The Earth is covered in plates that move in conjunction with the planet’s internal heat mechanism. Convection currents deep within the mantle form a conveyor belt, moving the plates and thereby the continents. Plate movement causes earthquakes, volcanic eruptions, mountain building events and deep sea trenches. These geologic features and events serve as evidence for the convection theory.
Holmes’ Convection Theory
In 1929, Arthur Holmes used Alfred Wegener’s observations to develop a plate tectonics mechanism theory. Holmes named convection as the driving force behind plate tectonics. His theory suggested that as the mantle became heated it became less dense and began to rise. As the material cooled, it would sink, forming what he described as a conveyor belt. He theorized that the pressure of heated magma broke continents apart, forcing pieces of the continents to move in opposite directions. He speculated further that when the material sank, continents were brought back together.
Sea Floor Spreading
Holmes’ theory received little attention in the 1920s because of a lack of evidence to support it. In the 1960s, Hess and Dietz built upon Holmes’ theory to create the idea of sea floor spreading. Hess and Dietz recognized that convection currents in the asthenosphere are strong enough to form a powerful mechanism such as plate tectonics. They analyzed maps and magnetic data of the ocean floor. They discovered that the mid-oceanic ridge was a weak zone where new crust was being created. In addition, they found deep-sea trenches along continental margins where crust was being destroyed. Through their research, they determined that the Atlantic Ocean was widening and the Pacific Ocean was shrinking. While Holmes’ theory lacked evidence, Hess and Dietz had a mechanism for plate tectonics and the evidence provided by deep sea research to back it up.
Analysis of the ocean floor indicates that heat flow on the ocean floor peaks at the crest of the ridge. It is also noted that the youngest rocks on the ocean floor exist at the ridge, and the rocks get progressively older moving away from the ridge in either direction. These facts suggest that upwelling occurs at the mid-ocean ridge, pushing the plates apart in both directions. Geologic activity such as earthquakes and volcanic eruptions are increased at the mid-ocean ridge.
Throughout Earth’s history, there have been a number of magnetic field reversals. During this time, the south magnetic pole will switch to be geographically in the north and vice versa. Rocks containing iron, such as the basalt on the ocean floor, will align themselves with the magnetic poles. The ocean floor shows the magnetic reversals in stripes where the minerals align differently. These magnetic field reversals are mirrored on either side of the mid-ocean ridge, indicating that sea floor spreading originates at the ridge.
Trenches and Island Arcs
Whenever crust is created, crust must also be destroyed. Hess and Dietz recognized that oceanic crust was being destroyed at deep sea trenches. When deep sea trenches occur adjacent to continental margins, volcanic mountain ranges can be seen on the continent. When deep sea trenches form within the ocean, volcanic island arcs are found adjacent to the trench. This occurs because when the crust is pushed deeper into the lithosphere, it heats. The heated material becomes less dense than the area around it, creating volcanic mountains and islands adjacent to the trench.