Question

This scene in Montana's Glacier National Park shows layers of Precambrian sedimentary rocks. The darker layer contained within the sedimentary layers is igneous. The narrow, light-colored areas adjacent to the igneous rock were created when molten material that formed the igneous rock baked the adjacent rock. a. Is the igneous layer more likely a lava flow that was laid down at the surface prior to the deposition of the layers above it or a sill that was intruded after all the sedimentary layers were deposited? Explain. b. Is it likely that the igneous layer will exhibit a vesicular texture? Explain. c. To which group (igneous, sedimentary, or metamorphic) does the light- colored rock belong? Relate your explanation to the rock cycle.

Short answer

a. The igneous layer is a sill. b. Unlikely to exhibit vesicular texture. c. The light-colored rock is metamorphic.

Step-by-step solution

Identify the Igneous Layer Type

The igneous layer is more likely a sill. A sill is an igneous intrusion that is parallel to existing rock layers. Since the molten material baked the adjacent sedimentary rocks, it suggests that the igneous rock was intruded after the sedimentary layers had formed.

Determine Texture of Igneous Layer

It is unlikely that the igneous layer will exhibit a vesicular texture. Vesicular textures form when lava cools rapidly on the surface, trapping gas bubbles inside. Since a sill forms beneath the surface, it cools more slowly, resulting in a non-vesicular texture.

Classify the Light-Colored Rock

The light-colored rock adjacent to the igneous intrusion is metamorphic. The baking process from the igneous intrusion indicates the sedimentary rock underwent contact metamorphism, altering its structure and mineralogy.

Key concepts

Igneous Rocks

Igneous rocks form when molten material cools and solidifies. There are two main types of igneous rocks: intrusive and extrusive. Intrusive igneous rocks, like sills and dikes, form below the Earth's surface. They cool slowly, allowing larger crystals to form. This slow cooling often gives them a more even texture, and they rarely have vesicular textures (full of holes or cavities). In the case of the igneous layer in Montana's Glacier National Park, it's likely a sill. A sill forms parallel to existing rock layers, which matches this description. Given that the molten material baked the nearby layers, it intruded after the surrounding sedimentary rocks had settled. On the other hand, extrusive igneous rocks form on the surface when lava cools quickly, often resulting in smaller crystals and sometimes vesicular textures. Examples include basalt and pumice. Understanding the cooling history is key to identifying the formation and texture of igneous rocks.

Sedimentary Rocks

Sedimentary rocks are formed from particles or sediments, when they accumulate in layers. These layers often come from weathered pieces of other rocks, mineral fragments, or organic materials. Over time, pressure compacts these particles, forming solid rock. Common types of sedimentary rocks include sandstone, shale, and limestone. In Montana's Glacier National Park, the layers of Precambrian sedimentary rocks illustrate how they are deposited in distinct strata. They hold clues about past environments, like ancient oceans or rivers, as they often show sedimentary structures like ripples, mud cracks, or fossils. When igneous activities occur, as seen with the intrusion of a sill, these sedimentary rocks may undergo metamorphism, altering their mineral composition and texture.

Metamorphic Rocks

Metamorphic rocks originate from pre-existing rocks—either igneous, sedimentary, or other metamorphic types—that have been altered by heat, pressure, or chemically active fluids. This alteration happens without the rock melting, which differentiates it from the process that creates igneous rocks. In the scenario at Glacier National Park, the light-colored rocks next to the igneous intrusion went through a transformation known as contact metamorphism. This occurs when heat from the nearby molten rock alters the minerals and structure of the adjacent sedimentary rock. This contact metamorphism results in changes such as recrystallization, foliation, or the development of new minerals. The new rock is thus substantially different from the original sedimentary layer, showcasing the dynamic nature of the rock cycle. Understanding these metamorphic processes helps geologists trace the history and changes that Earth's crust undergoes over geological time scales.