Why do continents move?
Diagram showing the layered structure of the earth. Information about the inside of the earth is derived from the observation of earthquake vibrations moving through the planet.
The story of kunanyi/Wellington is very closely aligned to the movement of continents around the surface of the earth. Forces created as a result of these movements are largely responsible for the mountain’s existence. But why do continents move in the first place?
Continents move as a result of a process we call plate tectonics. To understand how this works we need to know two things (1) that the earth is hotter at its centre than it is on the surface and (2) that the interior of the earth is layered.
The hot, metallic core at the centre of the earth is estimated to be around 5000 - 6000C. A solid inner core is encased by a liquid outer which is in turn surrounded by a layer of rocky material called the mantle. The outermost edge of the mantle is cooler and, together with the thin outermost layer of rock or crust, is known as the lithosphere.
The lithosphere is cool and brittle and is cracked into pieces called plates - much like the shell on a boiled egg. It is very different to the mantle where temperatures and pressures are high enough that rock is plastic and is able to flow. Heated from below by the core, rock deep in the mantle becomes buoyant and flows upwards towards the surface.
When these hot currents of rock get near the surface, they experience a drop in pressure that causes melting. Magmas push up into cracks along plate boundaries, solidifying to make new rock and shunting existing lithosphere sideways across the surface - like a giant conveyor belt. This process is called sea-floor spreading because it almost always happens underneath the ocean.
Continents move as a result of a process we call plate tectonics. To understand how this works we need to know two things (1) that the earth is hotter at its centre than it is on the surface and (2) that the interior of the earth is layered.
The hot, metallic core at the centre of the earth is estimated to be around 5000 - 6000C. A solid inner core is encased by a liquid outer which is in turn surrounded by a layer of rocky material called the mantle. The outermost edge of the mantle is cooler and, together with the thin outermost layer of rock or crust, is known as the lithosphere.
The lithosphere is cool and brittle and is cracked into pieces called plates - much like the shell on a boiled egg. It is very different to the mantle where temperatures and pressures are high enough that rock is plastic and is able to flow. Heated from below by the core, rock deep in the mantle becomes buoyant and flows upwards towards the surface.
When these hot currents of rock get near the surface, they experience a drop in pressure that causes melting. Magmas push up into cracks along plate boundaries, solidifying to make new rock and shunting existing lithosphere sideways across the surface - like a giant conveyor belt. This process is called sea-floor spreading because it almost always happens underneath the ocean.
Hot mantle rock rising from deep in the earth results in the process of plate tectonics and continental movement.
Of course, the earth is limited in size and it can’t just keep adding new crust. The conveyor belt of rock moving away from a spreading centre along the edge of one plate will always, eventually collide with another. At this point one plate is forced beneath the other, sinking back down into the mantle in a process called subduction. The down flow of cold, dense rock back into the mantle completes a cycle of heat movement known as mantle convection.
The movement of plates – and their continents - caused by mantle convection is what we mean by plate tectonics. It is important to understand that this is a process of continual change. Hot material rising from the mantle can both create and maintain plate boundaries; over geological time, shifting patterns in mantle convection are reflected in shifting patterns on the surface. This process is still happening today; the Australian continent is still moving north at rate around 5-6 cm/year. Eventually, collision with the Pacific and Sunda plates to the north will result in new stresses and faults that will, once again, reshape our ancient land.
The movement of plates – and their continents - caused by mantle convection is what we mean by plate tectonics. It is important to understand that this is a process of continual change. Hot material rising from the mantle can both create and maintain plate boundaries; over geological time, shifting patterns in mantle convection are reflected in shifting patterns on the surface. This process is still happening today; the Australian continent is still moving north at rate around 5-6 cm/year. Eventually, collision with the Pacific and Sunda plates to the north will result in new stresses and faults that will, once again, reshape our ancient land.
