Story behind the song
Subduction Zone
Subduction Zone is a piece for Alto Flute, Bass Clarinet, Guitar, Marimba, Finger Piano, and percussion. It is based on four chords in the Partch Tonality Diamond: F 4/3 minor; A 8/5 major; C 1/1 major; and A 5/3 minor. There are many notes in common between these scales, and many more that are only slightly different. The subduction takes place where the differences lie.
This piece exploits the challenging ratios between many of the notes in the four scales. Here is a chart that summarizes these differences:
Movement of notes
F minor bend A major
1 F 4 /3 9 :10 5 E 6 /5
2 G 16/11 77:80 6 G 7 /5
3 A 8 /5 1 :1 1 A 8 /5
4 B 16/9 81:80 2 B 9 /5
5 C 1 /1 1 :1 3 C 1 /1
6 D 8 /7 77:80 4 D 11/10
In this chart, the second degree of the F 4/3 minor scale, G 16/11, has to drop down by a 77:80 to reach the 6th degree of the A 8/5 major scale, which is G 7/5. That's about 1/2 of the normal 12 tone semi-tone. The movement from the 4th degree of the F 4/3 minor scale, B 16/9 to the 2nd degree of the A 8/5 major scale, B 9/5 is much smaller. It's a movement of an 81:80, nearly imperceptible. The thematic material is about how those changes come about, at the margins of the chord changes, the tectonic plates in the analogy to geological processes. There are similar charts moving from A 8/5 major to C 1/1 major, and then to A 5/3 minor.
I had a lot of fun with the glides within the scales as well. There are movements from one scale degree to another, up and down. The guitar, flutes, clarinets, and finger pianos move often from the 1,3,5 degrees to the 2,4,6 degrees. Triads made of the 1,3,5 sound like typical minor or major chords. The 2,4,6, sound pretty far out. The tension is in the movement from one to the other.
I also exploit 4 note chords based on scale degrees 1,4,6,3 or 2,5,1,4 or others. These sound more like typical fourth based harmonies rather than the triadic 1,3,5 and 2,4,6.
I also played around with trills, from one scale degree to the other.
The finger piano is made from samples taken from an instrument I built many years ago, consisting of spring steel tongues tuned with little bits of solder to ensure the overtones are in tune with the fundamental. The sounds are picked up with hand wound magnetic transducers.
The marimba plays either single notes or rolls on chords, but very fast rolls that sound more like bamboo gamelan instruments than a traditional marimba.
Plate tectonics - the basics
Lithospheric plates. The uppermost part of the Earth is subdivided into a small number of rigid plates which comprise about 85% of the surface. In places these are separated by non-rigid (deforming) zones. Elsewhere plate boundaries of three types exist: divergent or spreading (e.g., mid-oceanic ridges), convergent (e.g., subduction zones), and strike-slip (e.g., the San Andreas fault zone in California or oceanic transform faults).
Volcanism commonly is associated with the first two types of margins. An example of the first type is the currently active Axial Seamount which lies on the Juan de Fuca Ridge off the coast of Washington. Convergent margin volcanism produces the 'ring of fire' around the Pacific ocean, and is typified by the Cascade volcanic arc in the Pacific Northwest US.
Convergent margins are among the world's most seismogenic zones, and are characterized by progressively deeper earthquakes as one proceeds from trench to back-arc region - at most convergent margins, these earthquake foci define a dipping plane (the Wadati-Benioff zone, or WBZ) which corresponds to a fault zone between subducting oceanic lithosphere and the overriding plate. Such zones are characterized by chains of trench-parallel volcanoes (volcanic arcs) fed by magmas rising from depths of up to ~100 km.
