Why does volcanic ash cause aircraft flights to be cancelled?

The news is full of stories about the significant disruption to air traffic in Northern Europe due to the volcanic eruption in the south west part of Iceland's Eyjafjallajoekull glacier.

Aircraft are not permitted to fly into volcanic ash clouds because there is a risk of damage to aircraft components and systems by:

• Abrading forward facing surfaces including cockpit windows, control surfaces, wing leading edges. Cockpit windows can be pitted enough to cause significant hazards during landing.
• Causing engine failure due to accumulations forming on the turbine blades caused by fusing of the silica particles at around 1100 OC.
• Coating the plane enough to cause it to become tail heavy.
• Penetrating the aircraft, contaminating the passenger compartment and affecting the aircraft's internal systems.