Sheet-like intrusions that are mainly concordant with bedded sedimentary strata are called sills, and those that cross-cut strata are called dykes. Magma may also reach the surface at volcanoes, where it is erupted as lava or ejected explosively as fragments (including volcanic ash and volcanic bombs) that ultimately form pyroclastic rocks. Igneous rocks may be classified based on their silica content: those with low silica, for example basalt and dolerite, are termed basic rocks, whereas those with abundant silica, such as rhyolite and granite, are acid.
Intermediate compositions include andesite and trachyte. Examples of all of these compositions and forms of igneous rock are represented within Northumberland National Park.