The heating and cooling of the early earth led to the formation of the atmosphere and the oceans. The early atmosphere lacked free oxygen and probably consisted of nitrogen, carbon dioxide, methane, ammonia, and various sulfur gases. The photosynthetic activity of primitive plant- like organisms released free oxygen into the atmosphere. The earth's crust has been constantly changing since the time of its formation. Geology is a quantitative and interdisciplinary science which seeks to explain geologic phenomena and provide solutions to problems posed by geologic hazards.
Human activities cause or accelerate permanent changes in natural systems.
The impact of these activities is broadly proportional to the size of the population and the level of technological development. The world's population is predicted to exceed 6 billion within a decade and about 8 billion by the year 2025. Global population may be on the verge of exceeding the earth's carrying capacity.