Question

A granitic rock contains inclusions (xenoliths) of basalt. What can you say about the relative ages of the granite and the basalt?

Short answer

Based on the principle of inclusions, the basalt rock (xenoliths) is older than the granitic rock that contains the inclusions. The relative ages of the granite and the basalt would then be: the basalt is older, and the granite is younger.

Step-by-step solution

Understand the principle of inclusion

In geology, the principle of inclusion states that any rock (formation or unit) that contains an inclusion (foreign body) must be younger than the rock from which the inclusion was derived. This is because in order for the inclusion to be present in the granite, the basalt had to exist first for the granite to form around it.

Apply the principle of inclusion to given exercise

We are given that a granitic rock contains inclusions (xenoliths) of basalt. Based on the principle of inclusions, the basalt rock must be older than the granitic rock. This is because the basalt already existed when the granitic rock formed around and incorporated the basalt as inclusions.

State the conclusion

Using the principle of inclusions, we can conclude that the basalt rock (xenoliths) is older than the granitic rock that contains the inclusions. The relative ages of the granite and the basalt would then be: the basalt is older, and the granite is younger.

Key concepts

Principle of Inclusion

The principle of inclusion is a fundamental concept in geology. It helps scientists determine the relative ages of rocks and formations. This principle states that any rock containing pieces of another rock (known as inclusions or xenoliths) must be younger than the inclusions themselves. This is because, logically, the rock that forms the inclusion must predate the rock that surrounds it. Geologists use this principle to unravel the chronological sequence in which rocks form.

For example, if you find a granite rock that contains xenoliths of basalt, it indicates that the basalt rock pre-existed the granite. The granite must have formed around the basalt, thereby including fragments of it. This makes determining the relative age straightforward: the basalt must be older than the granite.

Relative Dating

Relative dating is a method used by geologists to determine the sequential order of geological events. Unlike absolute dating, which provides a specific numerical age, relative dating places rocks in a sequence of events.

It utilizes several principles, including the principle of inclusion. By studying rock formations and the composition of materials within them, scientists can infer whether one stratum is older or younger than another. This method is essential for understanding Earth's history and when major geological events occurred.

Factors like the presence of inclusions help geologists establish these timelines effectively. So, when a granitic rock has basalt inclusions, they conclude that the basalt is older without knowing the precise age in number of years.

Xenoliths

Xenoliths, also known as inclusions, are fragments of rock that have been enclosed within another rock. They are like small foreign bodies in a larger stone matrix and are very common in igneous rocks.

Xenoliths provide valuable clues about the geological environment of the past. When a xenolith is found, its presence tells geologists that there was already an existing older rock formation before the newer rock encased it. This is why understanding xenoliths is crucial when studying geological layers. They serve as natural records, allowing us to piece together past geological events and understand the history of an area.

For example, a basalt xenolith in granite highlights a process where the basalt was incorporated into the granite during its formation, offering insights into the history and timeline of these rocks.

Igneous Rocks

Igneous rocks are formed through the cooling and solidification of magma or lava. These are the kinds of rocks that often contain inclusions or xenoliths. They are classified into two main types: intrusive (or plutonic) and extrusive (or volcanic), depending on where they cool.

Intrusive igneous rocks, like granite, cool slowly beneath the Earth's surface, allowing large crystals to form. Extrusive igneous rocks, such as basalt, cool quickly on the surface, resulting in smaller crystals. These processes lead to diverse rock compositions and textures.

When you find a granitic rock with basalt inclusions, you're observing an intrusive rock that has incorporated pieces of an extrusive rock. This interesting interaction is part of what makes studying igneous rocks so fascinating, as it offers a window into the dynamic processes occurring beneath the Earth's surface.