Question

Why is the oldest seafloor on Earth generally never older than ~200 Ma (million years)?

a. When the oceanic crust becomes older than ~ 200 Ma, it becomes continental crust
b. When oceanic crust becomes older than ~200 Ma, it becomes dense enough to be subducted beneath a continent or another oceanic plate
c. The Earth is younger than 200 Ma

Answer 1

The oldest seafloor is generally not older than 200 Ma because dense, aged oceanic crust is subducted into the mantle at subduction zones, a process that recycles the crust and constantly renews the seafloor.

Step-by-step explanation:

The reason why the oldest seafloor on Earth is generally never older than approximately 200 million years (Ma) is because oceanic crust that becomes older than ~200 Ma grows dense enough to be subducted beneath a continent or another oceanic plate. This process occurs at subduction zones, where the oceanic plate is thrust down into the upper mantle.

The subducted plate melts and is recycled into a downward-flowing convection current, thus being removed from the earth's surface and renewing the oceanic crust through a continual process of sea-floor spreading and subduction. Oceanic plates are primarily composed of basalt, which becomes denser with age as it cools and contracts. Eventually, the increasing density relative to the underlying asthenosphere causes the plate to be driven down into the mantle at subduction zones. This contrasts with continental crust, which consists of less dense granite, and as such, is not typically subducted.