Symmetries have played an important role in a variety of problems in geology and geophysics. A large fraction of studies in mineralogy are devoted to the symmetry properties of crystals. In this paper, however, the emphasis will be on scale-invariant (fractal) symmetries. The earth's topography is an example of both statistically self-similar and self-affine fractals. Landforms are also associated with drainage networks, which are statistical fractal trees. A universal feature of drainage networks and other growth networks is side branching. Deterministic space-filling networks with side-branching symmetries are illustrated. It is shown that naturally occurring drainage networks have symmetries similar to diffusion-limited aggregation clusters.Symmetries appear in a wide variety of contexts in geology and geophysics. Rocks are made up of minerals and the symmetry aspects of minerals constitute a major part of any course in mineralogy. Many advances in x-ray diffraction were motivated by studies of minerals. The symmetries of crystalline structures are generally associated with translations, rotations, and reflections. In this paper, however, we will focus our attention on a fourth symmetry: scale invariance. Scale invariance is generally associated with fractals and self-similarity. The classic example is the Cantor set, illustrated in Fig. 1. At each order, the remaining line segments are divided into three parts and two are retained. The fractal dimension is D ϭ log(N 2 ͞N 1 )͞ log(r 1 ͞r 2 ), where N is number and r is length for the Cantor set D ϭ log2͞log3 ϭ 0.6309, intermediate between the Euclidean dimension of a line (D ϭ 1) and a point (D ϭ 0).It should be remembered that Mandelbrot (1) introduced the concept of fractals in terms of the length of a rocky coastline. The length of a rocky coastline scales with the length of the measuring rod used as a fractional inverse power (2). This is a statistical symmetry rather than a deterministic symmetry. An illustration of this is given in Fig. 2. On the left is a third-order Koch triadic island. This is a deterministic fractal with D ϭ log4͞log3 ϭ 1.262, intermediate between the Euclidean dimension of an area (D ϭ 2) and a line (D ϭ 1). On the right is the map of an actual island, Dear Island, Maine. Using either the measuring-rod method or the box-counting method (2), this island satisfies fractal statistics to a good approximation, with D Ϸ 1.4. Rocky coastlines generally exhibit statistical scale-invariant symmetries. Other examples of scale-invariant symmetries in geology and geophysics include fragments, faults, earthquakes, ore deposits, oil fields, and volcanic eruptions. The fractal dimension associated with a scale-invariant symmetry is a quantitative measure of texture; increased fractal dimensions correspond to increased roughness.Self-similar symmetries can also be associated with selfaffine fractals; some statistically self-similar time series are examples of self-affine fractals. A deterministic, self-affine fractal is illustrated in Fig....