From volcano top to deep sea...

Using the Canary Islands to show how the rock cycle works!

You’re probably familiar with the Canary Islands in one way or another. These islands off north west Africa are a popular holiday destination for British families because of their warm climate and spectacular scenery.

But there’s much more to the Canary Islands than meets the eye…if you’ve visited Tenerife or Lanzarote you will have noticed that the sand on most of the beaches is black rather than yellow – often a surprise (or disappointment!) to many holidaymakers! This is because the islands are actually volcanoes, made up of a black lava called basalt…read on to find out more....

The Canary Islands are perfect for illustrating how the rock cycle works in real life. Here you will see rocks and sediment from the different stages of the rock cycle, from the lava found at the top of the volcano to the fine mud at the bottom of the sea. On this page we will also show you how marine geologists go about investigating what the seafloor around the islands looks like, and how we use our data to interpret geological events.

Violent beginnings…

There are seven islands in the Canary Islands chain. All of them are volcanic in origin, formed by the buildup of lava erupting through the seafloor to form volcanoes. These volcanoes formed at slightly different times, so the islands in the east are much older than the islands in the west of the chain. Fuerteventura is the oldest island at 20 million years old, but Hierro is only 1.2 million years old.

Some of the Canary Islands are still volcanically active. Tenerife, La Palma, Lanzarote and Hierro have all erupted within the past 500 years – in fact, La Palma saw an eruption in 1971. Fuerteventura and Gran Canaria have not seen any volcanic activity for a very long time, and the small island of Gomera is thought to be volcanically extinct.

Tenerife is home to Pico del Teide, the enormous volcanic peak which stretches 3718 metres above sea level – that’s higher than any of the mountains in the Pyrenees! Teide is the third highest volcano on earth (after Mauna Kea and Mauna Loa in Hawaii) and the top of the volcano is occasionally covered with snow and ice.

The last major eruption to occur on Tenerife was in 1909. It started on 18th November and lasted 10 days. Since then the volcano has been quiet, but small vents and fissures on the sides of the main volcano steam quietly in preparation for the next event.

Heating and melting of material in the mantle below the ocean crust forms magma, which then spews out as lava over the seafloor and builds up into a volcano. The volcanic activity in the Canaries is not related to a mid-ocean ridge (which marks the join between two crustal plates), but to an isolated area of magma generation known as a hotspot.

So, the first stage in the rock cycle in the Canaries is the formation of the volcanic rock which makes up the islands.

What’s in a rock?

The volcanoes of the Canary Islands are made of basalt. Basalt is a very common igneous rock and it makes up the ocean floor as well as most of the volcanic islands found in the oceans.

All rocks are made up of minerals. Basalt contains only a few minerals: pyroxene, plagioclase feldspar and sometimes olivine. The dark colour of basalt is mainly due to the dark colours of pyroxene and olivine, although plagioclase is white. However, the size of the individual mineral crystals are often so small that it makes the rock look dark as well.

Basalt can appear in different forms. Lavas (formed when magma is extruded out onto the surface) have different textures, from ropey-looking lavas (called pahoehoe) to blocky, rubble-like textures (known as aa). Lumps of basalt can be thrown out of the top of the volcano to form dense volcanic bombs, or if gas is mixed in with the magma you might get a very light rock full of holes! When basalt is erupted under water, pillow lavas form – so-called because they form globules like oil in a lava lamp. Basaltic lava is erupted at about 1400 degrees centigrade, so when it makes contact with cold seawater, the outer skin of the lava globule cools and hardens very rapidly, forming a skin of volcanic glass.

Rock solid?

Since the Canary Islands poked their noses above the sea millions of years ago, they have been continually battered by nature. Basalt is pretty tough stuff, but combine the forces of the sun, wind, rain and sea and all rocks will eventually crumble into dust. This process is called erosion and it is a key force behind the rock cycle.

If you’ve been on holiday to the Canaries you will know that the sun can get pretty hot! The effect of the sun on black basalt causes the rock to expand by day, and cool and contract by night when the sun goes down. This process weakens the rock and makes it more vulnerable to the other erosional forces such as the wind and rain. In many places on the Canaries, the lavas already have a very blocky and rubbly appearance due to the way they formed, making it even easier for nature’s forces to break it down.

Particles of rock move slowly down the slopes of the islands towards the sea. There are virtually no streams or rivers on the islands to carry the particles downhill. On the coast, the waves continually batter the rocks and erosion is much faster. The beaches are composed of the fragments of the island’s basalt, which is why most of the beach sand is black.

A major factor in shaping the islands’ landscape is landslides. Enormous scars on the flanks of El Hierro, La Palma and Tenerife show that these landslides can be massive and potentially catastrophic. The most recent of these large slides occurred on the north west side of El Hierro (left) about 15,000 years ago and the scar left behind is very obvious. The landslides are thought to be triggered by small movements in the island’s structure caused by volcanic activity.

Journey to the bottom of the ocean

The journey of a sediment particle from the island doesn’t stop when it reaches the coast! In fact, this is really only the beginning!

Once on the beach, the sediment is washed around by waves and tides. You know that if you go for a paddle on a beach that the sand doesn’t stop at the waterline, but it extends out far beyond the shore. Currents in the coastal waters transport sediment around the seafloor: some sediment moves along parallel to the coastline; some is moved down towards the deeper ocean.

The most prominent geological features on the seafloor around the Canary Islands are the rocks and sediment deposited by the landslides. When a major landslide takes place on the side of an island most of the rock slides down directly into the sea as a debris flow or debris avalanche. This material usually forms a large deposit of rock and sediment in a distinctive lobe shape on the seafloor near the island. Large blocks of rock from the debris avalanche are dumped on the seafloor closest to the island, because they are heavy and require a lot of energy to move them.

As the debris avalanche hits the seafloor, sand and mud is disturbed and forms a dense, underwater cloud of sediment which rushes across the seafloor and down towards the deep sea. This cloud of sediment is called a turbidity current and resembles a snow avalanche in appearance. The massive amount of material disturbed by a large landslide means hundreds of cubic kilometres of this finer sediment can be transported up to 130 km away from the island, and the cloud of sediment can settle out over an area of thousands of square kilometres.

In the deepest parts of the ocean (the abyssal plains), sediment from landslides forms distinctive layers in the seafloor mud. Under normal conditions, the accumulation of sediment on the abyssal plain floor is extremely slow – much of it is wind-blown dust and the remains of microscopic marine organisms settling out through the water column. When a turbidity current comes rushing onto the abyssal plain, the coarsest grains settle out first, followed by finer and finer material until all the sediment has settled out and the environment returns to normal.

Using mud to discover the past

Scientists identify landslide events in the deep sea sediment record by taking sediment cores through the seafloor using special equipment on board a research ship. A sediment core is a column of mud and sand removed from the seabed like taking a cork out of a bottle. Once the core is back on board the ship, it is split in half lengthways and opened up.

In the area around the Canary Islands, many of the sediment cores have a striped appearance, which can be used to tell the difference between periods of normal deep sea sedimentation and landslide events. Normal deep sea mud is a brown or green colour, made up of very fine grains of clay and microscopic marine fossils. Turbidite deposits look very different and are usually much coarser grained. By looking at the composition of the sediment in these deposits, we can work out where the sediment came from and when it was deposited. The lateral spread of a turbidite deposit is worked out by looking for same sediment layers in cores from different locations.

We can also look at the surface of the seafloor to identify features created by landslides. Sound waves are used to measure the depth of the water. The depth of the water obviously changes depending on the topography of the seafloor, so the result is a clear, 3D bathymetric map of the seafloor which helps us to identify the major features.

Sidescan sonar shares the same principle but produces much more detail. It uses sound waves instead of light waves to build up a detailed image of the seafloor which looks rather like a black and white photograph. This technique allows scientists to identify textures and changes in the type of sediment present on the seafloor. We use an instrument called TOBI (Towed Ocean Bottom Instrument - pictured right), which not only gives us a picture of the seafloor, but also uses sonar pulses to look at the layers of sediment just below the seafloor. This gives us a seismic profile which shows the uppermost parts of the seafloor in cross section.

The end of the story?

So, you’ve seen how solid volcanic rock from the top of Teide volcano can end up as mud at the bottom of the ocean. Is that the end of the story? It’s not called a rock cycle for nothing…can you work out what happens next?

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February 2007