Question

What rock property would allow you to quickly distinguish an intrusive igneous rock from an extrusive one?

Short answer

Crystal size is the key property to distinguish intrusive from extrusive igneous rocks.

Step-by-step solution

Understanding Igneous Rocks

Igneous rocks are formed from the solidification of molten rock material. They are divided into two main types: intrusive and extrusive. Intrusive igneous rocks form below the Earth's surface, while extrusive rocks form at or above the surface.

Identify Key Differences

The primary difference between intrusive and extrusive igneous rocks is their texture. Intrusive rocks, such as granite, have a coarse-grained texture because they cool slowly beneath the Earth's surface, allowing large crystals to form. Extrusive rocks, like basalt, cool quickly on or above the surface, leading to a fine-grained texture with small crystals.

Determine Distinguishing Property

The distinguishing rock property between intrusive and extrusive igneous rocks is crystal size. Intrusive rocks have larger crystals due to slower cooling, while extrusive rocks have smaller crystals due to more rapid cooling.

Apply the Rock Property

To quickly distinguish an intrusive igneous rock from an extrusive one, you should look at the size of the crystals in the rock. Larger crystals indicate an intrusive origin, while smaller crystals indicate an extrusive origin.

Key concepts

Intrusive Rocks

Intrusive rocks, also known as plutonic rocks, form when magma cools and solidifies beneath the Earth's surface. This deep underground environment allows the magma to cool slowly, promoting the growth of larger crystals.

• Examples of intrusive rocks include granite and diorite.
• The coarse-grained texture of these rocks is due to the ample time crystals have to develop.

These larger crystals are easily observed with the naked eye, making it possible to distinguish intrusive rocks from their extrusive counterparts. Intrusive rocks often form significant geological features such as large batholiths and mountain ranges.

Extrusive Rocks

Extrusive rocks are formed when magma erupts onto the Earth's surface, typically through volcanic activity. Unlike intrusive rocks, extrusive rocks cool quickly because they are exposed to the cooler temperatures of the atmosphere.

• Common extrusive rocks include basalt and pumice.
• Their rapid cooling process results in a fine-grained or even glassy texture.

These rocks often have small crystals that are difficult to see without magnification, distinguishing them from the larger crystals in intrusive rocks. Extrusive rocks can create dramatic landscapes such as volcanoes and lava flows.

Crystal Size

The size of the crystals in igneous rocks is directly related to the rate at which the magma cools.

• Slow cooling, as seen with intrusive rocks, allows time for crystals to grow larger, resulting in coarse-grained textures.
• In contrast, extrusive rocks cool quickly, leading to small crystals or even a glassy texture without discernible crystals.

Crystal size is an essential property because it helps geologists identify the formation environment of the rock. Larger crystals typically suggest a deeper formation environment, while smaller crystals indicate a rapid cooling process near or at the surface.

Rock Texture

Rock texture refers to the size, shape, and arrangement of minerals within an igneous rock. The texture can reveal significant information about the rock's history and formation process.

• Coarse-grained textures with large, interlocking crystals indicate slow cooling within the Earth's crust, as seen in intrusive rocks.
• Fine-grained or glassy textures suggest rapid cooling, associated with extrusive rocks.

In some cases, the texture may be porphyritic, where large crystals are embedded within a finer matrix, indicating a complex cooling history. Thus, texture is a critical clue that helps geologists decipher the geological processes that formed a particular rock.