A fissure is a long, narrow crack or opening that splits through something solid. When an earthquake shakes the ground, it might create fissures in the earth's surface. Ancient volcanic fissures in Iceland can stretch for miles, releasing steam and lava from deep underground. Even glaciers develop fissures as they slowly move and shift.

You'll also find fissures in much smaller things. A rock face might have fissures running through it where water has frozen and expanded over many winters. Old paintings sometimes develop tiny fissures in their surface as the paint ages and dries. Dentists look for fissures in teeth, which are the natural grooves where cavities sometimes start.

The word suggests something more serious than a simple crack. A fissure goes deep and often signals something significant happening beneath the surface. Scientists study fissures in the ocean floor to understand how continents drift apart over millions of years.

As a verb, to fissure means to split or form deep cracks. People also use fissure to describe serious divisions between groups. When a political party splits over an important disagreement, newspapers might describe a fissure developing in the organization. This metaphorical use captures how a fissure separates things that were once connected, creating an opening that's difficult to close back up.