What type of rock would be under the basalt layer?

Under the basalt layer, you would typically find gabbro, which is a coarse-grained igneous rock formed from the same magma that cools slowly beneath the Earth's surface. Additionally, in some geological settings, you might encounter sedimentary rocks or metamorphic rocks, depending on the specific tectonic and environmental history of the area. If the basalt is part of an oceanic crust, it may also rest on peridotite, a mantle rock.