Hegyestű is the remains of the chimney of a volcano that was formed by erosion from its surroundings. The basalt magma, which breaks towards the surface of the earth, is stuck in the former chimney and solidifies into pentagonal and hexagonal basalt columns with a diameter of 20 to 40 cm, which are curved like organs and are close to each other towards the mountain top. In some places, a rock of the peperite type can also be observed at the edge of the chimney, which indicates a mixture of magma and pierced rock. In the so-called "cornstone" basalt with bubbles that can be observed in the lower levels, the bubble cavities that protect the earlier volcanic vapors and gases are sometimes directed. Although the nameplates on the geological demonstration report state that the very first tuff-strewn eruption occurred and a tuff ring was formed as a result near the eruption point, there is no trace of this tuff ring in the current excavations.
After volcanism, erosion primarily determined the appearance of the area. Erosion destroys softer, less elastic rocks and sediments faster and more intensively, while harder, more elastic rocks are destroyed more slowly: Accordingly, over millions of years, hard basalt bodies have developed from their softer environment, and these basalt bodies (e.g. a Vulkanhorn in the case of Hegyestű)) form the characteristic witness mountains of the Balaton highlands.
Source: Wikipedia