  Magnetars, isolated neutron stars with magnetic field strengths typically
$\gtrsim10^{14}$~G, exhibit distinctive months-long outburst epochs during
which strong evolution of soft X-ray pulse profiles, along with nonthermal
magnetospheric emission components, is often observed. Using near-daily NICER
observations of the magnetar SGR 1830-0645 during the first 37 days of a recent
outburst decay, a pulse peak migration in phase is clearly observed,
transforming the pulse shape from an initially triple-peaked to a single-peaked
profile. Such peak merging has not been seen before for a magnetar. Our
high-resolution phase-resolved spectroscopic analysis reveals no significant
evolution of temperature despite the complex initial pulse shape. Yet the
inferred surface hot spots shrink during the peak migration and outburst decay.
We suggest two possible origins for this evolution. For internal heating of the
surface, tectonic motion of the crust may be its underlying cause. The inferred
speed of this crustal motion is $\lesssim100$~m~day$^{-1}$, constraining the
density of the driving region to $\rho\sim10^{10}$~g~cm$^{-3}$, at a depth of
$\sim200$~m. Alternatively, the hot spots could be heated by particle
bombardment from a twisted magnetosphere possessing flux tubes or ropes,
somewhat resembling solar coronal loops, that untwist and dissipate on the
30-40~day timescale. The peak migration may then be due to a combination of
field-line footpoint motion (necessarily driven by crustal motion) and evolving
surface radiation beaming. These novel dataset paints a vivid picture of the
dynamics associated with magnetar outbursts, yet it also highlights the need
for a more generic theoretical picture where magnetosphere and crust are
considered in tandem.
