# Problems about combining melt and viscosity by Compositing model **URL:** \ **Category:** ASPECT\ **Created:** [May 30, 2025, 4:13pm UTC](https://community.geodynamics.org/t/problems-about-combining-melt-and-viscosity-by-compositing-model/4158 "2025-05-30T16:13:18Z")\ **Posts on this page:** 1\ **Showing post:** 2
**Author:** ![jbnaliboff](https://avatars.discourse-cdn.com/v4/letter/j/58956e/32.png) [@jbnaliboff](https://community.geodynamics.org/u/jbnaliboff)\ **Post date:** [June 3, 2025, 9:08pm UTC](https://community.geodynamics.org/t/problems-about-combining-melt-and-viscosity-by-compositing-model/4158/2 "2025-06-03T21:08:35Z")
Hi Junru, Thank you for posting your questions to the forum, which I’ve posted some initial responses to below. > I’d like to use the compositing model to combine the two models (viscoplastic model and melt model) to change the density. While you can use a compositing approach, I suggest using the reactive fluid transport material model ([see material model documentation](https://aspect-documentation.readthedocs.io/en/latest/parameters/Material_20model.html)). This material model directly composite a base model (such as visco plastic), with the functionality for handling fluid transport and reactions. This material model also uses a new [reaction module](https://github.com/geodynamics/aspect/tree/main/source/material_model/reaction_model) design, which will allow users to more easily develop and implement new reaction modules in a modular fashion. > But when I have more than one composition (like crust, mantle, etc.), how should I set the density for each composition? Which model should I choose for the Density option in the Compositing model (viscoplastic or melt model) to make the depletion-related density change considered? I think the best approach would be to do this in a new reaction model, where you use the name of the field and field type to determine how the densities (and other properties change). > I’ve noticed [previous discussions](https://community.geodynamics.org/t/confusions-about-the-melt-migration-and-the-porosity-compositional-field/2944/5) that suggest modifying the code to achieve this. I would like to seek some advice on how to implement this functionality. Can you specify what additional information is needed beyond the advice provided in the linked discussions? I think the first step would be to write out the equations and workflows for a simple analytical case, such as the reactions and change in material properties as P-T conditions change at a single point. This benchmark then provides a reference case that you can try to reproduce in ASPECT with the new functionality. Cheers, John --- _[View the full topic](https://community.geodynamics.org/t/problems-about-combining-melt-and-viscosity-by-compositing-model/4158)._