Principle of Inclusions

The Principle of Inclusions is an important concept in geology used to establish the relative age of rocks and geological formations. It is one of several principles used in the field of stratigraphy to help geologists understand the order in which rock layers and geological features formed over time.

What is the Principal of Inclusions

The Principle of Inclusions states that any rock fragment (called an "inclusion") enclosed within another rock must be older than the enclosing rock. Essentially, for an inclusion to be present within another rock, it must have existed before the formation of the host rock.

The geologic principle that a rock fragment within another rock is older than the host rock.

Formation of Inclusions

Inclusions form when fragments from pre-existing rocks, called clasts, are transported by natural forces such as water, wind, or volcanic activity. These clasts become embedded within a new rock layer, either through sedimentation in sedimentary rocks or by being trapped in magma or lava that cools around them in igneous rocks.

Explanation

Inclusions are fragments of one rock type enclosed within another. According to the Principle of Inclusions, these fragments must have existed before the formation of the surrounding, or "host," rock, making them older than the rock in which they are found.

For example, if magma intrudes into an existing rock formation, it may break off fragments of the surrounding rock. As the magma cools, these fragments are preserved within the newly formed igneous rock as inclusions. This process shows that the surrounding rock must have existed prior to the magma intrusion, allowing geologists to establish the relative ages of the rocks involved.

The Principle of Inclusions: Schist fragments within sandstone are older than the sandstone, and both are older than the granite dike containing their fragments.

The Principle of Inclusions where schist fragments within sandstone are older than the sandstone layer, and all are older than the granite dike that contains fragments of both sandstone and schist.

Practical Example

Consider an outcrop where a sandstone layer containing pieces of granite is observed. According to the Principle of Inclusions:

The granite fragments must have formed before the sandstone layer.

This relationship indicates that the sandstone is younger than the granite, even without knowing the absolute ages of these rocks.

By analyzing inclusions and applying this principle, geologists build a relative sequence of geological history, which can later be combined with other dating methods to construct a more complete understanding of Earth’s history.

Types of Inclusions

Clasts in Sedimentary Rocks: Erosion breaks down rocks into fragments which can then be transported and deposited in new layers. Through the process of deposition and lithification, these fragments become inclusions in sedimentary rocks. For instance, pebbles of granite found within a sandstone layer serve as evidence that the granite predates the sandstone.

Xenoliths in Igneous Rocks: During volcanic activity, magma can ascend through existing rock, fracturing and incorporating pieces of it. These fragments, known as xenoliths, are enveloped by the magma as it rises. When this magma cools and solidifies, the xenoliths are left as inclusions, clearly older than the igneous rock they are embedded in. This inclusion of older rock material provides a direct visual cue of the relative ages of the rocks involved.

White limestone Inclusion in porous red-brown lava from Mt. Vesuvius, Pompei Italy.

Applications in Geology

The Principle of Inclusions allows geologists to determine the relative ages of rock formations, piecing together the sequence of geological events without needing to know the rocks' absolute ages.

This principle complements other stratigraphic techniques:

Principle of Superposition: Layers of rock are deposited in a time sequence, with the oldest layers at the bottom and the youngest at the top.
Principle of Cross-Cutting Relationships: Any geological feature (like a fault or an igneous intrusion) that cuts across a rock formation must be younger than the rock it cuts through.

Together, these principles underpin stratigraphy, the study of rock strata and their relationships over time.

Evidence of Formation Processes: Inclusions serve as markers of geological processes. In igneous rocks, they might suggest tectonic events or magma intrusions. In sedimentary rocks, they can indicate erosion, transportation, and depositional histories, offering clues about ancient environments or paleoenvironments.

Identifying Unconformities: An unconformity represents a break in the geological record often due to erosion or lack of deposition. The presence of inclusions from an older rock within a younger layer above such a gap can pinpoint these unconformities, aiding in understanding the timing and nature of geological interruptions.

Older fragments within sandstone, limestone inclusions in breccia, and a basalt xenolith within granite.

Examples of inclusions

Diamonds in kimberlite: Diamonds are often found in kimberlite rocks, which are thought to have formed as a result of the rapid ascent of magma from the Earth's mantle.

Pyroxene in eclogite: Pyroxene is often found in eclogite rocks, which are thought to have formed as a result of the high-pressure metamorphism of basaltic rocks.

Petroleum fluid inclusions trapped within quartz crystals.

Fluid inclusions in quartz: Fluid inclusions are often found in quartz rocks, which can provide information about the conditions under which the quartz formed.

These examples illustrate how the Principle of Inclusions enables geologists to construct a relative timeline of Earth’s history, which can later be expanded with other dating methods.