What is the cleavage of basalt?

Basalt: Cracking the Code of This Volcanic Rock

Basalt. You’ve probably seen it – that dark, unassuming rock making up huge swathes of our planet, from the ocean floor to places like Hawaii. It’s volcanic rock, plain and simple, and there’s a lot of it. In fact, it makes up over 90% of all volcanic rock on Earth! But don’t let its commonness fool you. Basalt has secrets to tell, especially when you start looking at its cracks.

Now, before we dive in, let’s clear up some jargon. Geologists like to distinguish between “cleavage” and “fracture.” Think of cleavage as a mineral’s preferred breaking point – like mica, which peels off in perfect sheets. Fracture? That’s just any old break that doesn’t follow those neat, predictable lines.

So, where does basalt fit in? Well, basalt itself doesn’t really do cleavage. Instead, it fractures. And the type of fracture you usually see is called “conchoidal.” Imagine breaking a piece of glass – those curved, shell-like surfaces? That’s conchoidal fracture, and that’s how basalt tends to break too.

But here’s the thing: these fractures aren’t just random cracks. They’re a big deal! They tell us a lot about how the basalt formed and how it interacts with the world around it.

Think about it: when molten lava cools down, it shrinks. And when it shrinks, it cracks. These cracks, or joints, are what give us those amazing columnar basalt formations you see in places like the Giant’s Causeway in Ireland or Devil’s Postpile in California. The faster the cooling, the smaller the columns – pretty neat, huh?

But cooling isn’t the only culprit. Earthquakes and other tectonic shenanigans can also create fractures. And even just the daily grind of temperature changes can cause stress and cracking over time.

Why does all this fracturing matter? Let me tell you:

• Erosion: Fractures are like erosion’s little helpers. The more cracks, the easier it is for water to pluck away chunks of rock. Fewer cracks? The rock might be tougher to pluck, but it’ll slowly wear down from abrasion and weathering.
• Lava Flows: Fractures even influence how lava flows move! They can dictate how the flow advances and ultimately limit its reach.
• Volcanic Hazards: Believe it or not, understanding fracture patterns can even help predict where new volcanic vents might pop up on the sides of volcanoes. That’s pretty important for hazard planning!
• Water Flow: And let’s not forget groundwater! Fractures act like tiny highways for water to move through the rock, affecting everything from our water resources to how the rock weathers.

So, what kinds of fractures are we talking about?

• Columnar Joints: These are the rock stars of the fracture world – the ones that create those stunning columns.
• Sheet Joints: Imagine giant, flat cracks running parallel to the ground. That’s sheet jointing, caused by the rock releasing stress.
• Tectonic Fractures: These are the fractures with a story to tell about the Earth’s movements.
• Microfractures: Don’t underestimate the little guys! These tiny cracks can have a huge impact on the overall strength and how easily water can seep through the rock.

In conclusion, while basalt doesn’t have cleavage like some fancy minerals, its fractures are where the real action is. They’re a window into the rock’s past, its present, and its future. So next time you see a chunk of basalt, take a closer look at those cracks. They’re telling you a story!