Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks and magma fueled by mantle convection, flows up to fill these cracks. This magma cools in the seawater to form igneous rock, a new part of Earth’s crust. This oceanic crust slowly moves away from mid-ocean ridge and the site of seafloor spreading. If the Earth's crust is continually growing, then why isn't Earth expanding? Hypothesize as to what is happening at the opposite tectonic plate boundary. Select a model that predicts the opposite boundary.