The theory of seafloor spreading, proposed by geologist Harry H. Hess in the early 1960s, offered a mechanism to explain how continents move across the Earth’s surface. This concept posits that new oceanic crust is continuously created at massive underwater mountain ranges known as mid-ocean ridges. Here, molten material rises from the mantle, solidifies, and pushes the existing seafloor away from the ridge axis in both directions. The slow, persistent movement of this newly formed crust acts as the conveyor belt that drives continental drift. The definitive confirmation of this process relies on specific, measurable evidence locked within the rocks and sediments of the ocean floor.
Paleomagnetic Signatures in Oceanic Basalt
The most compelling evidence for seafloor spreading comes from the magnetic record preserved in the basaltic rock that makes up the ocean crust. As magma cools and crystallizes at the mid-ocean ridge, iron-rich minerals within the molten rock align themselves with the Earth’s magnetic field. This alignment is permanently locked into the rock once the temperature drops below the Curie point, creating a stable thermoremanent magnetization. This magnetic orientation records the direction of the magnetic field at the moment of the rock’s formation.
Scientists discovered that the Earth’s magnetic field has undergone numerous geomagnetic reversals throughout geologic history, where the magnetic North and South poles have effectively swapped places. By mapping the magnetic properties of the ocean floor, researchers found a striking pattern of magnetic anomalies. These anomalies appear as alternating stripes of “normal” polarity (aligned with the present-day field) and “reversed” polarity (opposite to the present-day field).
The most telling feature of this magnetic evidence is its perfect symmetry on either side of the mid-ocean ridge. The magnetic stripes form a mirror image, indicating that new rock is created continuously and equally at the ridge axis before being split and carried away. This consistent, symmetrical pattern acts like a continuous magnetic tape recording of Earth’s polar history, supporting the Vine-Matthews-Morley hypothesis.
Progressive Aging of the Ocean Floor
The process of seafloor spreading predicts a clear pattern in the absolute age of the oceanic crust, which has been verified through rock dating. If new crust is constantly being generated at the mid-ocean ridges, then the youngest rocks should be found at the center of the ridge axis. Conversely, the crust should become progressively older the farther away it is from the spreading center.
This age gradient was confirmed by collecting rock samples from various locations across the ocean floor. Scientists utilized radiometric dating techniques, such as potassium-argon dating, on the basalt samples to determine their precise crystallization age. Samples taken directly from the ridge crest were found to be extremely young, often near-zero age.
As researchers sampled the seafloor moving toward the continental margins, the age of the basalt was found to increase systematically. The oldest oceanic crust discovered is approximately 200 million years old, a stark contrast to continental rocks that can be several billion years old. This relative youth of the entire ocean floor is explained by the spreading theory, as older crust is eventually recycled back into the mantle at deep-sea trenches.
Deep-Sea Sediment Thickness and Distribution
The distribution and thickness of the material lying on top of the basaltic crust offers support for the continuous creation of new seafloor. Deep-sea sediments consist of fine particles, including wind-blown dust, volcanic ash, and the remains of microscopic marine organisms, which constantly accumulate on the ocean floor. The rate of this accumulation is slow, measured in just millimeters per thousand years.
If the crust near the mid-ocean ridge is newly formed, it has not existed long enough to gather substantial layers of sediment. Therefore, the thickness of the sediment layer should directly correlate with the age of the underlying crust. Observations confirm that the sediment cover is thin directly over the mid-ocean ridge.
Moving away from the ridge axis, the sediment layer progressively thickens as the underlying crust has had more time to accumulate this falling debris. This thickening continues toward the continental margins, where the oldest oceanic crust resides beneath the thickest sediment piles. This observed pattern of sediment distribution is precisely what the theory of seafloor spreading predicts, serving as an independent confirmation of the age gradient established by rock dating.