The movement of plates at mid-ocean ridges does not leave a deep gap in the lithosphere because new crust is constantly being created at these locations. The process of seafloor spreading occurs when magma rises from the Earth's mantle and fills the gap created as plates move apart. This magma cools and solidifies, forming new oceanic crust. As the plates continue to move apart, the newly formed crust is carried away from the ridge and replaced by more magma. The continuous creation of new crust at mid-ocean ridges prevents the formation of deep gaps in the lithosphere.