The moral lesson of Journey to the Center of the Earth may be found in Professor Lidenbrock's admonition to his nephew, "Enough! When Science has spoken, it behooves us to be silent."
Lidenbrock is contemplating a descent into a volcanic crater; Harry, his nephew, thinks this is a bad idea. But Lidenbrock is certain the volcano is dormant. He points out the plumes of steam issuing through cracks in the ground as evidence. He is compelled to make his attempt.
Verne combines in Journey to the Center of the Earth a love for hard science, particularly geology and paleontology, and a romantic love of adventure. It seems that science takes on a moral aspect for Verne, in that scientific "truth" is something that must be sacrificed for and that science itself becomes an alternative to myth and religion.
Lidenbrock's casual personification of science as something that "speaks" is part of this; the steam is a kind of clue that science can explain, and the role of the scientist (quite literally, in Lidenbrock's case) is to follow the clues wherever they might lead. The quest for scientific knowledge is a quest to uncover the mysteries of the planet. Though secular in nature, the "voice of science" is presented as not that far from the voice of God, and the scientific passion of Lidenbrock is charged with a certain religious intensity.
For Verne, morality has to do with the pursuit of knowledge, because such knowledge will lead to larger truths about creation and man's place in it.