What is alteration and is the same as metamorphism?

Question

this is a geology question. Alteration of sedimentary rock wall by intrusions?

Answer 1

Alteration is typically thought about in terms of chemical change of the bulk rock and the resultant change in mineralogy.  It is typically a fluid-mediated change: fluids (water, water-co2 or other various mixes of volatiles and dissolved salts) migrate through rocks and change the chemistry by adding and removing constituents.  I have trouble thinking of an alteration event that is not fluid-related.

I would consider alteration a subset of metamorphism.  Although alteration may be associated with a regional process, such as when carbonate minerals degas and the CO2-water fluids migrate away, taking some dissolved salts with them, it is normally a localized process.

For me, alteration differs from general temperature-pressure change metamorphism by the role of fluids in the process.  A dry contact metamorphism is not normally called alteration, but changes to the rocks in the immediate vicinity of an intrusion due to magmatic fluid influx, or even the circulation of heated groundwaters caused by the intrusion of the hot magma, are a type of metamorphism that is normally called alteration.

Skarns are a type of altered rock formed in the immediate vicinity of an intrusive magma that result from the migration of fuids through the rock under very high temperatures.

Answer 2

Alteration is not the same thing as metamorphism, although on the surface they sound the same.

Alteration is the changing of one mineral to another by the means of chemical or physical weathering (for example, water, acid, or wind).  It does not change the original rock.  IE, granite will still be granite (and sandstone will stay sandstone), even if the biotite in them has altered to chlorite.  You cannot alter anything by intrusion, unless you're talking about water 'intruding' into the sedimentary rock - however, in geology that's not what we use the term 'intrusion' for.  We use it to stand for some igneous body (such as magma) intruding into another rock - which leads to the next paragraph:

Metamorphism is the changing of one mineral to another by the means of temperature and/or pressure.  It DOES change the original rock, as it changes the entire structure of its composition.  IE, granite will be transformed to gneiss and sandstone will be transformed to quartzite.  It's also WAY more widescale, and can cover entire regions.

Answer 3

sorry, dunno the meaning of those words.