Lava
Lava is molten rock that has erupted onto Earth's surface from a volcano or fissure, cooling into extrusive igneous rock.
Definition
Lava is magma that has reached Earth's surface, glowing red or orange from its heat before cooling into hard rock. Basaltic lava is fluid, erupting at $1{,}100$ to $1{,}200^\circ C$ and forming smooth ropy pahoehoe or rough blocky aa flows, while andesitic and rhyolitic lavas are thicker, move slowly, and build steep domes, and because lava cools quickly at the surface it forms fine-grained or glassy rock such as basalt, obsidian, and pumice. Viscosity depends on temperature, silica content, crystal fraction, and dissolved volatiles, ranging from about $10^2\ \text{Pa}\cdot\text{s}$ for hot basalt to $10^{11}$ or more for rhyolite, and basaltic flows can advance as sheet flows, channels, or insulated lava tubes that carry lava tens of kilometers with little cooling.
Example
In Hawaii, lava from Kilauea has flowed across roads and into the sea, hissing as it hardens into new land, and pillow lava forms when basalt erupts underwater, as it does along the Mid-Atlantic Ridge; ancient lava flows and ash from an island-arc volcano over $500$ million years ago are preserved in the volcanic rocks around Charlotte and the Uwharrie Mountains. The Columbia River Basalt Group in the Pacific Northwest erupted about $175{,}000\ \text{km}^3$ of lava between $17$ and $6$ million years ago, with single flows traveling over $500\ \text{km}$, while the 2018 Kilauea eruption released roughly $1\ \text{km}^3$ of lava and destroyed more than $700$ homes.
Key Insight
Lava can be runny like syrup or thick like peanut butter, and it usually moves slowly enough, a few kilometers per hour for basalt, that a person can walk away from it, though it has been clocked at highway speeds down steep slopes. Flood basalts are the dangerous exception: the Siberian Traps and Deccan Traps coincide with the end-Permian and end-Cretaceous mass extinctions, and the $CO_2$ and $SO_2$ they released are leading suspects in the climate disruptions that followed.