A slab avalanche of hard, dense snow.
Hard Slab Avalanche:

Hard slabs are stiff, cohesive slabs, usually deposited by strong wind drifting or the slabs may be old, hardened layers of snow. Think of them like a pane of glass on top of potato chips. The good news is that hard slabs are more difficult to trigger than soft slabs, but the bad news is that they tend to propagate farther and make a much larger and more deadly avalanche. Also, the stiffer the slab, the farther above you the fracture line will usually form, and the harder it will be to escape. The reason for this is that stiff slabs tend to spread a person's weight over a larger area. Even with very poor weak-layer bonding directly beneath the person, a stiff slab can bridge outward to places where the slab might be better bonded to the bed surface, like a drunkard stumbling along supported on the shoulders of two sober friends.

Hard slabs are especially tricky because the stiffness and/or thickness of slabs can vary a lot from place to place, so just because you may not be able to trigger a slab in a thick spot, as soon as you get to the edge of the slab--for instance where it may thin near a ridgeline--you may be able to trigger the whole slope.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab