What is Seafloor spreading

Seafloor spreading normally happened at large tectonic plates whereby the plate (lithosphere) try to split apart from each other.

Underneath the large tectonic plate, there is mantle that carries heat from core to lithosphere via convection process. As the heat reaches lithosphere, it made the lithosphere become more plastic and less dense. The less dense material rises forming mountain or elevated area (mid ocean ridge) of the seafloor. 

Seafloor spreading occurs at mid ocean ridge. As magma reaches the ridge, the magma will cool offs and creates basalt. The basalt created will then move away from mid ocean ridge as new magma emerged from the convection process like a conveyor belt.

In South East Asia, the mid ocean ridge is believed started at Dangerous Ground during early Paleocene. The seafloor spreading causes opening of South China sea. the plate was then subducted under Borneo.