Magma: Molten rock deep within Earth that may cool and crystallize to form plutonic igneous rocks or be ejected to the surface as volcanic lava.
Lava: Magma that has been ejected to the surface through a volcanic vent or fissure and then cools to form volcanic igneous rocks.
Pyroclast: Fragments of volcanic material ejected with lava during volcanic eruptions.
Crater: Area surrounding a volcanic vent that has sunken in or eroded away as a result of weathering and volcanic eruptions.
Caldera: When the ceiling of a magma chamber is too weak to support the immense mass of the volcanic rock formation above, it collapses, and the entire volcano sinks into the magma chamber forming a caldera (a really, really, big crater.)
Diatreme: Sometimes referred to as a “volcanic neck." When a volcano settles down after an explosive eruption, the vent may become filled with volcanic breccia which solidifies into igneous rock. If the surrounding material is weathered away, the resulting structure looks like a massive tower of black stone. Shiprock of New Mexico is a great example of a diatreme or volcanic neck.
Lava Dome: When lava is incredibly viscous, it may pile up over the vent and pool in the crater of the volcano instead of flowing over during volcanic eruptions. Here, lava will solidify to form a volcanic dome (or cork) that will plug the vent of the volcano until the next explosive eruption.
Parasitic Cone: A smaller cone on the flank of a large volcano with a feeder vent that connects to the main volcanic vent or magma chamber.
Fissure: A volcanic eruption that emits lava through a rift or elongated fissure in the ground instead of a centralized vent.
Fumarole: A small vent or geyser that periodically emits volcanic gases and hot water steam. The surrounding pool is rich in sulfur and carbon dioxide.
Batholith: A great, irregular mass of plutonic igneous rock that forms when a chamber of magma cools and crystallizes below the surface.
Dike: A vertical intrusion of igneous rock that cuts through the surrounding rock layers—forms when a vein of magma cools and crystallizes.
Sill: A horizontal intrusion of igneous rock that runs between the surrounding rock layers—forms when a vein of magma cools and crystallizes.