Question

One of the main attractions at this park is a forest of large petrified trees. These trees were preserved by being replaced with silica from ash layers within the Triassic (Mesozoic) sandstones. Based on the geologic history and features of the area, which of the following statements about these sedimentary rocks is TRUE? a. The clastic sand that forms these rocks was weathered and eroded from the Appalachian Mountains. b. The ash was produced from volcanoes associated with the Yellowstone Hotspot. c. The clastic sand that forms these rocks was weathered and eroded from the Rocky Mountains. d. The clastic sand that forms these rocks was weathered and eroded from the Sierra Nevada Mountains.

Short answer

The correct statement is: a. The clastic sand that forms these rocks was weathered and eroded from the Appalachian Mountains.

Step-by-step solution

Understand the Geological Setting

The petrified trees are found within Triassic sandstones, which means they are part of sedimentary rock formations from the Mesozoic era. This helps us focus on the sources of the clastic material that formed these sedimentary rocks.

Evaluate the Triassic Time Period

During the Triassic period, the geography of North America was different. The Appalachian Mountains had already formed and were eroding, while the modern Rocky Mountains had not yet uplifted. This historical context is essential for determining the source of the sediment.

Eliminate Implausible Options

Option b involves the Yellowstone Hotspot, which did not exist during the Triassic period. Option c refers to sediment from the Rocky Mountains, which were not yet formed in the Triassic. Likewise, option d posits sediment from the Sierra Nevada, which also had not yet formed.

Identify the Likely Source of Sediment

The Appalachian Mountains were a significant source of sediment during the Triassic period. These older mountains were eroding, providing clastic material that could be transported by rivers to form sedimentary rocks in other regions.

Confirm the Accurate Statement

Given the geological timeline and sediment sources, it is logical that the clastic sand forming these rocks was weathered and eroded from the Appalachian Mountains. Thus, option a is the correct statement.

Key concepts

Triassic Period

The Triassic period marks a pivotal era in Earth's history. It began around 252 million years ago and lasted until about 201 million years ago. During this time, the Earth was undergoing significant changes, both in terms of climate and continental shifts. This period is the first era in the Mesozoic Era, which also includes the Jurassic and Cretaceous periods.
The Earth's landmasses were part of the supercontinent Pangaea. This enormous landmass influenced global climates, making them warmer and more arid. It was a time when life was recovering from the Permian-Triassic extinction event, the most severe extinction event in Earth's history.

• Marine life began to flourish again, with the introduction of new species like ammonites and corals.
• On land, the first dinosaurs and mammals appeared, gradually diversifying.
• The Triassic period set the stage for the Earth's future biodiversity.

It played a crucial role in the geologic and biological narrative of our planet, influencing sedimentation processes that eventually led to the petrification of trees in specific regions.

Sedimentary Rocks

Sedimentary rocks are fascinating structures formed through the compaction and cementation of sediments. These sediments might include fragments of other rocks, minerals, and organic material that have accumulated over time.
To understand sedimentary rocks, it's essential to know that they form through processes that happen over millions of years, often at the bottom of bodies of water. The process begins with the weathering and erosion of existing rocks, which are then transported by wind, water, or ice.

• Once deposited, these materials gradually build up in layers, known as strata.
• Over time, the pressure from the weight of the layers above compacts these sediments, turning them into rock.
• Cementation binds the sediments together with minerals like silica, calcite, or iron oxides serving as natural glues.

In the case of the petrified trees mentioned in the exercise, silica preserved the trees by replacing their organic material, thus encasing them in stone. This fascinating transformation is a testament to the power of geological processes.

Appalachian Mountains

The Appalachian Mountains are a significant range located in Eastern North America. They are among the oldest mountain ranges on the planet, forming over 480 million years ago during the Ordovician period. These mountains have a rich history that deeply influenced the geology of North America.
By the time of the Triassic period, the Appalachians were already ancient, having undergone extensive erosion. This erosion process played a crucial role in shaping the geographical and geological landscape of the period.

• As they eroded, they provided a massive source of sediment, which rivers carried across the continent.
• These sediments contributed significantly to the formation of sedimentary rock formations, such as the Triassic sandstones where petrified trees are found.
• Thus, understanding the history of the Appalachian Mountains helps explain the origins of sedimentary materials in specific regions.

The sedimentary rocks formed from these materials serve as natural records, helping us decipher Earth's geologic past.

Mesozoic Era

The Mesozoic Era is a significant phase in Earth's history. Spanning from about 252 million to 66 million years ago, it is often called the "Age of Reptiles." This era is divided into three periods: Triassic, Jurassic, and Cretaceous, each with its own unique characteristics.
During the Mesozoic Era, the Earth experienced dramatic shifts in its geological and biological landscapes. The continents gradually drifted apart from the supercontinent, Pangaea, leading to continental fragmentation and the establishment of modern global geography.

• The climate during this era was generally warm, with no polar ice caps, allowing for lush vegetation and diverse ecosystems.
• It was a time of evolutionary innovation, with the rise of dinosaurs, the first birds, and flowering plants.
• The Mesozoic also saw significant volcanic activity contributing to the composition of the Earth's crust and the formation of new landforms.

Understanding this era provides insights into the processes that shaped much of the Earth's present configuration, including the conditions that led to the preservation of petrified trees.