Ocean water is salty primarily due to the presence of dissolved salts, while most freshwater sources on land, such as lakes, rivers, and groundwater, have lower salt concentrations. Here are the main reasons for the difference in salinity:
Dissolved minerals: Water bodies on land, including lakes and rivers, are mostly fed by freshwater sources such as rainfall, snowmelt, and groundwater. As freshwater flows over the land, it picks up relatively fewer dissolved minerals and salts from the rocks and soil it encounters. This results in a lower concentration of salts compared to ocean water.
Geological processes: The Earth's crust is composed of various minerals and rocks that contain salts. Over geological timescales, weathering and erosion break down these minerals and release salts into the surrounding environment. Rivers transport these salts and minerals to the ocean, gradually increasing its salinity over millions of years. In contrast, most freshwater sources on land do not have extensive geological processes continually contributing salts to maintain high salinity.
Evaporation and concentration: The ocean covers about 70% of the Earth's surface and receives a significant amount of sunlight. When seawater is heated by the sun, it evaporates, leaving the salts behind and increasing the concentration of salts in the remaining water. This process, called evaporation, contributes to the high salinity of the ocean. In contrast, freshwater bodies on land experience evaporation as well, but they are usually replenished by rainfall and other freshwater sources, preventing significant concentration of salts.
It's worth noting that there are exceptions to the generalization that land water is not salty. Some landlocked seas, such as the Dead Sea and the Great Salt Lake, have high salinity levels due to limited outflow and high evaporation rates, resulting in saltier water. Additionally, certain geological formations can lead to underground saltwater reservoirs, known as saline aquifers, which can be tapped as sources of salty groundwater.