The earth’s surface is a very active environment, so after a lava flow cools and becomes a hard rock we need to look at a smaller scale. When we examine a thin section of a typical volcanic rock, such as a from Hawaii, we see that the rock is made up of different parts. These include: several different minerals, void space (pores), and glass.

Now the typical minerals in a basalt lava would be tiny crystals of feldspar, pyroxene, olivine and magnetite. The glass present is basically a frozen liquid. Now as soon as the lava solidifies and cools to room temperature chemical reactions begin to break down the rock. For example, the oxygen in the air begins to react with the magnetite crystals to make a mineral called hematite: You would call hematite rust! If it rains, then the rainwater, which is slightly acidic due to dissolved carbon dioxide, begins to chemically weather the rock. That is the glass begins to dissolve and the minerals react with the rainwater to produce new minerals such as clay minerals that are softer. Meanwhile winds blow dust and pollen and small seeds unto the lava flow that is beginning to break down into soil. Perhaps insects burrow through the rock since it is porous and rainwater will fill the pores… basically, the biosphere will invade the rock as it breaks down by chemical and mechanical weathering (freeze thaw cycles adds to mechanical weathering).

There have been islands in the oceans that form from below sea level and within a few years such islands become habitats for life. Once microbes start to grow, their waste products aid in further chemical degradation of the rock….. Eventually enough soil forms so seeds can germinate and begin to grow. Many plants produce humid acid which when combined with rain water enhances the dissolution rate of minerals and glass.

If you want to learn more find a geology book and look up. Weathering and erosion: chemical weathering, mechanical weathering, soil formation as KEY WORD on internet search.
Good luck!

Some scientists tested which nutrients are important for plants to grow on old volcanoes. So they took a lot of different fertilizers with different nutrients. Then they traveled to old lava flows on Hawaii and fertilized the plants to see which nutrient helped plants the most. One of the main ingredients in fertilizers, nitrogen, actually ended up helping plants grow the fastest. Plants grow differently on old volcano flows based on weather, height above sea level, and lava type as well. That means that the environment already at a volcano helps decide how plants will grow back after an eruption.

When lava freezes and becomes rock, it is actually very similar to other common rocks like granite and sandstone. The minerals and nutrients in lava rocks and most other common rocks are pretty similar. But most plants grow in soil, not rock. So the main change that has to happen to the surface of a volcano before most plants can grow is that the lava rocks have to break down in the wind and the rain to make soil. Soil forms from rocks when they are slowly broken down by wind and rain. When rocks break down to make soft soil that plants can grow in, we call it “weathering" of the rocks.

Weathering happens to all rocks, but it is very slow. After a volcano erupts and makes new lava rocks, it can take a long time for them to weather enough for plants to start growing; in some places it takes a few years, in other places it takes hundreds of years! Weathering happens faster in places that are hotter and have more rain (which is also where plants like to grow! This is not a coincidence!). So a volcano in the tropics will grow plants faster than a volcano in the desert.

All that being said, some plants don't need as much soil as others. There are some plants that grow directly on rocks and actually help break the rocks apart with their roots, which makes more soil for other plants to grow in!

So the main change that has to occur to the surface of a volcano in order for most plants to grow is that the lava rocks have to weather: the lava rocks have to break down in the wind and rain to make soft soil that the plants can grow in.
I hope this was helpful! Great question!

Volcanoes covered with plants aren't all that different from any other pile of rock covered by soil, except that the magma beneath will sometimes come out. This means that the changes to the surface of a volcano that allow plants to grow on it are basically the same as those anywhere else. You might be thinking of a volcano soon after a lava flow, in which case the surface is all hard rock. Wind, rain, and other weathering processes erode that rock into soil. Over time, nutrients that plants use are built up in the soil by bacteria, other plants, and animals that travel through the area. The ash and magma from the eruption can also be very rich in nutrients essential for plant growth, so the soil around volcanoes is often some of the most fertile on the planet. Seeds are carried to the soil by wind and animals, and eventually the volcano may be covered by dense vegetation.

The first plants to grow on rocks are flat lichens . Lichens are plants with both algae and fungus growing together. I read that it takes 100 years for a flat circle of lichen the size of a quarter to grow on a bare rock. As the lichens grow up above the rock a bit, they will collect some dust and water from wind and rain. They also break down a bit of the rock under them. Water sits there and breaks down the rock some more, creating bits of soil. Cracks in rocks also hold some water and are a good place for other plants to start to grow. Moss grows on cracks in rocks, and seeds can fall into the cracks and sprout. And as the primitive plants grow and die, they make soil for seed plants to grow in. You can look at what grows on rocks that are mostly bare.

Ash from volcanoes can also fall on plants, in thick layers. That happened with Mt. St. Helen's in the US, and I've read that trees grew great there.

Volcanic soils are generally very mineral rich but volcanic ash drains very well, so plants growing on fresh volcanic ash are likely to be desert-type plants.

Thanks for the great question!

The soil on and around volcanoes can be some of the best in the world for growing plants. When volcanoes erupt, ash and lava cover the surrounding land. While devastating in the short term, in the long term an eruption creates new soil. The new soil that forms after a volcanic eruption is often very rich in all sorts of minerals and nutrients that plants need to take root. These minerals include sulfur, potassium, and phosphates, which plants often have a hard time getting.

When combined with heavy rainfall, volcanic soil becomes very fertile, or good for growing plants. For instance, think about Hawaii and how green its rainforests are. That is in part due to the rich volcanic soil of the islands and the great deal of rainfall there.