  The Earth's surface is composed of a staggering diversity of
particulate-fluid mixtures: dry to wet, dilute to dense, colloidal to granular,
and attractive to repulsive particles. This material variety is matched by the
range of relevant stresses and strain rates, from laminar to turbulent flows,
and steady to intermittent forcing, leading to anything from rapid and
catastrophic landslides to the slow relaxation of soil and rocks over geologic
timescales. Geophysical flows sculpt landscapes, but also threaten human lives
and infrastructure. From a physics point of view, virtually all Earth and
planetary landscapes are composed of soft matter, in the sense they are both
deformable and sensitive to collective effects. Geophysical materials, however,
often involve compositions and flow geometries that have not yet been examined
in physics. In this review we explore how a soft-matter perspective has helped
to illuminate, and even predict, the rich dynamics of Earth materials and their
associated landscapes. We also highlight some novel phenomena of geophysical
flows that challenge, and will hopefully inspire, more fundamental work in soft
matter.
