Serpentinite and Plants - It Is A Harsh World

Geology is one factor that shapes the plant world around us. Geology can influence the types and locations of plants. Geology contributes to plant endemism. Geology does by creating harsh soil conditions that discourage plant growth. Serpentinite rock is one well documented example where the chemical (and physical) characteristics of the rock discourage plant growth. Recall that I am not a botanist, so what I say is seen through the eyes of a geologist.

In this note, I share observations made on the former Munro Mine (Photo 1), located east of Matheson (Photo 2), Ontario, Canada, in the famous Abitibi Greenstone Belt (Photo 3). These observations demonstrate that serpentinite rock is a very challenging substrate for plants to grow on. I also illustrate how coarse cobble-sized rock fragments, in a waste rock dump, also create a substrate that is challenging for plants to grow on, regardless of the chemical composition of the rock type.

Munro Mine

The Munro Mine opened in 1949-1950 and closed in 1964. The mineral mined was asbestos (Photo 4). The asbestos was contained in a rock called serpentinite (Photo 5). Serpentinite is a modified form of original ultramafic rocks that geologists call dunite and peridotite. These ultramafic rocks contain a lot of nickel, copper, iron, and other metal elements. Ultramafic rocks have virtually no plant nutrients like nitrogen, phosphorous and potassium (standard plant fertilizers). This rock chemistry creates very challenging growing conditions for most plants; however, some specialized plants called “serpentine endemics” are adapted to grow in these harsh chemical conditions.

The following is the simplified process by which asbestos is extracted from the rock, which creates waste, or “left over” rock. Although I am not an expert of the milling of asbestos at the Munro Mine, I suspect the milling process I describe also applies to the Munro Mine.

Step 1: remove the asbestos-bearing rock from the ground using mining methods that create an open pit. When mining is completed, the pit fills with water and forms a lake (Photo 6).

Step 2: in the mill, the mined serpentinite rock is crushed to various grain sizes in order to liberate the asbestos mineral. Crushing creates a lot of “left-over” rock of various sizes that does not contain asbestos. That “left over” rock is stored as surface waste rock dumps (Photo 7).

Step 3: I don’t know the exact milling process used at the Munro Mine, but the simplest way to separate asbestos from the crushed serpentinite rock is to use large vacuums to suck up the asbestos fibre. No chemicals are used during this process. At the Munro Mine, the most finely crushed waste rock is stored in a large hill locally named “JM Mountain” (Photo 7, background).

That is a very simple explanation of the steps used to extract the mineral asbestos from waste serpentinite rock.

Different Personalities of Serpentine-rich Rock

In geological language, serpentine is the name of a family of secondary minerals. Geologists just call them all serpentine for simplicity. A rock composed mostly of serpentine is called serpentinite. Recall that serpentinite formed from pre-existing ultramafic rocks named dunite and peridotite.

Dunite and peridotite are not common on the Earth’s surface, but they are common in the Earth’s mantle, deep below the land surface. They were the original rocks that found themselves on, or close to, the Earth’s surface in northeastern Ontario (and elsewhere) about 2,700 million years ago. Dunite and peridotite: a) contain a primary mineral named olivine, which is not happy on, or near, the Earth’s surface; and b) contain a lot of magnesium and iron, very little potassium and calcium, a lot of nickel, copper, cobalt and chromium, and virtually no phosphorous and nitrogen. Olivine is the gemstone called peridot, which is the birthstone for the month of August.

On, or near, the Earth’s surface, the primary olivine reacts with water and is transformed to the secondary mineral called serpentine. That reaction takes place when great geological forces ravage the rock, sometimes during mountain-building, when continents smash into each other, and sometimes when the Earth is torn apart by faults. When serpentine is created, it inherits many of the chemical features of the primary olivine. So, the chemical personality of the serpentine-rich rock remains, more or less, the same as the primary dunite and peridotite.

Serpentine and serpentinite rock come in a range of colours: white, light green, blue-green, grey, blue-black, and black. The specific type of serpentine mineral dictates the mineral colour.

So, walking over the crushed rock waste dump at the Munro Mine, your eyes are treated to a cornucopia of coloured rocks, most of which contain serpentine (Photo 8)!

Modern Uses of Serpentine

In addition to being the host rock for the industrial mineral called asbestos, a fibrous form of serpentine, serpentine and serpentinite rock: a) are used as architectural stone; b) can be transformed into a semi-precious gemstone; c) can be carved into rock art (Photo 9); d) sometimes contains pockets of jade, also used for carving; and e) is being investigated as a possible rock to sequester atmospheric CO2. Serpentine is a mineral and serpentinite is a rock that have many personalities.

Harsh Soils

Now, the physical characteristics of mine waste rock on the former Munro Mine area can create an habitat that is too “harsh” for plants to grow.

Some define harsh soil as having too much, or too little, of something essential for plant growth. Excess, such a soil nickel, may lead to plant toxicity. “Too little”, such as the absence of nitrogen, may lead to nutrient deficiency that will also negatively affect a plant. That definition works well for plant nutrients, but how does it apply to physical characteristics of the plant substrate? Well, “harsh soil” can also be described as being shallow soil with poor structure or texture, or even the absence of soil.

We would not consider rich, dark, moist soil on the floor of a deciduous forest as being harsh. Conversely, we would likely consider a substrate to be “harsh” if it consisted of fist-sized rock cobbles packed against each other in a way that left lots of spaces between the cobbles. We would describe the cobble pile as having high porosity, meaning many spaces between the cobbles, AND high permeability, meaning the spaces are interconnected. We would likely say the cobble habitat was really “harsh” if there was absolutely no silt- and clay-sized soil particles or organic material between the cobbles. Such a cobble substrate offers little in the way of nutrients for plant roots or for soil fungi mycelium needed to support healthy plant growth. One example of this type of cobble habitat, where virtually no organic soil exists between the cobbles, exists by the Montreal River Provincial Park (Ontario, Canada), where an old, raised cobble beach stands about 30 m (100 feet) above the present shore of Lake Superior. Few trees or other plants grow on much of the raised cobble beach (Photo 10).

A second “harsh soil” example occurs on the old Munro Mine property (Photo 11) where cobble-sized crushed rock occurs as a waste dump. On this rock dump, there is virtually no fine-grained or organic soil between the crushed rock fragments. Nutrient level is low and moisture retention is poor. Interestingly, where some trees have taken root (Photo 12), it is where a thin veneer of fine grained, crushed mine waste rock was dumped and duff has accumulated. That fine grained, organic-rich veneer of material provides some nutrients for plant roots and soil fungi.

So, from a geologist’s perspective, coarse-grained, crushed mine waste rock is a “harsh” place for plants to grow, regardless of the composition of the cobbles.

Really Harsh Soils

Now, recall when we discussed the definition of the phrase “harsh soil”, we recognized that some “harsh” soils can have too much, or too little, of something essential for plant growth. That harshness can come from the chemistry of the substrate soil. A really good example of this condition is seen on the hill of finely crushed waste serpentine rock at the former Munro Mine site (Photo 7). Very little vegetation grows on that finely crushed serpentine waste hill. The lack of vegetation cannot simply be attributed to high porosity and permeability of the hill, because the hill consists of finely crushed waste rock. So, we have to appeal to something else, like the chemical characteristics of the finely crushed serpentinite waste rock, to explain the lack of vegetation. The rock chemistry creates an habitat that is too harsh for most plants to grow in abundance. Recall that I am not a botanist, so what I say is through the eyes of a geologist.

Chemistry of Ultramafic Rock and Plants

Recall that serpentinite is a modified ultramafic rock. Ultramafic rock has an unusual chemistry: a) high concentrations of magnesium and the metals nickel, copper, cobalt, and cadmium; b) very low concentrations of calcium and very high concentrations of magnesium so that the substrate has a very low calcium to magnesium ration, in chemical terms; and c) virtually no concentrations of nitrogen, phosphorous or potassium. Nitrogen, phosphorous and potassium are essential plant nutrients. Calcium is needed for cell walls and cell membranes, but when faced with substrates having a very low calcium to magnesium ratio, such as on serpentinite substrates, plants do not do well. In addition, high concentrations of the metals nickel, copper, cobalt, and cadmium are poisonous to many plants. So, serpentinite, and derived soils, have a nutrient imbalance from a plant perspective. Simply, serpentinite rock and its derived soils, including finely crushed serpentinite, is a really “harsh” substrate for plants, except for a few specially adapted serpentine endemic plants.

When I look at the surface of the fine-grained serpentinite waste rock hill (Photo 13), I am struck by the lack of vegetation, despite the fact that the Munro Mine closed 37 years ago, which is a long time for plants to gain a root-hold on the material.

Of the sparse vegetation does does eek out an existence on the hill, the trees are stunted and consist predominantly of balsam poplar (Populus balsamifera; photo 14), accompanied by rare paper birch (Betula papyrifera; Photo 14), and rare white spruce (Picea glauca; photo 15). I also saw one occurrence of bearberry (Arctostaphylos uva-ursi; photo 16) and one occurrence of the orchid broad-leaved helleborine (Epipactis helleborine; photo 17).

Summary

It is interesting that this hill of finely crushed serpentine waste rock has been inactive and exposed for more than 35 years, yet the density of plant growth is low and the few plants that are present appear to be stunted. Few trees reached more than 1 metre in height (Photo 18). Weather likely contributes somewhat to the poor plant growth on the hill and some of the white spruce trees appear to show shape features associated with Krummholz trees - a type of stunted, deformed vegetation encountered in the subarctic and subalpine tree line landscapes, where the tree form is shaped by continual exposure to fierce, freezing winds. Yet, the virtual absence vegetation, even at the base elevation of the hill, strongly suggests the chemistry of the finely crushed serpentinite waste rock is the more important factor that discourages plant growth.

So, from a geologist’s perspective, the finely crushed serpentinite rock dump is a “really harsh” place for plants to grow.

Do You Have A Question of Comment?

Nov 13/21