Explore the Deep Sea

Volcanoes & Vents

Thin Sections

Putting rocks under the microscope to find out what they're made of, where they came from and how they formed.

Crystal sizes and shapes

Thin section of gabbro under cross-polarized light, showing vivid blue crystals. Image courtesy of M. Perfit

Sometimes, when magma cools very slowly, the crystals that form from it are large enough to be seen with the naked eye. But often, magmas cool so fast that crystals can only be seen clearly with a microscope.

Coarse-grained gabbroic rocks (gabbros) make up the thickest and deepest part of the oceanic crust. They are rarely exposed on the seafloor but sometimes crop out along faults, fractures and rift zones. Gabbros can also be found in ophiolites: rare sections of the oceanic crust that have been thrust up on land by plate tectonic forces. The colorful image on the left shows a thin section through a gabbro: different types of crystals can be clearly seen, especially with a microscope. Because this rock cooled slowly, deep within the ocean crust in a magma chamber, there was enough time for various elements to migrate within the melt to form these large interlocking crystals (the red bar shows 1mm).

Glassy rind on a basaltic rock. Image courtesy of M. Perfit

By contrast, basalts are the most common rock found at the surface along mid-ocean ridges. Basalts are very fine grained: they have small crystals, or none at all. When hot basalt magma is erupted into near freezing-seawater, its surface cools so rapidly that no mineral crystals can form. Instead, a glassy rind (typically 2-4cm thick) forms on the surface of the flowing lava. This can be seen in the photograph to the left (not a thin section, but a photo of a rock about the size of a football): the rind is the deep grey/black layer on the outside of the rock. Inside the glassy rind, the rock cools a little more slowly, so some tiny crystals have time to form (these are in the lighter grey area of the rock in this photo).

Throwing light on the problem

Geologists can learn a great deal about rocks by shining a special sort of light through thin sections (the slices are so thin that they allow some light through) and examining them under a microscope.

Diagram illustrating polarization of light

Light travels away from its source (such as the sun, or a light bulb) in waves that vibrate in all planes perpendicular to the direction of the light's travel. But the light can be passed through a filter that only lets through the light waves vibrating in a single plane: this light is plane polarized. If plane polarized light is passed through a thin section, the internal structures of the crystals will refract (bend and split) the light, changing the plane of its vibration. If the crystal is rotated, the orientation of the crystal structures will change relative to the plane of light: the crystal will refract the light more, or less. Different types of crystal will change the light differently.

Another polarizing filter can be inserted into the microscope, at right angles to the first, but above the crystal. So the light which has been bent by the crystal passes through this second, cross-polarizing filter. If the crystal has not bent the light at all (because the internal planes of the crystal are parallel to the plane of light polarization), then none of the light will get through the second filter: the crystal will appear dark. But if the crystal has changed the plane of the light's vibration, some of the light will get through the second filter. Often, this means the crystal appears colored: the color depends on the type of crystal, and the angle at which it is oriented relative to the planes of the first and second filters.

Crystals under plane polarized light. Image courtesy of M. Perfit The same crystals under cross polarized light. Image courtesy of M. Perfit

The photos on the left illustrate how a thin section viewed through a cross-polarizing filter can appear different to the same thin section viewed under plane polarized light. Both photos show exactly the same the boundary between two crystals. The first photo was taken under plane polarized light, the second under cross-polarized lenses. Notice how, in the second photo, the upper crystal is largely dark while the lower crystal is colored.

See our image gallery for more colorful photos of thin sections.

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