Tag Archives: Subduction

How Crystals Form: Slow and Steady

Ever picked up a glittering rock and wondered how those beautiful crystals form? Here on Catalina we have lots of shiny igneous rock, and some metamorphic and sedimentary rock with crystal (igneous) elements. Any time you see smooth, reflective rock, think about the enormous temperatures and pressures it took to cook it up!


A hundred million years ago, the Pacific Oceanic tectonic plate, made largely of dense basalt, subducted under the North American continental tectonic plate, made of lighter granite. Like oil floating on top of water, that which was heavier or more dense (the basalt) sunk beneath what was less dense (the granite). Magma oozed up from the lithosphere, the zone directly under the Earth’s crust. Rock types rich in magnesium and iron, called mafic rock, and rock types made of lighter elements, such as silicon, aluminum, and oxygen, called felsic, melted at extremely high temperatures and pressures in the subduction zone.


In the next 80 million years after the islands began to form, the tectonic plates continued to settle and shift, and 20 million years ago, an intense period of volcanism around the Channel Islands created even more igneous intrusive rock.

screen-shot-2016-10-11-at-2-45-44-pmWhen magma cools slowly underground, intrusive igneous rock is formed. Extrusive rock, by contrast, forms when magma reaches the surface and cools quickly as lava. Trapped underground, that melted rock cooled slowly, allowing molecules to move past each other until they found another of the same shape to which to stick, forming crystals. The molecular properties of the elements involved will determine what shape the crystals form. One such example is quartz, made of silica and oxygen molecules stuck together in an ordered trigonal pattern.screen-shot-2016-10-11-at-2-46-01-pm

Quartz is the most abundant mineral crystal, comprising 12% of the Earth’s crust, and is often mixed in with other types of igneous, sedimentary, and metamorphic rock. Mineral hybrids containing crystals have been used around the world for art, science, and their believed healing properties for centuries. One such crystal is called diorite, and is often found with quartz intermixed, called quartz diorite. Catalina’s unique geology allows us to find quartz diorite along our cliffs and beaches. Quartz diorite was used in ancient Egypt for artistic sculptures, statues, and mausoleums.



Was Catalina Island formed by a volcano?

True or False?
Catalina Island was formed long ago by an ancient volcano.

Volcanoes form some islands, however Catalina Island was formed by geologic activity that pushed the Earth’s mantle to the surface forming the island. Once that super heated rock was pushed up, it was cooled by the ocean water, which formed Catalina Island as well as the other Channel Islands. This type of geologic movement is called subduction, specifically between two of the Earth’s tectonic plates. About 200,000 million years ago, the Farallon plate, sitting under the Oceanic Plate, began to subduct, or go under the North American Plate. This movement scraped up rock and sediment from the bottom of the ocean bringing it to the surface, forming what we know today as The Channel Island chain. This type of plate subduction took about 100 million years to break the surface, and to become the islands that we walk and roam present day.

Because of this type of subduction formation, Catalina Island is made up of three different types of rock; Igneous, Metamorphic, and Sedimentary which require heat, pressure, and sediment in order to form. The most primary rock composition of the island is made up of a type of metamorphic rock called blue and green schist, located mainly on the western and central parts of the island. Catalina is also made up of quartz, located on the east end of the island and in the middle canyon.

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All of this rock talk can be lumped into a big category known as geology, or the study of rocks and how they change over time. I know, sounds kind of boring, but geology actually allows us to look into the past and understand our timeline as well as Earth’s history and place within the cosmos as we know it. Think of geology as Earth’s well-kept diary, shedding light on the secrets of land and ocean formation on our big blue planet.


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