How are agates formed?
HOW DO AGATES FORM? Agates develop as secondary deposits in hollow cavities, called vesicles. Although they can form in all types of host rock, most of the world’s agates developed in ancient volcanic lava. When the continents were first forming, layers of molten lava pushed toward the earth’s surface through rift zone cracks. Within the lava, there were pockets of trapped gases. Later, these gases escaped through cracks that formed as the igneous rock cooled and hardened, leaving hollow cavities. Other cracks and seams also formed when adjoining sections of lava cooled at different rates.
The hardest question of all. The answer can only be supplied when science can fully answer the following: What is the temperature of agate formation? What is the initial form of the deposited silica? How does silica get into the gas cavities? What is the process of crystallisation that allows the bands to form? Apart from isotope evidence to suggest that agates form at temperatures < 100oC, the answers have not yet been fully established; • Why has the agate in Fig. 4 taken on this squashed appearance? In an igneous setting, agates are mainly the result of a gas-cavity infill. The molten lava flows and the gas bubbles can take up some weird shapes with the flow movement. The cavity is later filled with agate. • Why is the agate in Fig. 8 surrounded by host rock as part of the nodule? These agates are formed in a rhyolite host rock and unusual conditions prevailed to create these agates. • Why do some bands form horizontally? It is assumed that these bands are due to the influence of gr
