Why is the vertical cross section along a tectonically active region of the coast so steep

Tectonically active regions of the coast, are so steep because they also have frequent earthquakes and/or volcanic activity.

Volcanic activities lead to the eruption of molten magma. Given its high viscousity, it flows very slowly along the vertical sections of the volcano/topography and as it cools leaves even a more vertical section of solidified igneous rocks.

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