mechanical layers?Show answer
Mechanically (by how the rock behaves) the outer Earth is divided differently. The lithosphere is the rigid outer shell, made of the crust plus the uppermost, rigid part of the mantle; it is brittle and forms the tectonic plates. Beneath it the asthenosphere is a weaker, partially molten, ductile layer of the upper mantle that can flow slowly. The rigid lithospheric plates move over the mobile asthenosphere, which is why this mechanical distinction is the one that matters for plate tectonics.
What is the seismic evidence?Show answer
We cannot drill to the core, so the structure is deduced from seismic waves from earthquakes. P waves (primary, compressional) travel through solids and liquids and are the faster waves. S waves (secondary, shear) travel only through solids, because liquids have no shear strength. Two key observations follow: waves speed up or change direction sharply at boundaries called discontinuities (the Moho, and the core-mantle boundary), revealing the layers; and there are shadow zones where waves do not arrive.
What is q1?Show answer
Distinguish oceanic crust from continental crust by composition and thickness. [2 marks]
What is q2?Show answer
Why can S waves not travel through the outer core? [1 mark]
What is q3?Show answer
State what the lithosphere is composed of. [2 marks]
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