Continental drift describes the large-scale movement of continents across the Earth’s surface. This movement is a surface manifestation of plate tectonics, the theory that the planet’s rigid outer layer, the lithosphere, is broken into large slabs. The North American and Eurasian continents sit on their respective tectonic plates. These two massive slabs are currently being pulled apart, a slow, continuous separation responsible for the widening of the Atlantic Ocean basin. The geological forces driving this separation originate deep within the planet’s interior.
Identifying the Mid-Atlantic Ridge
The separation between the North American and Eurasian plates occurs along the Mid-Atlantic Ridge (MAR), a massive underwater mountain chain. This geological feature is a divergent plate boundary where two tectonic plates are moving away from one another. The MAR is recognized as the longest mountain range in the world, stretching approximately 25,000 miles across the ocean floor. The ridge runs down the center of the Atlantic, roughly equidistant from the surrounding coastlines.
At this boundary, the North American Plate moves westward while the Eurasian Plate moves eastward. The MAR is mostly submerged, rising about 3 kilometers above the adjacent seafloor, with a deep rift valley running along its crest. Iceland represents one of the few places where this spreading center is visible above sea level. The island nation is bisected by the Mid-Atlantic Ridge, allowing observers to see the rift valley and its geological activity.
The Mechanism of Seafloor Spreading
The physical process by which the plates separate and the ocean basin expands is called seafloor spreading. This mechanism occurs at the crest of the Mid-Atlantic Ridge, where the diverging plates fracture the lithosphere. The separation reduces pressure on the hot rock below, allowing molten material, or magma, to rise from the underlying mantle.
This magma erupts onto the ocean floor and quickly cools and solidifies in the cold seawater, forming new oceanic crust primarily made of basalt rock. The continuous upwelling and solidification of this material acts like a conveyor belt, pushing the older crust away from the ridge axis in both directions. This constant creation of new crust is the direct cause of the separation between North America and Eurasia.
Since the Earth is not expanding, the creation of new crust at this divergent boundary must be balanced by the destruction of old crust elsewhere. This recycling occurs at convergent boundaries, where plates collide and one is forced back down into the mantle.
The Driving Force of Plate Movement
The energy source powering this system is the intense heat within the Earth’s core and mantle, largely from the decay of radioactive elements. This heat drives mantle convection, where hotter, less dense material rises, and cooler, denser material sinks. These slow-moving currents within the semi-fluid mantle act as a dynamic conveyor belt beneath the tectonic plates.
While convection provides the underlying thermal engine, the mechanical forces that directly move the plates are primarily driven by gravity. One of these forces is “ridge push,” which occurs because the Mid-Atlantic Ridge is elevated due to the hot, buoyant material rising beneath it.
The newly formed, hot lithosphere at the ridge is higher than the older, cooler lithosphere farther away. Gravity acts on this elevated mass, causing the lithosphere to slide away from the ridge crest and pushing the entire plate in front of it.
Another contributing force is “slab pull,” which involves the weight of a cold, dense plate sinking into the mantle at a distant subduction zone. In the Atlantic, the ridge push force is a significant factor in the divergence of the North American and Eurasian plates. The combination of convection and ridge push creates the power necessary to overcome friction and resistance, ensuring the continuous movement of the continents.
Measuring the Separation and Geological Evidence
The separation of the North American and Eurasian plates is a gradual process, not a sudden event. The average rate of spreading along the Mid-Atlantic Ridge is about 2.5 centimeters per year. This rate is comparable to the speed at which human fingernails grow, making the movement imperceptible over short time spans.
Over millions of years, this slow movement has resulted in the formation of the vast Atlantic Ocean basin. Scientists confirm this continuous spreading through two primary lines of evidence: the age of the oceanic crust and magnetic striping.
Dating the rocks collected from the ocean floor shows that the youngest rocks are found directly at the ridge crest. The rock age increases symmetrically as one moves farther away from the ridge, providing a clear record of the spreading over time.
The second piece of evidence is the pattern of magnetic striping in the basaltic rock. As magma cools, iron-rich minerals align with the Earth’s current magnetic field, locking in that polarity. Since the Earth’s magnetic field reverses periodically, the new crust formed at the ridge records a sequence of normal and reversed magnetic polarities. This record appears as symmetrical “stripes” on both sides of the Mid-Atlantic Ridge, confirming that new crust is constantly being created and pushed outward.