Mount St. Helens
Left: Mount St. Helens area after eruption 1980
Right: Mount St. Helens 2008
We can see from these two pictures of the Mount St. Helens area, one from 1980 and the other from 2008, that in less than 30 years, it was covered again by plants. This would not be possible in such a short lapse of time if there was no natural fertilizer present or if there wasn't one created by the volcanic eruption.
Volcanic soil is indeed extremely fertile after the decomposition of solid lava by weathering. Nutrients such as calcium, magnesium, sodium, potassium, phosphorus and sulphur are brought up to the surface from deep within the Earth to the surface. This process takes a minimum of a few years up to centuries, if for example the erupted products are mainly ashes in a warm and humid climate. Indeed, ash weathers much more quickly than lava.
Volcanic soil is indeed extremely fertile after the decomposition of solid lava by weathering. Nutrients such as calcium, magnesium, sodium, potassium, phosphorus and sulphur are brought up to the surface from deep within the Earth to the surface. This process takes a minimum of a few years up to centuries, if for example the erupted products are mainly ashes in a warm and humid climate. Indeed, ash weathers much more quickly than lava.
Section 1: How is volcanic ash produced and where does its composition come from?
Volcanic ash comes from magma. It is composed of fine pieces of glass and rock. Magma is a mix of liquid rocks and gases at a very high temperature (800 to 1200°C) in the mantle. Due to Archimedes’ buoyancy, this liquid is less dense than its surrounding solid environment and is therefore uprising and fractures the upper rocks. If this fracture reaches the surface of the Earth, we observe a sudden drop in the pressure from the weight of the crust (depressurization) and the gas pressure increases. The liquid magma is then pulverised into fine drops from the conduct into the atmosphere and solidified due to the contact with cold air. Transported by the wind, the ash particles are spread over a large area near the volcano.
The composition of volcanic ash therefore completely derives from the original magma, which contains calcium, magnesium, potassium, phosphorus and sulphur. The effect of these components on the development of vegetation is seen in the next section.
Section 2: In what way is volcanic ash a fertilizer?
Fertilizer label- we see a high percentage of calcium, magnesium and sulphur in this example of fertilizer. It shows the link between the composition of volcanic ash and its role as a natural fertilizer.
Volcanic ash is normally rich in nutrients which act as natural fertilizers for the plants in the area, such as calcium, magnesium, potassium, phosphorus and sulphur. These elements are all macronutrients essential for plant growth and survival. They therefore enhance the development of plants in tropical regions where the soil would not normally contain these essential nutrients, added to the high rainfalls and temperatures. Some of the main functions of these elements in the development of crops are outlined below:
PHOSPHORUS: enhances the early growth and the formation of the root, accelerates the development of and strengthens the plant, fosters the production of seed.
POTASSIUM: plays an important role in the enzyme systems activity of the plant. It also promotes the resistance of the plant against external threats, such as cold or diseases.
CALCIUM: promotes the carbohydrate movement in plant cells, responsible for energy transport and storage in the plant. It has a key role in the root system and cell membrane’s strength.
MAGNESIUM: key element of photosynthesis as it is the central core of the chlorophyll - without chlorophyll no photosynthesis
SULPHUR: promotes the production of certain proteins and oil.
Section 3: Variation of effect
Soil development in volcanic areas depends on the content of silica (SiO2) in the ash, which itself depends on the original composition of the magma.
BASALTIC ASH: quickly makes the soil very fertile, one year after the eruption (e.g. on Central American volcanic areas corn is often planted less than a year after the eruption.)
ANDESITIC ASH: takes 10-20 years under humid conditions for the soil to nurture (e.g. the Soufrière volcano on the island of St. Vincent, where a forest grew 30 years after the 1902 eruption)
RHYOLITIC ASH: takes up to 5000 years to be able to support vegetation. (e.g. El Salvador and Guatemala.)
This difference in soil development according to the silica composition is made by its variation in capacity of alteration by weathering.
Basalt Andesite Rhyolite
MgO 9.54% 7.30% 3.33%
CaCO 10.32% 10.36% 6.79%
K2O 0.84% 0.43% 1.62%
Total 20.7% 18.09% 11.74%
We can also determine the relative concentration of magnesium, calcium and potassium in these three different types of volcanic ash through their concentration in the original volcanic rock. It seems as if there are more nutrients in te ask, the soil will be fertilized faster.
BASALTIC ASH: quickly makes the soil very fertile, one year after the eruption (e.g. on Central American volcanic areas corn is often planted less than a year after the eruption.)
ANDESITIC ASH: takes 10-20 years under humid conditions for the soil to nurture (e.g. the Soufrière volcano on the island of St. Vincent, where a forest grew 30 years after the 1902 eruption)
RHYOLITIC ASH: takes up to 5000 years to be able to support vegetation. (e.g. El Salvador and Guatemala.)
This difference in soil development according to the silica composition is made by its variation in capacity of alteration by weathering.
Basalt Andesite Rhyolite
MgO 9.54% 7.30% 3.33%
CaCO 10.32% 10.36% 6.79%
K2O 0.84% 0.43% 1.62%
Total 20.7% 18.09% 11.74%
We can also determine the relative concentration of magnesium, calcium and potassium in these three different types of volcanic ash through their concentration in the original volcanic rock. It seems as if there are more nutrients in te ask, the soil will be fertilized faster.
Damaging Effects of Volcanic Ash after Eruption
This image shows the Montserrat volcano in the Caribbean destroyed by a pyroclastic flow. The ash in the cloud has covered and destroyed all the vegetation on its path. A delta along the shore has been created by the accumulation of the ash.
Volcanic ash has short-term damaging effects on vegetation. Pyroclastic flows (fast moving avalanche flowing down the side of a volcano consisting of a mixture of volcanic ash and debris) are hot and poisonous and all the vegetation caught in its path is therefore killed and flattened.
After an eruption, the layer of ash deposited is very thick (up to 100mm), and many plants find themselves buried. The weight of ash on the leaves is problematic for the survival of the plant and the lack of exposure of the plant to sunlight makes life for the plant impossible, as it cannot photosynthesize. The ash plume after an eruption also reacts with the rain passing through it, to produce acids, resulting in acid rains, which is very destructive for many plants. Furthermore, ash has toxic components that are spread on the soil and poison it. After the eruption, the soils have a low water-holding capacity and lack essential nutrients such as phosphorus and nitrogen. The toxicity of the ash extinguishes a soil fungi called mycorrhizae which is in many cases essential for the absorption of the nutrients by the plant roots. Therefore, at this stage, vegetation is generally destroyed and the development of plants is very limited.
However it is possible for life to recover from disasters such as these. A gelatinous film of blue-green algae ash substrate was discovered on top of the volcanic ash around the Krakatoa volcano three years after the cataclysmic eruption of 1883. It was assumed that it was advantageous to the germination and development of the first ferns. Twenty years later a large forest had grown. This process is called succession: primary succession is the colonization of a bare ground that has never built up soil by plants. Secondary succession is the development of a forest derived from the latter.
Volcanic ash has short-term damaging effects on vegetation. Pyroclastic flows (fast moving avalanche flowing down the side of a volcano consisting of a mixture of volcanic ash and debris) are hot and poisonous and all the vegetation caught in its path is therefore killed and flattened.
After an eruption, the layer of ash deposited is very thick (up to 100mm), and many plants find themselves buried. The weight of ash on the leaves is problematic for the survival of the plant and the lack of exposure of the plant to sunlight makes life for the plant impossible, as it cannot photosynthesize. The ash plume after an eruption also reacts with the rain passing through it, to produce acids, resulting in acid rains, which is very destructive for many plants. Furthermore, ash has toxic components that are spread on the soil and poison it. After the eruption, the soils have a low water-holding capacity and lack essential nutrients such as phosphorus and nitrogen. The toxicity of the ash extinguishes a soil fungi called mycorrhizae which is in many cases essential for the absorption of the nutrients by the plant roots. Therefore, at this stage, vegetation is generally destroyed and the development of plants is very limited.
However it is possible for life to recover from disasters such as these. A gelatinous film of blue-green algae ash substrate was discovered on top of the volcanic ash around the Krakatoa volcano three years after the cataclysmic eruption of 1883. It was assumed that it was advantageous to the germination and development of the first ferns. Twenty years later a large forest had grown. This process is called succession: primary succession is the colonization of a bare ground that has never built up soil by plants. Secondary succession is the development of a forest derived from the latter.
Island of Surtsey
Island of Surtsey today.
Surtsey rose out of the sea on November 14, 1963 near the southern coast of Iceland. After its eruption, the magma transformed into volcanic ash was quickly cooled by the surrounding cold water this island was formed. By the end of January 1964 the island was 174 m high (300 m above the sea floor where the volcano started).
This diagram shows the different stages of the creation of a volcanic island, starting from the sea-floor to being elevated hundreds of meters above the surface.
Until recently, the Surtsey's ground was only inhabited by organisms that were well-adapted to the poverty of the soil but gradually, with weathering, a combination of the decay of plants and the dispersed volcanic ash was formed, creating a new, rich layer of soil, capable of sustaining increasingly varied forms of life. This example shows well how a volcanic area can host a diversity of organisms over time.
Soil fertility around volcanically active areas such as Central and South America and the Philippines results in plantings of coffee, cotton, sugar etc. Human populations will always tend to settle in the areas where the agriculture is the most promoted. Indeed, many of the richest farmlands are situated on a thin ash layer near a volcano.
Until recently, the Surtsey's ground was only inhabited by organisms that were well-adapted to the poverty of the soil but gradually, with weathering, a combination of the decay of plants and the dispersed volcanic ash was formed, creating a new, rich layer of soil, capable of sustaining increasingly varied forms of life. This example shows well how a volcanic area can host a diversity of organisms over time.
Soil fertility around volcanically active areas such as Central and South America and the Philippines results in plantings of coffee, cotton, sugar etc. Human populations will always tend to settle in the areas where the agriculture is the most promoted. Indeed, many of the richest farmlands are situated on a thin ash layer near a volcano.