According to the geology website Plate Tectonics a divergent boundary is an area of the Earth's crust where two tectonic plates are moving away from one another. There are three types of plate boundaries called divergent, convergent and transform. As their names suggest, convergent boundaries move closer together as they move. Transform boundaries are areas where plates meet and slide horizontally around each other.
Earth is made up of various plates over the crust of the planet that move independently of each other. Plate Tectonics reports that despite the fact the plates move independently they are interconnected with the movement of one plate causing the movement of another---possibly thousands of miles away. Where plates meet, massive amounts of energy can be released with the movement of the plates. When divergent plate boundaries move apart, this energy is released in the form of tremors felt on the land around the plate.
The Center for Educational Technologies at Wheeler Jesuit University reports divergent boundaries cause the surface layer of the Earth, called the lithosphere, to break apart leaving a deep chasm called a rift. In places where molten lava, or magma, is able to fill the rift the lava eventually cools to form a new crust. Eventually, where magma is forced to the surface in areas of divergent boundaries, volcanic islands are formed.
An example of a divergent boundary given by the science information website Universe Today is the country of Iceland, through which the Mid-Atlantic Ridge runs. Beneath Iceland a divergent boundary between the North American tectonic plate and the Eurasian plate is slowly opening a rift through the center of Iceland. As these plates move apart the land above them moves with the plate, which will eventually cause the country of Iceland to split in two and form two separate islands. As these two islands are formed the waters of the Atlantic Ocean will fill the area between the islands causing a boundary of water between the two parts of Iceland.
According to the Universe Today website the Mid-Atlantic Ridge formation of mountains was formed by the movement of the North American plate and the Eurasian plate along its divergent boundary. The Mid-Atlantic Ridge formation of mountains is the longest in the world and lies at the bottom of the Atlantic Ocean, growing up along the ocean floor where the new crust of the planet was formed by volcanic lava.
The Tectonic Plates website reports that the oceans of the planet Earth were formed by the movement of tectonic plates at divergent boundaries. By opening large rifts that were not filled by magma the oceans were both born and grew wider because of the divergent boundaries of tectonic plates.