# Using a free surface in a model with a crust

*This section was contributed by William Durkin*.

This cookbook is a modification of the previous example that explores changes
in the way topography develops when a highly viscous crust is added. In this
cookbook, we use a material model in which the material changes from low
viscosity mantle to high viscosity crust at $z = z_j = \texttt{jump height}$,
i.e., the piecewise viscosity function is defined as
```{math}
\begin{aligned}
  \eta(z) = \left\{
    \begin{matrix}
      \eta_U & \text{for}\ z > z_j, \\
      \eta_L & \text{for}\ z  \le z_j.
    \end{matrix}
  \right.
\end{aligned}
```
where $\eta_U$ and $\eta_L$ are the viscosities of
the upper and lower layers, respectively. This viscosity model can be
implemented by creating a plugin that is a small modification of the `simpler`
material model (from which it is otherwise simply copied). We call this
material model "SimplerWithCrust." In particular, what is
necessary is an evaluation function that looks like this:

``` c++
template <int dim>
    void
    SimplerWithCrust<dim>::
    evaluate(const typename Interface<dim>::MaterialModelInputs &in,
             typename Interface<dim>::MaterialModelOutputs &out ) const
    {
      for (unsigned int i=0; i<in.n_evaluation_points(); ++i)
        {
          const double z = in.position[i][1];

          if (z>jump_height)
            out.viscosities[i] = eta_U;
          else
            out.viscosities[i] = eta_L;

          out.densities[i] = reference_density * (1.0 - thermal_expansion_coefficient * (in.temperature[i] - reference_temperature));
          out.thermal_expansion_coefficients[i] = thermal_expansion_coefficient;
          out.specific_heat[i] = reference_specific_heat;
          out.thermal_conductivities[i] = thermal_conductivity;
          out.compressibilities[i] = 0.0;
          out.entropy_derivative_pressure[i] = 0.0;
          out.entropy_derivative_temperature[i] = 0.0;
          for (unsigned int c=0; c<in.composition[i].size(); ++c)
            out.reaction_terms[i][c] = 0.0;
        }
    }
```

Additional changes make the new parameters `Jump height`, `Lower viscosity`,
and `Upper viscosity` available to the input parameter file, and corresponding
variables available in the class and used in the code snippet above. The
entire code can be found in
[cookbooks/free_surface_with_crust/plugin/simpler_with_crust.cc](https://github.com/geodynamics/aspect/blob/main/cookbooks/free_surface_with_crust/plugin/simpler_with_crust.cc).
Refer to {ref}`sec:extending:idea-of-plugins` for more information about writing and running plugins.

The following changes are necessary compared to the input file from the
cookbook shown in {ref}`sec:cookbooks:free-surface` to include a
crust:

-   Load the plugin implementing the new material model:

    ```{literalinclude} free_surface_with_crust.part1.prm
    ```

-   Declare values for the new parameters:

    ```{literalinclude} free_surface_with_crust.part2.prm
    ```

    Note that the height of the interface at 170km is interpreted in the
    coordinate system in which the box geometry of this cookbook lives. The
    box has dimensions $500\text{ km}\times 200\text{ km}$, so an interface height
    of 170km implies a depth of 30km.

The entire parameter file is located in
[cookbooks/free_surface_with_crust/free_surface_with_crust.prm](https://github.com/geodynamics/aspect/blob/main/cookbooks/free_surface_with_crust/free_surface_with_crust.prm).

Running this input file generates a crust that is 30 km thick and 1000 times
as viscous as the lower layer. {numref}`fig:freesurfaceWC` shows that adding a crust to
the model causes the maximum topography to both decrease and occur at a later
time. Heat flows through the system primarily by advection until the
temperature anomaly reaches the base of the crustal layer (approximately at
the time for which {numref}`fig:freesurfaceWC` shows the temperature profile). The
crust's high viscosity reduces the temperature anomaly's velocity
substantially, causing it to affect the surface topography at a later time.
Just as the cookbook shown in {ref}`sec:cookbooks:free-surface`,
the topography returns to zero after some time.

```{figure-md} fig:freesurfaceWC
<img src="freesurface_with_crust.*" alt="Screenshot"  width="100%"/>

Adding a viscous crust to a model with surface topography. The thermal anomaly spreads horizontally as it collides with the highly viscous crust (left, white solid line). The addition of a crustal layer both dampens and delays the appearance of the topographic maximum and minimum (right).
```