Blueschist

Some rocks have a pretty blue color.

This rock forms deep under the ground. It is pushed down by heavy weight. This weight is very high. But the ground is not too hot there.

The blue color comes from tiny bits inside. It can also look gray or black. We find these rocks in many lands. They are found in Greece and Japan. People once used them to pave streets. It is a very rare rock to find.

Blueschist is a special kind of rock. It often has a blue color.

This color comes from minerals called glaucophane and lawsonite. The rock can also look gray, black, or blue-green.

This rock forms in a very specific way. It starts as basalt, which is a volcanic rock. The basalt must go deep under the Earth. It must reach a depth of 15 to 18 kilometers. At this depth, the pressure is very high. However, the temperature stays relatively low. It stays between 200 and 500 degrees Celsius.

Because these conditions are rare, the rock is rare too. It forms where one plate slides under another. This is called subduction. To find the rock at the surface, it must move up quickly. If it moves too slowly, it will get too hot. If it gets too hot, it changes into other rocks. You can find blueschist in places like Greece, Japan, and Turkey. Long ago, people in Crete used these rocks to pave streets.

Blueschist is a very special type of metamorphic rock. Metamorphic rocks are made when existing rocks change under heat and pressure. This rock is famous for its blue color. This color comes from minerals called glaucophane and lawsonite.

The rock can also look black, gray, or blue-green. It often forms from a volcanic rock called basalt. Because it needs such specific conditions, it is quite rare to find.

To make blueschist, a rock must go very deep underground. It must travel down at least 15 to 18 kilometers. At this depth, the pressure is extremely high. We measure this pressure as being over 0.6 gigapascals.

Even though it is deep, the temperature stays relatively low. It stays between 200 and 500 degrees Celsius. This is a strange mix of high pressure and low heat. This specific way of forming is called the Franciscan facies series.

This rock forms during a process called subduction. This happens when one tectonic plate slides under another. The rock must move back up to the surface very quickly. This movement is called exhumation.

If the rock moves up too slowly, it will get too hot. If it heats above 500 degrees Celsius, it changes into different rocks. It might become a green rock called greenschist or a rock called eclogite. Fast movement through faults or flowing helps keep the rock cool.

Scientists have studied these rocks in many different places. You can find well-exposed blueschist in Japan and New Zealand. They also appear in Greece, Turkey, and New Caledonia.

In 1962, a man named Edgar Bailey helped define this rock. He worked for the U.S. Geological Survey. He explained the exact pressure and temperature needed to make it. His work helped geologists understand how these rocks change.

Even ancient people used these rocks in their daily lives. In Minoan Crete, people used blueschist to pave streets. They also used it for courtyards between 1650 and 1600 BC.

They likely got the stones from Agia Pelagia on the north coast. Today, we look at these stones to learn about the Earth. They tell us a story about how the deep crust moves. They show us how much pressure exists far below our feet.

Blueschist is a rare and fascinating type of metavolcanic rock. It is classified as a schist, which is a rock with a layered or flaky structure. This rock is most famous for its distinct blue color. This color is caused by the presence of specific minerals, mainly glaucophane and lawsonite.

While it is often blue, blueschist can also appear black, gray, or blue-green in the field. It typically forms from the metamorphism of basalt or similar volcanic rocks. Because the conditions required to create it are so specific, blueschist is quite uncommon.

To understand how blueschist forms, we must look at the relationship between pressure and temperature. Most rocks deep in the Earth are very hot. However, blueschist forms under a unique "low temperature, high pressure" path. This process requires pressures exceeding 0.6 gigapascals. This level of pressure is found at depths of more than 15 to 18 kilometers. At these depths, the temperature must stay between 200 and 500 degrees Celsius.

This specific combination of conditions is known as the Franciscan facies series. This name comes from the west coast of the United States, where these rocks are often found.

The mechanism that creates these conditions is called plate subduction. This happens when one tectonic plate slides beneath another plate into the Earth's interior. As the rock is pushed down, the weight of the plates creates immense pressure. Because the rock is moving down quickly, it does not have time to heat up significantly. This allows the rock to reach high pressures while remaining relatively cool. If the rock stays at these depths for too long, it will gain heat through conduction from the hotter rocks below. If the temperature rises above 500 degrees Celsius, the minerals will change into different types. The rock would then become greenschist or eclogite instead of blueschist.

Because of this heat risk, blueschist must undergo a process called exhumation to be found at the surface. Exhumation is the movement of the rock back up toward the Earth's crust. This must happen swiftly to prevent the rock from reaching thermal equilibrium with the hot surroundings. Scientists believe this rapid movement happens through faulting or flow within accretionary wedges. Sometimes, buoyancy helps the rock rise if it is associated with low-density continental crust. If the rock moves too slowly, the mineral assemblages will metamorphose into new forms. This is why seeing blueschist at the surface is a sign of very fast geological movement.

Blueschist is defined by a specific group of minerals. In basaltic rocks, the classic assemblage includes glaucophane, lawsonite, or epidote. Other minerals might include albite, titanite, garnet, or quartz. The rock often has a microstructure that is lepidoblastic or schistose. This means the minerals, such as chlorite or phengitic white mica, have an elongated or platy shape.

The grain size of the minerals is rarely coarse. This is because the low temperatures and the swift metamorphic path slow down mineral growth. However, some porphyritic varieties, which have larger crystals, can still occur.

Different types of starting rocks produce different mineral sets under blueschist conditions. For example, ultramafic rocks might produce serpentinite containing talc or zoisite. Pelitic sediments, which are clay-rich, will form different minerals like kyanite or chloritoid. Even limestones change during this process. In limestone, the mineral calcite transforms into aragonite due to the high pressure.

However, aragonite often reverts back to calcite once the rock is exhumed to the surface.

Humans have interacted with these rocks for thousands of years. In Minoan Crete, people used blueschist and greenschist for construction. Between 1650 and 1600 BC, they used these stones to pave streets and courtyards.

They likely quarried the stone from the north coast of central Crete at Agia Pelagia. In modern science, the study of these rocks changed significantly in 1962. Edgar Bailey of the U.S. Geological Survey introduced the formal concept of "blueschist." He created the precise definition for the pressure and temperature conditions that define this rock type.

Today, geologists study blueschist to understand the history of tectonic plates. Well-exposed examples are found in many parts of the world. These include Greece, Turkey, Japan, New Zealand, and New Caledonia. These rocks serve as a record of the deep movements of the Earth's crust. They show us how material is recycled through subduction zones and brought back to the surface.