Boulder trail – granite porphyry

Granite porphyry is a type of rock with a unique appearance. It features large reddish feldspar crystals embedded into a finer-grained mass or matrix. This structure gives the rock an attractive speckled look, with the large crystals standing out distinctly against the finer base of rock.

In Estonia, granite porphyry is a common type of glacial erratic. The term “porphyry” originates from the Greek word for “purple”, but granite porphyry can come in a range of colors, including pink, red, and gray.

Granite porphyry resembles granite but has a more varied texture due to the contrast between the large crystals and the finer base material.

Task: Examine this large rock closely, particularly its polished surface. What crystals can you find here? Think about the crystals you saw in the other rocks and how you recognised them. Do you see anything similar here? You can identify feldspar by its red color, quartz by its grayish or transparent haze and mica by its black and shiny appearance. Use your phone’s flashlight and camera zoom for a closer look, if needed.