Shiprock in New Mexico

Molten rock does not always erupt onto the surface. Sometimes it stalls and cools just beneath the crust, forming what’s called an intrusion. Intrusions often push up and fracture the overlying surface, creating a system of cracks. Erosion sometimes wears away the overlying material, exposing the hardened intrusion. 

Shiprock in New Mexico is a preserved core of a volcanic vent exposed by erosion. The vent was active 30 million years ago.  Several sheet-like lava intrusions called dikes, in the foreground, radiate from the core on the left. Image courtesy of Wikimedia.