Obviously the older sedimentary rock is denser because it is much more compacted due to overburden.
Oceanic lithosphere gets older as it moves away from the mid-ocean ridges where new lithosphere is formed. This process, known as seafloor spreading, causes older lithosphere to be pushed further from the ridge, resulting in a gradient of lithosphere age with the oldest being farthest from the ridges.
The Earth's lithosphere is generally thicker in the vicinity of trenches than around mid-ocean ridges. At mid-ocean ridges, where new oceanic crust is formed through volcanic activity, the lithosphere is relatively thin. Conversely, at subduction zones near trenches, the lithosphere is thicker due to the presence of older, denser oceanic crust being forced down into the mantle.
The process of plate tectonics drives the creation and destruction of lithosphere. As new lithosphere is formed at mid-ocean ridges through seafloor spreading, older lithosphere is consumed at subduction zones, creating a balance between formation and destruction over time. This equilibrium maintains a relatively constant amount of lithosphere on Earth.
The older, denser oceanic lithosphere will subduct beneath the younger, less dense oceanic lithosphere. This process can lead to the formation of deep ocean trenches, volcanic island arcs, and earthquakes. The descending plate may partially melt, contributing to the formation of magma that can erupt as volcanoes.
It gets younger.
New lithosphere is constantly being produced at the oceanic ridges. Lithosphere is consumed at the subduction zones. A balance is maintained because older, denser proportions of oceanic lithosphere descend into the mantle at a rate equal to seafloor production.
Convection ridge push and slab pull work together to produce plate motion within the Earth's lithosphere. Ridge push occurs at mid-ocean ridges where newly formed crust pushes older crust away, while slab pull occurs at subduction zones where denser oceanic crust sinks into the mantle, pulling the plate along with it. These forces combined drive the movement of tectonic plates.
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Younger. The mid-ocean ridge is formed from new material being thrust up from inside the Earth.
The lithosphere can have two main types of crust: continental crust and oceanic crust. Continental crust is thicker, older, and less dense than oceanic crust. Oceanic crust is thinner, younger, and denser, primarily composed of basaltic rocks.
As the oceanic lithosphere moves away from a mid-ocean ridge, it cools and thickens. This process causes the ocean floor to become older and denser, resulting in its gradual subsidence. The farther it gets from the ridge, the deeper the ocean floor becomes due to the increased thickness and weight of the lithosphere.
Cooler, older oceanic lithosphere sinks into the mantle at subduction zones where two tectonic plates converge. As the denser oceanic plate descends into the mantle, it creates deep ocean trenches and may eventually cause volcanic activity. This process is essential for the recycling of oceanic crust and plays a key role in plate tectonics and the Earth's geological processes.