I see that there’s an in-depth answer from a geologist, but I’ll add this for some “like I’m five” flavor:
The reason they haven’t disappeared is simply because the part of the world they’re in has a climate that doesn’t disturb it. It’s a very dry dessert, it gets very little rain. Without the rain it doesn’t wash away, plants don’t grow on top of them, rivers and creeks don’t wash through them, there aren’t a significant amount of animals running around on them, and there aren’t as many humans there either. Whatever wind or other weather they get is evidently not enough to destroy them either.
They’re made from brushing the soil aside to reveal a different color of earth below. In every way they are extremely specific to the landscape they are on.
I know it’s weird to think something so delicate could last so long but sometimes, with the right conditions, they do. Another example is the cave art in Lascaux France. It’s some of the older known art period. What it is made of is essentially charcoal and various mud and minerals smeared on a rock wall. But it happens that the cave was sealed and not entered for thousands of years. The climate inside barely changed, and the most delicate form of art possible just happened to last longer than anything else. They are so sensitive to climatic changes that when they were discovered, just having people in the caves breathing made them start to disintegrate. Now you can’t visit without special clearance and special suites to keep your body from destroying them.
Geologist here.
In long-term arid environments, alluvial surfaces develop what is called ‘desert pavement’ over time. It is where eons of wind and occasional rain blow and wash all the light-colored silt and fine sand from the desert surface and leave behind only the dark-colored rocky cover (gravel and pebbles). When people disrupt that dark-colored rocky cover it exposes light-colored soil underneath. The Nazca lines are places where prehistoric peoples intentionally made lines and graphic features like the hummingbird, by moving the dark-colored surface rocks to expose the light-colored soil beneath. It took eons for that desert pavement to develop, so it will take eons for the disturbed pavement to re-establish that uniform dark color.
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