What is a petrified forest?

How a forest of green trees turns to stone and rubble

Baking in the Arizona sunshine is a forest frozen in stone. Spanning 896 square kilometres, the Petrified Forest National Park is a vast, arid site filled with undulating hills and rocky outcrops all staying dry in the hot weather. But it wasn’t always this way.

Travelling back more than 225 million years, the almost-barren land today once flourished with towering conifers and trickling rivers. However, during Earth’s evolutionary journey to the modern day, the land was stripped of its once-luscious green foliage. Instead it has been left with the petrified shadow of a forest that once was. As if having stared into the eyes of Medusa, solid stone tree trunks litter the ground. They are the same trees that once stood tall in a living forest, but over time have undergone a type of preservation known as petrification.

Unlike the creation of tree fossils, where the carbon-rich wood is compacted by mounting mud and rocks, petrification is a process whereby minerals are displaced from one source and incorporated into the remains of a onceliving organism. This form of natural preservation occurs not only in ancient trees, but across the spectrum of life on Earth. For example, the bodies of migrating birds that have fallen into Lake Natron in northern Tanzania have undergone a calcification whereby their remains are chemically preserved with calcium from the alkaline lake water – albeit in a much shorter time frame than the stone wood in the Petrified Forest. It’s taken millions of years for the types of preserved wood found in Arizona to form, and it’s not the only place in the world to showcase forests of the past. Throughout America and as far as Greece, these stone forests offer a glimpse into a world long forgotten.

Prehistoric forests Conifer trees that once stood as lush, green forests in prehistoric Arizona reached heights of around 60 metres, concentrated near rivers and streams. The Petrified Forest was designated a national park in 1962 Volcanic eruptions “Timber!” When these trees died or were damaged they would naturally fall into the surrounding water systems. Over time volcanic explosions caused tonnes of ash to enter into the atmosphere, settling into water that trees had fallen into. Absorption The minerals that make up volcanic ash, such as silica, are then soaked up by the submerged wood.

Transformation Over millennia the silica and other trace minerals, such as iron, crystallise into an array of coloured hard quartz. The National Park is a beautifully haunting sight Global shift During this petrification process, the world’s once-joined continents separated, changing the landscape and ultimately draining this area of water to reveal the now-stony remains of forests long forgotten.