Re: What type of rock is slate classified as?

Dear Sue,

Slate is a metamorphic rock.

Metamorphic rocks are rocks that started out as either sedimentary or 
igneous rocks but have been altered by heat and/or pressure at depth in 
the Earth’s crust.  The alteration takes the form of a reorganization of 
the chemical constituents of the original rock to form new minerals.  No 
new material is added to the rock, and it does not heat up so far as to 
melt – all the changes take place in a solid state!  Also, especially 
where significant pressure is involved in metamorphism, the new minerals 
will usually have a preferred alignment at right angles to the direction 
of maximum pressure.

Slate usually starts as shale, which is a fine-grained sedimentary rock 
typically deposited as mud in a quiet marine environment, often in deep 
water.  The fact that slate starts out as shale may be the source of the 
confusion you mention.  The original shale will have been deposited in 
layers, so that shale can be split along the bedding planes.  When 
metamorphosed to slate, however, the growth of new minerals under pressure 
will cause a new cleavage to develop, not usually in the same direction as 
the original bedding planes.  So slate can also be split (which is why 
some slates make good roofing material) but in a different direction to 
the original bedding planes of the parent shale.

Metamorphic rocks can be grouped according to the degree to which they 
have been altered by heat and/or pressure.  Slate is the lowest grade 
(that is, least changed) of all the metamorphic rocks.  The range of 
metamorphic rocks in order from least to greatest alteration is as follows:

Slate – a fine-grained rock with almost perfectly flat foliation surfaces
Phyllite - a fine-grained rock  with a sheen of minute mica flakes and 
wrinkled foliation surfaces
Schist – medium to coarse grained rock with abundant crystals of mica and 
moderately spaced, undulating foliation surfaces
Gneiss – the highest grade metamorphic rock, showing bands of its 
constituent minerals including feldspar, quartz and mica.  Gneiss has 
essentially the same mineral composition as granite and resembles granite 
except for the banding.  Unlike granite, gneiss is formed not by cooling 
from a liquid magma, but by the application of extreme heat and pressure 
to another rock just short of melting.

Geologists predict that when a shale undergoes metamorphism, provided the 
application of heat and pressure are great enough and last long enough, 
all of the metamorphic rocks from slate to gneiss will be produced in 
sequence.  Which one we find at the surface later tells us at 
approximately what combination of heat and pressure the metamorphic 
episode ceased. 

I hope this answers your question.

Best wishes,

David Scarboro