Which description best characterizes eclogite?

• A. A high-pressure metamorphic rock
• B. An igneous rock
• C. A sedimentary rock
• D. A volcanic glass

Answer: (A)

Eclogite is a high‑pressure metamorphic rock that forms deep in subduction zones, when basaltic rocks are altered under great pressures (and temperatures) without melting. Its distinctive mineral pairing of garnet and pyroxene, often with omphacite, creates a dense, crystalline rock that records high‑pressure conditions. This is different from igneous rocks, which crystallize from molten magma, from sedimentary rocks formed by cementing or lithifying sediments, or from volcanic glass, which is glassy and forms from rapid surface cooling. The presence of well-formed crystals and the garnet–pyroxene assemblage are telltale signs of high‑pressure metamorphism, making that description the best fit.