This is a great question – it is surprising how fast vegetation can grow on lava flows after they have cooled and hardened. We know that grass and trees need soil to grow, but fresh hardened lava flows do not contain any soil. So where does the soil come from?

Soil is a mixture of minerals and organic matter derived from the decayed remains of plants and other organisms. The minerals that make up soil come from rocks. So to make soil, we have to start with rock. As you know, the rock on Hawaii is hardened lava which we call basalt. But in rainy climates, the basalt that is in contact with the atmosphere does not stay fresh and hard for long. When basalt is exposed to a lot of rain, it will start to physically erode and break down into small mineral particles. Some of these minerals will chemically react to form new minerals which we more commonly find in soil. For example, the mineral feldspar which is abundant in basalt will react with rain water and air to form clay minerals, commonly found in soil. The amount of time it takes to break down basalt into clay-rich soil depends on the amount of rain fall in a particular area. Lava flows which form on the rainy parts of Hawaii will break down to produce soil within a couple of years, whereas lava flows which form on dry parts of Hawaii can take hundreds of years to break down and produce soil.

Soil is not just made from rock though, it also contains organic matter from decayed organisms. The first organisms to grow on a basalt rock are mosses and lichens, because they can live without soil. Moss and lichen will start to grow on freshly cooled lava flows before soil has started to form. When these plants die, their decayed remains become part of the soil, along with the broken down minerals from the basalt rock. Once there is a small amount of soil on the lava flow, more plants can grow, contributing more material to the soil. Eventually enough soil will build up to support forests and grasslands.