To distinguish between intrusive and extrusive rocks, one of the most prominent factors to consider is the crystal size. Crystal size is primarily influenced by the cooling rate of the molten rock, also known as magma.
Extrusive rocks, also referred to as volcanic rocks, are formed from magma that reaches the Earth’s surface and cools rapidly. Due to the quick cooling process, extrusive rocks typically have very small or even no visible crystals. This occurs because the magma does not have enough time to grow large crystals before solidifying. An example of an extrusive rock with a fine-grained texture is basalt. Basalt is commonly found in volcanic regions and exhibits a dark color due to its high iron and magnesium content.
On the other hand, intrusive rocks, also called plutonic rocks, form from magma that cools slowly beneath the Earth’s surface. The slower cooling process allows sufficient time for the molten rock to crystallize and form larger crystals. As a result, intrusive rocks generally have a coarse-grained texture. Granite is a well-known example of an intrusive rock. It often contains visible crystals of minerals like quartz, feldspar, and mica. The variety of minerals in granite contributes to its diverse colors and patterns.
Apart from crystal size, there are other characteristics that can help differentiate between intrusive and extrusive rocks. These include:
1. Color: Extrusive rocks often have darker colors due to the presence of minerals like olivine and pyroxene, which are rich in iron and magnesium. In contrast, intrusive rocks can exhibit a wider range of colors depending on the minerals present.
2. Density: Intrusive rocks tend to be denser than extrusive rocks. This is because the slower cooling process in the Earth’s interior allows for the formation of denser minerals.
3. Texture: In addition to crystal size, the overall texture of the rock can provide clues about its origin. Extrusive rocks commonly have a glassy or vesicular texture, indicating rapid cooling and the presence of air bubbles. Intrusive rocks, on the other hand, often have a granular or phaneritic texture due to the visible crystals.
4. Geological Setting: The location where the rock is found can also provide valuable information. Extrusive rocks are commonly associated with volcanic areas or areas of recent volcanic activity. Intrusive rocks, on the other hand, are more likely to be found in regions of past or present mountain building, where magma has intruded into the crust.
5. Rock Associations: Observing the rocks surrounding the specimen can also be helpful. Extrusive rocks are often found as part of volcanic formations, such as lava flows or volcanic ash deposits. Intrusive rocks are commonly associated with larger masses of rock, like batholiths or dikes, which have intruded into existing rock formations.
In my personal experience as a geologist, I have encountered various examples of both intrusive and extrusive rocks. I vividly remember a field trip to a volcanic region where we examined lava flows composed of basalt. The rock had a very fine-grained texture, and its dark color indicated its extrusive nature. On another occasion, during an exploration of a mountainous region, we came across large granite outcrops. The coarse-grained texture and diverse colors of the granite clearly indicated its intrusive origin.
The key differences between intrusive and extrusive rocks lie in crystal size, cooling rate, color, density, texture, geological setting, and rock associations. By considering these characteristics, geologists can accurately determine the origin and type of a rock specimen.