Sea Floor Spreading
The theory of Sea Floor Spreading states that new oceanic crust is being formed continuously at mid-oceanic ridges, while the older rocks move away from the ridge. That is, it explains why the age, thickness, and density of the oceanic crust increases with distance from the mid-oceanic ridge.
Sea Floor Spreading
In this article series, we are learning about the theories which explain the present distribution of oceans and continents, the concepts which explain the distribution of earthquakes and volcanoes, folds and faults. In the previous article, we have learned about the Continental Drift Theory given by Alfred Wegener. Although Wegener is right in stating that the continents drift over time, but his assumption that the continents were blocks of rock that slid across the ocean floor has been found to be incorrect. What we know today is that the continents and ocean floor are part of the same layer of the earth.
In this article, we will learn about Sea Floor Spreading which explains continental drift by the help of the theory of plate tectonics. But, before coming directly into the concept of Sea Floor Spreading, we must understand some basic concepts that explain Sea Floor Spreading.
These concepts are Ocean Floor Mapping, Distribution of earthquakes and volcanoes, Convectional Current theory and Paleomagnetism.
Ocean Floor Mapping
The ocean floor is found to be having mountain ranges, plains, canyons, submarine ridges, deep trenches and other relief features. It has also been found that along the mid-oceanic ridges, volcanic eruptions are most active. Further, the dating of rocks suggests that the oceanic crust rocks are much younger than the continental rocks. Also, the rocks which are equidistant from the crest of oceanic ridges on both sides have been found with utmost similarities in terms of their age, constituents, chemical composition and magnetic properties. And the sediments on the ocean floor near the ridge has been found to be thinner.
Figure showing typical relief features of the ocean floor
Convectional Current Theory
- Convectional current theory was proposed by Arthur Holmes.
- First of all, it is to be noted that the heat which is generated from the radioactive decay of substances deep inside the Earth (the mantle) creates magma which consists of molten rocks, volatiles, dissolved gases among other material.
- The Convectional current theory states that this magma, heat and gases seek a path to escape which leads to the formation of convection currents in the mantle.
- According to the theory of Seafloor spreading, convectional cells are the force behind drifting of continents.
- Also, please note that the ocean plates get subducted under the continental plates (since ocean plates are denser than continental plates), when these two types of plates converge.
- The collision of plates is followed by earthquakes and volcanoes.
The Distribution of Earthquakes and Volcanoes
Another observation is that, if we map the distribution of earthquakes and volcanoes, it is surprising to know that volcanic activities, earthquakes and other associated activities at plate margins are a result of convection currents in the mantle.
Do observe in the map shown below, that the volcanic activity in the oceans is almost parallel to the coastlines.
Since magma is less dense than the crust, it rises up to the oceanic crust with convection currents, leading to the formation of a volcano.
Figure showing ridges as the hot-spots of the earthquake and volcanic activity
Palaeomagnetism, in simple words, refers to the study of the earth's magnetic properties. The magnetic studies have revealed that the ocean floor consists of parallel bands of oceanic crust which have alternating magnetic polarity. These magnetic bands are symmetric and are mirrored around the mid-oceanic ridge.
Alternating magnetic polarity or the reversal of magnetic polarity refers to change in the Earth's magnetic field, that is, the north magnetic pole becomes south magnetic pole and vice versa.
These alternating magnetic bands had happened because the new rocks which are formed near the ridge, while the older rocks, which formed millions of years ago when the magnetic field was reversed, have been pushed farther away.
Hence, this further explains the seafloor spreading.
Figure showing parallel bands of oceanic crust with alternating magnetic polarity around the mid oceanic ridge
The concept of Sea Floor Spreading
The concept of seafloor spreading was put forward by H.Harry Hess, an American geologist.
He suggested that new sea-floor forms at the oceanic ridges and spread outwards from the line of origin. Further, he claimed that continents would be pushed aside by the same forces that cause the ocean to grow. That is, constant eruptions at the crest of oceanic ridges cause the oceanic crust to rupture and new lava to wedge out of it, pushing the oceanic crust on either side. The ocean floor thus spreads.
Figure showing sea-floor spreading
The intense heat generated by radioactive substances in the mantle beneath the lithosphere seeks a path to escape and forms convection currents. The rising convectional currents continuously bang thick but strong oceanic SIMA crust and the hammering effect leads to the development of cracks, joints, fractures and disintegration of oceanic ridges. Pyrospheric material oozes out and pushes the older oceanic SIMA away from the rift zone. Seafloor spreading occurs at diverging plate boundaries.
Upwelling of the magmatic material leads to the formation of mid-oceanic ridges and substitution of older material by newer one. Successive eruption results into seafloor spreading.
The rate of movement is slow, 2.5 cms per year and resembles two giant conveyor belts carrying sea floor from the zones of accretion(divergence area- mid-oceanic ridge) to zones of consumption ( convergence area- trenches).
The theory of seafloor spreading solved many of the unsolved problems
- It solved the problem of younger age crust found at the mid-oceanic ridges and older rocks being found as we go away from middle part of the ridges.
- It also explained why the sediments at the central parts of the oceanic ridges are relatively thin.
- The sea-floor spreading also proved the drifting of continents as propounded by Alfred Wegener and helped in the development of the theory of plate-tectonics.
In the next article, we will learn about a related concept, the theory of plate-tectonics.