Types of Rocks Found In Costa Rica: A Guide To The Most Common You’ll Spot

Rocks in Costa Rica are interesting, and some of the most common types of rocks found in Costa Rica are igneous.

If you are planning on visiting there, here are three of the most common types that you’ll come across. 

Types of Rocks Found In Costa Rica

Dacite

Dacite is an igneous rock that is commonly found in pyroclastic debris, lava flows, domes, and dikes.

It is typically a light colored rock when consisting of plagioclase, with a composition between andesite and rhyolite.

However, it contains more plagioclase than rhyolite and more quartz than andesite.

Plagioclase is the most common mineral in many dacite specimens, although biotite, augite, enstatite, and hornblende, as well as quartz, can also be present. 

While specimens of dacite with high plagioclase content tend to be light colored, when hornblende and biotite content is higher, the color can be light gray or light brown.

Darker specimens tend to be more abundant in enstatite or augite. 

The dacite magma which produces these rocks, forms in subduction zones where younger oceanic plates are moving under a continental plate.

As the oceanic plate is forced under, the friction creates heat which causes a partial melting to occur. 

Dacite is sometimes also associated with explosive eruptions where viscous magma with a high gas content erupts violently.

However, dacite magma with lower gas content tends to produce thicker flows that slowly seep out of vents producing step domes. 

In the past, dacite has been used to make ancient tools such as spear and arrow heads.

While it isn’t as sharp as obsidian, it is much more durable. 

Turrialba, a basaltic-to-dacitic stratovolcano, is located in Turrialba Volcano National Park, which is open to visitors when it is safe.

Here, outcrops of pyroclastic material from at least 20 basalt and dacite eruptions have been recorded around the summit area.

This park is somewhere to definitely visit, if you want to learn more about the volcanic history of Costa Rica, as well as how igneous rocks have helped shape the landscape. 

Rhyolite

Rhyolite is another igneous rock with a very high silica makeup.

It can be either gray or pink in color, with small grains that require a hand lens to be able to see effectively.

This stone is made up of plagioclase, quartz, and sanidine, although smaller amounts of biotite and hornblende may be present.

Unlike dacite, rhyolite can contain opal, crystals, or glassy material when split open. 

Many specimens of rhyolite form when granitic magma partially cools in the subsurface.

When erupted, specimens with two types of grain sizes can be formed.

Larger crystals which form beneath the earth are known as phenocrysts, and the smaller crystals that form on the surface are known as groundmass. 

Rarely produced from oceanic eruptions, rhyolite is usually only formed in continent-margin or continental eruptions. 

Many gem deposits can be found in rhyolite, and these are formed when the thicker granitic lava cools quickly with pockets of gas trapped inside.

As the cooling continues, these pockets form hollow cavities known as vugs.

As the lava flow cools further and ground water or hydrothermal gases pass through, material necessary for gem formation can be introduced into the cavity.

Some of the world’s greatest deposits of topaz, agate, opal, jasper, and red beryl are formed this way. 

Unlike dacite, rhyolite doesn’t make for a construction material, although there have been ancient scrapers, spear heads, arrow heads, and blades found.

However, the construction of these tools was probably out of necessity, not choice, as it is easy to fracture and not as durable as dacite or other igneous materials. 

Andecite

Yet another igneous rock, andesite, is the name given to a family of fine grained extrusive rocks which can be light or dark gray in coloration.

Typically found in lava flows, similar to the other rocks mentioned above, they are produced by stratovolcanoes that exist above subduction zones.

Because the lava produced is rapidly cooled on the surface, tiny crystals are formed, and most are so small that they can only be properly seen with the use of a hand lens. 

When exposed to weathering, andecite can turn to various shades of brown, and when found in the field it will need to be broken in order to see its proper color and composition. 

These specimens tend to be rich in plagioclase, amphibole, and feldspar.

However, pyroxene minerals and quartz can also be present in smaller quantities along with biotite or muscovite.

Along with diorite, andesite is usually found above subduction zones, and they form as the oceanic plate heats up during its subduction under a continental plate.

This gives it a composition somewhere between granite and basalt, as the parent magma is formed from the partial melting of the basaltic ocean plate.

Its granitic composition can be attributed to granite rocks either mixing during accession or mixing with granitic magma. 

With all of this being said, the exact formal definition of andesite can be difficult to pinpoint.

Many classify it based on it’s chemical and mineralogical composition, although there is no singular agreement.

For many fine grained rocks, like andesite, these exact classifications can be difficult to pinpoint in the field or classroom without the use of chemical or mineralogical analysis, which can be neither affordable nor practical.

However, if you find a specimen that you think is an andesite, you can call it an anesitoid, meaning that it looks like an andesite, but a chemical or microscopic analysis is needed for exact identification.  

Can You Take Rocks From Costa Rica? 

With all the types of rocks found in Costa Rica, it can be tempting to try and take some home for further investigation or collecting.

However, if you are visiting Turrialba Volcano National Park, you should know that it is prohibited to remove anything, including rocks, plants, and wood, from inside the park.

On beaches — outside of parks and wildlife refuges — only collectors with permits are allowed to remove things like shells.

Some travelers have even reported having things like sea glass confiscated from their carry-on by Costa Rican airport security. 

If you really want to bring home some rock samples, it may be best to try and find a shop where local rocks are sold, or you can always find specimens online to add to your collection.

When you are in doubt, always double check with a guide about whether or not something is appropriate to pick up and take with you. 

A Firey Volcanic History

Costa Rica is rich with volcanic history, and it can be one of the best places to find igneous rocks and see volcanoes first hand.

However, take care when trying to locate specimens to bring home, as many areas prohibit it. 

types of rocks found in costa rica