When molten rock (magma) reaches Earth's surface, it solidifies or hardens. Scientists call the resulting solid rocks "extrusive" igneous rocks. Extrusion is the process of pushing material out to the surface of the Earth's crust. At some volcanoes, the extrusive rock flows as lava across the ground before it hardens; the ripples in the lava may freeze in place. Hot, rapidly expanding gases within other volcanoes' vents can force the magma out explosively, forming pumice: low-density rock full of vesicles, or frozen bubbles. Extrusive igneous rocks are easy to find near many volcanoes, such as Mount St. Helens in Washington state. Hawai`i Volcanoes National Park, home of two active volcanoes, contains lava flows that cooled only a few decades, or minutes, ago.

You can tell a lot about the history of igneous rocks by looking at the size of crystals within them. Rocks that cool quickly contain small crystals, while slow-cooled rocks are filled with large crystals. When magma erupts at the Earth's surface, heat radiates out from the lava allowing it to cool rapidly and the atoms and molecules do not have time to grow into large crystals before the lava solidifies. The resulting rock has such small crystal grains that humans have a difficult time distinguishing them, even with a handheld lens. Geologists describe the texture of these fine-grained igneous rocks as "aphanitic," from the Greek word meaning "unseen." Deep inside the Earth's crust, the magma cools much more slowly because the surrounding rocks insulate the magma from rapid heat loss. This allows the crystals to grow into mineral grains that are easier for humans to see. Geologists describe the resulting coarse-grained rock texture as phaneritic, from the Greek word meaning "visible." Some igneous rocks contain crystals that are much larger than the crystals in the matrix surrounding them. Scientists call these specimens, which resemble a chocolate-chip cookie, porphyritic rocks, and the larger crystals are called phenocrysts. The phenocrysts had started to form within the magma before it later cooled rapidly, probably due to that magma erupting at a volcano.

More than 2 million years ago, early humans began to strike stones against each other to shape them into the first tools. Early toolmakers used chert, quartzite, basalt, obsidian, and similar rock types because their crystalline structure gives the stone tools their sharp edges. About 7,500 years ago, humans in southeastern Europe learned how to melt native copper and cast it in molds to make tools and ornaments. About 6,000 years ago in North America, Native Americans were also making tools and ornaments from copper that came from rich deposits in the Lake Superior area. Next, people discovered how to extract copper and other metals from rocks by crushing the rocks and sorting out the bits of metal. The bits were then smelted to create a pure metal. People also noticed that by combining different metals, they could make a new one that might be stronger or a different color. When copper is mixed with tin the result is bronze, which is much harder than pure copper or tin and has a more golden color. More than 5,000 years ago, humans in Asia made a few ornamental items from iron-nickel meteorites - the only kind of native iron available. Only about 3,300 years ago did humans figure out how to smelt iron from ore. The first iron that was produced was soft, but when carbon was added to the iron it produced steel, which is much harder. The downside was that there was little control over how to get the right balance of carbon and iron, and the steel could be brittle. The Romans solved this problem by "piling" or layering steel and iron and then forging and quenching it to make the metal durable and less brittle. In 200 CE (Common Era), people improved the process by pattern welding, taking rods of steel and twisting them together and then hammering them into a flat sheet.