By Kevin Connolly BBC Dublin correspondent

As many as 500 rioters on the streets of north Belfast, 136 petrol bombs thrown, 48 police officers injured - as always the bald figures do something to convey the scale of the trouble on the streets of Northern Ireland.

What they can not explain though is why the news from that part of the province obstinately refuses to change even as the political institutions created by the peace process begin to bed down.

North Belfast is one of the crucibles of the Troubles and more than a fifth of all those murdered in more than 30 years of political violence died within one or two square miles of the Holy Cross school on Ardoyne Road.

The two sides never agree on exactly how or why trouble has broken out

It is a patchwork of small Protestant and Catholic enclaves where many people who are close neighbours are also bitter enemies.

In these circumstances minor disputes sometimes involving just a handful of people can rapidly escalate, sucking in larger and larger numbers.

Often the results are unpleasant bouts of stone-throwing or fighting which die away relatively quickly.

Just occasionally though, when the political circumstances, or the mood on the ground are right, it can escalate into full scale rioting.

Wednesday was a case in point.

Permanent tension

Loyalists say the trigger for trouble was an attack by nationalists in which a floral tribute to a Protestant taxi driver who was murdered just over a year ago was torn off a lamp-post and destroyed.

Nationalists say the fighting escalated because Protestants had again begun to abuse Catholic families on their way to and from the Holy Cross school.

The truth is that the proximity of the two communities in which many people hold uncompromising views generates a permanent atmosphere of tension... it is in the nature of this deeply divided society that the two sides never agree on exactly how or why trouble has broken out.

Republicans and loyalists blame each other for the trouble

It does seem that the Holy Cross school was not this time the original focus of the dispute but inevitably it quickly became involved, possibly simply because of its position on the Ardoyne Road which is divided into Catholic and Protestant sections.

Politicians have inevitably called for calm and attempts to establish some sort of dialogue between the two communities are never wholly abandoned.

But the deal which ended the loyalist protest at Holy Cross late last year dealt only with that specific issue and could hardly even have hoped to address the broader and deeper problems of the area which are so deeply rooted in Northern Ireland's history.

Often when trouble suddenly flares and becomes big news, it is tempting to believe there must be a major reason behind it... but the truth is in North Belfast atmosphere of hatred and mistrust provides the catalyst to turn small events into headline news.