...All very close in the circle-of-fifths...completely unrelated chord in the circle of fifths...
The circle of fifths isn't the "be all end all" of harmony. You should also learn about counterpoint and voice leading. But, let's stick with the circle of fifths for now, but with one change: rather than a literal circle of fifths consider root progression by descending perfect fifth.
C-Minor -> F-Minor -> Bb-Major -> Eb-Major -> C-Minor ... All very close in the circle-of-fifths
The root progressions are
↓P5 ↓P5 ↓P5 ↓m3, and the descending third motion connections relative major/minor chords.
C-Minor -> F-Minor -> Bb-Major -> G-Major -> C-Minor ... seems a completely unrelated chord in the circle of fifths
The root progressions are
↓P5 ↓P5 ↓m3 ↓P5, again the descending third motion connections relative major/minor chords.
Both progressions give the tones for a key signature of three flats with
C minor being the sensible tonic. The progression
G-Major -> C-Minor nominally gives us a cadence. Strictly speaking a cadence would depend on voice leading and rhythm, but the progression is tonally very important. Such cadential harmony would be one of the very first topics covered in a harmony textbook.
Identifying the cadential harmony is the basic answer to your question, but I think you should also consider your basic premise about the harmony that one of the progressions is more related to the circle of fifths. Both have the same number and type of root progressions. Both use chords from
C minor. The only difference relative to the circle of fifths and roots by descending fifths is the placement of the descending third progression. The two progressions aren't very different on that basis.
I'm guessing you regard the
G major chord as unrelated, because you are expecting it to be a
G minor chord. That would be the case if you used only the purely diatonic chords from a key signature of three flats. But that is not how minor key harmony works. In the key of
C minor the basic circle of fifths would be:
Cm Fm Bb Eb Ab Ddim G Cm. The
G chord is chromatic in terms of the key signature, but it does that by convention in
C minor. The seventh degree in minor keys is always raised to one half step below the tonic for dominant chord. Some say this minor key harmony uses chord derived from the harmonic minor scale, but that just makes a different muddle of minor key harmony. Suffice to say minor key harmony is considered a more advanced harmony topic usually covered in the later sections of a textbook or course.
Your first progression is a plain vanilla diatonic progression that returns to the tonic with a relative major/minor progression.
Your second progression "works well" because you inadvertently put it into conventional minor key harmony with the standard dominant chord and a descending fifth, cadential progression back to the tonic.
Also, I was wondering whether I can use this phenomenon to modulate to another progression close to G-Major (E-Minor?)? And if so, how? (and why?)
Yes. Basically harmony is about relative relationships between chords. They typical thing is label chords with Roman numerals to analyze and list progressions. Your two would be:
Cm: i iv VII III i
Cm: i iv VII V i
Just transpose them to another minor key:
Em: i iv VII V i
Em Am D B Em
Nothing about the progression changes in the transposition except the tonic.
I'm not sure if you really meant to take your minor progressions and put them into a major key (
G major.) That won't always be a mechanical process of transposition. Sometimes you can just "flip" the chord qualities of major, minor, diminished from those of minor keys to those of major keys:
Major: i iv VII III VI iio V i
Minor: I IV viio iii vi ii V I
o means "diminished." So, in major:
Em: i iv VII V i
G: I IV viio V I
G C F#o D G
...which should work just fine. Other progressions might need tweaking to flip from minor to major and vice versa, but that would involve a deeper study of harmony.
Some harmony resources:
The music of Handel would also be a great resource. Historically Baroque composers will provide for minor key examples than classical composers like Mozart. This style may not be your style, but it would be the stuff to study to learn common practice for minor key harmony.
- first read up on the rule of the octave, it will provide a concise rule of thumb for practical harmony and was part of how harmony was taught historically
- Kostka, Tonal Harmony is a well known textbook, you would need to purchase it
- Some older textbooks can be found in Google Books (use the preview and free ebook filters) or the Internet Archive. They aren't always great - teaching and writing approaches change over time - but they are free. This one - Hamilton, A Manual of Harmony - seem pretty good, it gives a lot of major/minor scale harmonization examples side by side and has a section on cadences.
I think it's best to compare several textbooks and then reconcile what they say in regard to real historic scores. Ultimately what great composers did is what matters most. Textbooks attempt to describe that with different wordings, some better than others, sometimes contradictory. But you can always rely on real composer's scores.