Ever since 1960 when the rooftop garden on the Kaiser Center’s garage, largest in the world at the time, was built, it’s been heralded by American landscaping pros, Kaiser Center employees and appreciative residents. After the garden’s recent rehabbing, now (specifically, any weekday) is a good time to visit.
Putting a garden on a roof, even one made of the best Kaiser reinforced concrete, takes careful design and execution. The heaviest plantings and features were situated directly over the strongest supporting columns.
Even so, the designers had to minimize the weight of everything. The soil, a blend of lava rock, expanded shale and peat moss, is practically potting mix. The concrete is a lightweight formula. And then there are these impressive boulders.
Each one is a generously sized hunk of pumice, the nearest thing to balsa wood in the mineral kingdom. Well, diatomite is lighter, but so friable you can crumble it in your hands. Pumice, basically a foam of expanded lava, stands up to weather beautifully with its relatively inert glassy composition. And close up, it’s handsome.
If you see this stuff in a landscaping-supply yard, kick it around or pick it up. It’s so light, you’ll feel like Superman — but pumice will cut your bare skin, so be careful.
Oakland Geology 
4 February 2020 at 4:14 pm
The pumice boulders are in their natural shapes. I managed that garden for almost 15 yrs so they hold a special place in my rocky heart.
26 September 2016 at 2:52 pm
I’m sure they’re natural. I once cut specimens from a boulder of this stuff (bought at a yard supply place) and tools can’t handle it. Only centuries of weathering can sculpt this.
26 September 2016 at 12:33 pm
Nice piece of information. Is the pimice in it’s natural shap or is it carved or shaped somehow.