Question

This rock sample consists of intergrown crystals. How would you determine whether the rock is sedimentary or igneous? If it is sedimentary, what term describes its texture?

Short answer

The rock is igneous due to intergrown crystals, not sedimentary.

Step-by-step solution

Identifying Rock Type

First, examine the physical characteristics of the rock sample. Sedimentary rocks typically have layered structures, while igneous rocks often consist of intergrown crystals or a glassy texture due to the cooling of molten rock. Since the rock in question consists of intergrown crystals, it is more likely to be igneous.

Assessing Crystal Formation

Note the size and arrangement of the crystals. Igneous rocks have crystals that interlock in a tightly packed manner due to crystallization from magma or lava. This texture is not common in sedimentary rocks, which often have grains that are cemented together without an interlocking pattern.

Determining Sedimentary Texture

In case the rock was determined to be sedimentary with a similar characteristic, it would have a texture termed as crystalline. However, the presence of intergrown crystals, as described, strongly suggests an igneous origin.

Key concepts

Sedimentary Rocks

Sedimentary rocks are formed by the accumulation and compaction of mineral and organic particles, typically in a water environment. Over time, these particles or sediments, often transported by water, wind, or ice, settle and layer up under pressure to form rocks.

Here are some distinctive features of sedimentary rocks:

• **Layers**: One of the most noticeable characteristics is their layered structure. These layers, or strata, can vary greatly in thickness and color, corresponding to different periods of sediment deposition.
• **Fossils**: They often contain fossils, offering clues about the Earth's past environments and life forms.
• **Texture**: Although sedimentary rocks are not characterized by intergrown crystals, they can have variable textures such as clastic texture, referring to broken fragments of minerals or rocks cemented together.

By identifying these features, geologists can easily distinguish sedimentary rocks from others, including igneous rocks, which are typically formed from cooled magma or lava.

Rock Texture

Rock texture refers to the appearance and feel of a rock's surface, which provides critical clues about its history and formation process. For instance, the texture of a rock can reveal its cooling history or the environment in which it formed. Here are some common types of rock textures:

• **Grain Size**: The size of the individual grains or crystals in the rock can range from very fine to coarse. For example, rocks with large, easily visible grains often cooled slowly beneath the Earth's surface, while those with finer grains may have cooled more quickly.
• **Grain Shape and Arrangement**: The shape can be rounded, angular, and the arrangement can be random or aligned. In igneous rocks, interlocking crystals often resemble a tightly packed puzzle, indicating a crystalline texture.
• **Surface Feel**: Descriptions such as smooth, gritty, or glassy describe a rock's texture. A glassy texture, for example, suggests rapid cooling of the rock.

Understanding rock texture is key to identifying the type of rock and its formation process.

Crystal Formation

Crystal formation in rocks is a fascinating process that plays an essential role in determining the texture and type of the rock. This process primarily occurs when minerals crystallize from a solution or when molten rock cools and solidifies.

In igneous rocks, crystals form as magma or lava cools, either underground or on the surface of the Earth. The rate of cooling directly influences the size of the crystals:

• **Slow Cooling**: When magma cools slowly deep within the Earth, it allows crystals to grow large. This results in coarse-grained rocks with visible crystals, like granite.
• **Rapid Cooling**: When lava cools quickly on the surface, there's less time for crystal growth, leading to rocks with small, fine crystals such as basalt.

Crystals can interlock tightly, forming a solid and durable rock structure. Understanding crystal formation helps geologists pinpoint the origin and history of rocks, distinguishing, for instance, between igneous and sedimentary origins.