What rocks are extrusive?

Extrusive Rocks: Straight from the Volcano’s Mouth!

Ever wondered how some rocks are born in fire? That’s the story of extrusive rocks, also known as volcanic rocks. These guys are formed when molten rock – magma from deep inside the Earth – bursts onto the surface as lava and cools down super fast. Think of it like nature’s own rapid prototyping! This quick cooling is what sets them apart from their “slow-burn” cousins, the intrusive rocks that take their sweet time solidifying underground.

So, how does this fiery birth happen? Deep down, near those restless plate boundaries or over volcanic hotspots, magma is cooked up. Being lighter than the solid rock around it, this molten stuff starts to rise. When it finally punches through to the surface – boom! – it’s lava. Now exposed to the air or ocean, the lava faces a sudden temperature change. It’s like jumping from a hot tub into an ice bath, geologically speaking.

This rapid chill-out has a huge effect on what the rock looks like. Unlike intrusive rocks, which boast big, showy crystals thanks to their leisurely cooling period, extrusive rocks are usually fine-grained, or even glassy. There’s just no time for those big crystals to form. Sometimes, the cooling is so lightning-fast that crystals don’t even get a chance to appear, resulting in a sleek volcanic glass like obsidian. Pretty cool, huh?

And that’s not all. You’ll often find little holes, called vesicles, in extrusive rocks. These are basically the ghosts of gas bubbles that got trapped as the lava solidified. Pumice, for example, is so riddled with these bubbles that it can actually float! I remember once finding a piece of pumice on a beach in Iceland and being amazed that something so rocky could be so light.

Now, let’s talk about some of the rock stars in the extrusive world:

• Basalt: This is the heavyweight champion, the most common extrusive rock out there. It’s dark, fine-grained, and packed with iron and magnesium. Basalt makes up most of the ocean floor and pops up in volcanic areas on land.
• Rhyolite: Think of this as granite’s fiery twin. It’s light-colored, fine-grained, and has a similar chemical makeup. But watch out – rhyolitic lavas are thick and sticky, often leading to explosive eruptions!
• Andesite: This one’s often found hanging out at continental margins near subduction zones. It’s a fine-grained, usually gray rock.
• Dacite: Sitting between rhyolite and andesite in terms of composition, dacite is another light-colored extrusive rock.
• Obsidian: This volcanic glass is formed by super-fast cooling. It’s got a smooth, glassy look and is usually black, although you can find some cool green or purple ones too.
• Pumice: We already mentioned this one! Light, full of holes, and floats on water. What’s not to love?
• Scoria: Similar to pumice, but darker and denser. Still pretty bubbly, though!
• Tuff: This is like the concrete of the volcanic world, made from volcanic ash and other bits and pieces.

Why should you care about extrusive rocks? Well, they’re a window into the Earth’s volcanic activity and its long, fascinating history. They build those dramatic volcanic landscapes we all know and love – lava flows, plateaus, and towering mountains. By studying these rocks, geologists can piece together what’s happening deep inside our planet and understand how volcanoes work. Plus, some extrusive rocks, like obsidian, have been used by humans for tools and weapons for thousands of years. So, next time you see a dark, glassy rock, remember its fiery origins and the story it has to tell!