Chihuahua Dog Breed. First discovered in Mexico, the Chihuahua is best known for being the smallest dog breed in the world. Extremely loyal to its owner, the breed has recently become a popular culture icon in the United States, most notably Paris Hilton's Chihuahua, Tinkerbell. Physical Characteristics.
Earth formed around 4.54 billion years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and then the ocean; but the atmosphere contained almost no oxygen and so would have been toxic to most modern life including humans.
This continuous process of precipitation and evaporation is called the water cycle, or hydrologic cycle. It's been going on ever since oceans were formed on this planet 3.8 billion years ago. Earth was formed 4.6 billion years ago, but water was not present at the start. At some point, possibly because of the heating of hydrogen and oxygen as Earth developed, water vapor began to form in the atmosphere. Oceans formed and the cycle began. Seventy percent of Earth is covered by water.