Hi, what a great question! Plants have an amazing repertoire of adaptations which allow them to grow in harsh conditions all over the world-- deserts, icy mountains, and even on rocks and trees.
Many parts of Hawaii are lush and tropical, but the lava fields are actually quite arid, just like a desert! Over time, plants have evolved special traits to allow them to survive on these lava rocks. For example, many of the plants found on lava rocks are very hairy. These little hairs help plants retain their moisture and protects them from the sun (just like how your hair protects the top of your head from getting sunburned). Often, these plants also have long roots which allow them to suck up water from deep underground. These adaptations and others allow plants to make the lava rocks their home.
But why would plants "decide" to grow in such a harsh place when there are wet forests with soft soil nearby? One explanation is that millions of years ago, a random seed landed on the lava rocks and, even though it may have been hard, it was able to survive and grow on the lava rocks. When that plant put out its seeds, some of those seeds would randomly have slightly different adaptations (like being a little bit hairier) and be better suited to grow on the lava rocks. These plants would grow up to make seeds of their own, and the process continues. After many generations, the plants that are able to grow look very different from the original seed that landed on the lava rocks. Eventually, they might become so specialized that they can only live on the lava rock and become a new species of plant.
Another explanation might be that because the wet forest was such a nice place to live, it became very crowded! The plants living in the forest have to compete for water, sunlight, and nutrients. On the edges of the forest, where it meets the lava rocks, plants that were able to tolerate drier conditions might take advantage of the lava rocks to climb into the sun.
Like the previous explanation, over time, the plants that were able to take advantage of the empty lava rocks survived, reproduced, and became more specialized until they became new species.
During a volcanic eruption, lava spews out and gradually cools down. This process forms the lava rocks that you mentioned in your question. The mineral composition of these rocks can vary depending on the volcano.
After being exposed to the elements (sun, rain, and wind), the lava rocks will begin to naturally break down, or erode, into smaller pieces. The time it takes for lava rocks to erode depends on whether these elements are present. For example, rocks in a windy and rainy environment will erode faster than ones that are in a sunny and less windy environment.
Once these rocks erode, it creates a good environment for some plants, such as lichens, to grow. The wind or animals, such as birds, can carry seeds from different places and drop them on the eroded rock, which gradually becomes soil as plant matter mixes with the minerals found in the lava rock. With the help of the sun and rain, these plants will grow, break down, and contribute to the soil for more plants to grow in.
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