Petrified forest

A petrified forest is the fossil remains of a forest where the organic material of the wood is replaced by minerals, usually silicates such as quartz. They form when sediment or volcanic ash buries wood underground and dissolved minerals eventually replace the organic materials in the trees. Perhaps the most famous petrified forest is Petrified Forest National Park in Arizona, which contains 221,390 acres of a fossilized Triassic-era forest. As petrified forests provide convincing evidence that the world is more than 6,000 years old, creationists have to devote a large part of flood geology to trying to come up with an explanation.

Formation
Petrified wood occurs when a tree is buried rapidly, usually by volcanic flows but sometimes by other sediment as well. It is important that it is buried in an area without oxygen, as that prevents aerobic decay that will result in the tree rotting before it is able to crystalize. Some creationists will make a big deal about how there are no animal fossils found alongside petrified wood and therefore the wood must have been buried during the global flood, when in reality the tree wouldn't have petrified in the first place if there were animals around when the tree was being petrified. As a result of the displacement that is necessary for petrified wood to form, damage to the trees is common; in particular, it is extremely rare for a tree's root system to be preserved.

The next step in the process is for water rich in minerals to fill in parts of the tree as its lignin and cellulose slowly decay. Quartz and similar silicates are by far the most common mineral for petrified wood to be made of, but calcite and pyrite are also sometimes present, and mineraloid silicates like opal are also sometimes found. While quartz is typically colorless, impurities of elements like iron and manganese give petrified wood its variety of colors. Over millions of years, eventually all of the organic matter is replaced by inorganic matter. Petrified forests are subject to the same geological forces as anything else, which can also leave an impact on them — notably, the trees in Petrified Forest National Park were fractured during the uplifting of the Colorado Plateau 60 million years ago.

Notable discoveries
A petrified forest more than 300 million years old has been found in the remains of mine workings. The forest is so large that it underlies parts of Illinois, Indiana, and Kentucky and extends over thousands of hectares. The aforementioned Petrified Forest National Park contains fossilized trees that lived in the Late Triassic period about 225 million years ago. While the Lesbos petrified forest in Greece is just 17-20 million years old, it has a tree with intact branches, leaves, and roots, and petrified trees that are that well-preserved are incredibly rare. The age of these forests alone refutes claims that it could have been buried in the global flood or that the Earth is a few thousand years old.

B-b-but flooddidit!
Yeah, no. If you believe that all petrified forests were deposited in the same global flood, numerous inconsistencies arise. First of all, petrified forests reflect how forests were like during the time they were deposited. For example, Petrified Forest National Park has the remains of coniferous trees and ginkgoes, but no deciduous trees, as they hadn't evolved yet whereas today, Arizona has common deciduous species like aspens that you would expect to show up if the petrified forest was only 6,000 years old, and hell, the fact that there was a large forest in a place that is today as arid as northern Arizona should say something. The much more recent Lesbos forest, on the other hand, has plenty of angiosperms and the petrified trees there are similar to the species that grow there today. Similarly, petrified forests can be very different from each other — some like the one in Arizona were heavily shaped by later geological events, some like the one in Yellowstone contain many forests layered on top of each other, and there's of course the differences in trees that were buried by volcanic ash and those that were buried by other sediments. In short, petrified forests show nowhere near the level of consistency that you would expect if they were all formed by the same event.