1. Upwelling of Magma:
- The Earth's mantle is constantly moving in convection currents.
- At mid-ocean ridges, these currents bring hot magma from deep within the mantle towards the surface.
2. Formation of a Rift Valley:
- As magma rises, it pushes apart the existing oceanic crust, creating a deep rift valley.
3. Eruption and Solidification:
- Magma erupts from the rift valley, flowing out onto the ocean floor.
- As it cools and solidifies, it forms new oceanic crust, primarily composed of basalt, a dark, fine-grained volcanic rock.
4. Seafloor Spreading:
- As new crust is formed, it is pushed away from the rift valley in opposite directions.
- This continuous process, known as seafloor spreading, creates new ocean floor and expands the ocean basins.
5. Magnetic Stripes:
- As the magma cools and solidifies, it records the Earth's magnetic field at the time of formation.
- The magnetic field flips its polarity over time, resulting in alternating bands of normal and reversed magnetic polarity in the ocean floor. This pattern provides crucial evidence for seafloor spreading.
6. Lithospheric Formation:
- The newly formed oceanic crust, along with the upper layer of the mantle, forms the oceanic lithosphere.
- The lithosphere is relatively rigid and moves as a single unit.
7. Ocean Basin Expansion:
- The process of seafloor spreading continues, constantly adding new oceanic lithosphere and expanding the ocean basins.
8. Subduction:
- Eventually, the older, denser oceanic lithosphere will be subducted back into the mantle at convergent plate boundaries.
This continuous cycle of seafloor spreading, creating new oceanic lithosphere, and subduction, destroying old oceanic lithosphere, is the driving force behind plate tectonics.
