  We used PdBI observations of SiO (2-1) to investigate the morphology and
profile of the SiO emission within several massive dense clumps (MDCs) in
Cygnus-X. We find that most molecular outflows are detected in both SiO and CO,
although there are some cases of CO outflows with no SiO counterpart. We find a
significant amount of narrow line SiO emission that appears to be unrelated to
outflows. The fraction of the total SiO luminosity that is not associated with
outflows is highly variable in the different MDCs (from 10% to 90%); this might
be a problem when extrapolating outflow properties from SiO luminosities
without resolving individual outflows. The extent of the narrow SiO emission
varies from rather compact (~ 0.03 pc) to widespread (~0.2 pc), and its
kinematics often differs from those found by other high-density tracers such as
H13CO+. We find that the least centrally concentrated clumps with the least
massive protostellar cores have the most widespread narrow SiO emission. In
line with previous evidence of SiO emission associated with low-velocity
shocks, we propose an evolutionary picture to explain the existence and
distribution of narrow SiO line profiles. In this scenario, the least centrally
condensed MDCs are at an early stage where the SiO emission traces shocks from
the large-scale collapse of material onto the MDC (e.g. CygX-N40). As the MDC
collapses, the SiO emission becomes more confined to the close surroundings of
cores, tracing the post-shock material from the infalling MDC against the dense
cores (e.g. CygX-N3, N12, and N48). At later stages, when single massive
protostars are formed, the SiO luminosity is largely dominated by powerful
outflows, and the weaker narrow component shows perhaps the last remnants of
the initial collapse (e.g. CygX-N53 and N63).
