In much the same way that geographic borders have separated, collided and been redrawn throughout human history, tectonic plate boundaries have diverged, converged and reshaped Earth throughout its geologic history. Today, science has shown that the surface of Earth is in a constant state of change. We are able to observe and measure mountains rising and eroding, oceans expanding and shrinking, volcanoes erupting and earthquakes striking.

Before the Tharp-Heezen map of the seafloor was published in 1977, scientists had little understanding of the geological features that characterized the seafloor, especially on a global scale. The data and observations represented by the Tharp-Heezen map became crucial factors in the acceptance of the theories of plate tectonics and continental drift. The theory of plate tectonics states that Earth’s solid outer crust, the lithosphere, is separated into plates that move over the asthenosphere, the molten upper portion of the mantle. Oceanic and continental plates come together, spread apart and interact at boundaries all over the planet.

Each type of plate boundary generates distinct geologic processes and landforms. At divergent boundaries, plates separate, forming a narrow rift valley. Here, geysers spurt superheated water, and magma, or molten rock, rises from the mantle and solidifies into basalt, forming new crust. Thus, at divergent boundaries, oceanic crust is created. The mid-ocean ridge, Earth’s longest mountain range, is a 65,000 kilometers (40,390 miles) long and 1,500 kilometers (932 miles) wide divergent boundary. In Iceland, one of the most geologically active locations on Earth, the divergence of the North American and Eurasian plates along the Mid-Atlantic Ridge can be observed as the ridge rises above sea level.

At convergent boundaries, plates collide with one another. The collision buckles the edge of one or both plates, creating a mountain range or subducting one of the plates under the other, creating a deep seafloor trench. At convergent boundaries, continental crust is created and oceanic crust is destroyed as it subducts, melts and becomes magma. Convergent plate movement also creates earthquakes and often forms chains of volcanoes. The highest mountain range above sea level, the Himalayas, was formed 55 million years ago when the Eurasian and Indo-Australian continental plates converged. The Mediterranean island of Cyprus formed at a convergent boundary between the African and Eurasian plates. Hardened mounds of lava, called pillow lavas, were once on the bottom of the ocean where this convergence occurred, but have been pushed up and are now visible at the surface.