Geology of the Tofino Botanical Gardens
By Jacqueline Windh, PhD
The west coast of Vancouver Island is, in the geological sense, a very dynamic place. It is the zone of convergence between two giant tectonic plates, the Pacific oceanic plate (including the smaller Juan de Fuca plate) and the North American continental plate. Oceanic plates are thinner and heavier than continental plates – that is why they sit lower and are covered with water. When they collide with lighter continental plates, they slide under and sink, forming a subduction zone.
The intense compression between the plates has caused uplift, forming mountains throughout the province of British Columbia and as far east as the Rocky Mountains in western Alberta. Melting of the subducting oceanic plate causes magma to rise, forming volcanoes that were once active in BC, and that are still active today in Washington and Oregon. As the plates push and grind against one another, they get stuck and then suddenly slip, causing the earthquakes that are typical of the entire Pacific Rim. Mountain ranges, active volcanoes and earthquakes are all part of the geological excitement you can experience out here on the plate margin.
200 million years ago, Canada’s west coast was somewhere around Calgary. All of the rocks that form British Columbia have been added to North America’s west coast since that time. As oceanic plates under the Pacific Ocean subducted under the North American continent, the muddy oceanic sediments resting on them and any volcanic islands stuck on top of them could not subduct. Instead, they got scraped off of the plate and smeared out and stuck on to the western edge of the continent. Some of the rocks in BC’s interior actually formed as oceanic islands up to 2000 km offshore, and contain marine fossils typical of creatures that lived in Asia hundreds of millions of years ago. Every time an island crashed into North America, the edge of the continent grew outward a little bit (accretion), and so British Columbia gradually came into existence.
Each island chain or patch of sedimentary rock that has accreted is known as a terrane. By 100 million years ago, Canada’s coastline reached almost to where Vancouver is now. Then a volcanic island chain carried on top of the subducting plate crashed into the coast, adding what is now much of BC’s lower mainland, as well as the Queen Charlotte Islands and Vancouver Island on to the continent. This terrane is known as Wrangellia.
The last terrane to hit the continent was the Pacific Rim Terrane – it arrived here 55 million years ago, and is composed of sedimentary rocks and volcanic rocks that were deposited on the ocean floor between about 200 and 100 million years ago. The entire coastline between Tofino and Ucluelet, including the southern part of Vargas Island, consists of black mudstone and grey sandstone and greywacke of the Pacific Rim Terrane. However, the boundary between it and Wrangellia lies only just to the east of the coast. The location of this ancient giant fault is within sight of both Tofino and the Botanical Gardens, roughly following the Pacific Rim Highway then cutting out across the south-facing forested slopes of Meares Island before heading out to sea.
This geological map of the Tofino area and of Meares Island shows the Pacific Rim Terrane and the various rock types that comprise Wrangellia.
The Ice Ages
About 2 million years ago, a series of ice ages began. This period of time is called the Pleistocene Epoch. The causes for this cool period are numerous, and include changing ocean current patterns due to continental drift, as well as continents drifting closer to the poles allowing ice to accumulate. Other causes are cyclical changes in the shape of the Earth’s orbit, affecting how cold winters are (and how much snow accumulates) and how hot summers are (and whether all of the winter’s snow melts). Interestingly, once an ice age starts, it encourages itself; as more snow accumulates, the white icecaps reflect more of the Sun’s heat, causing the Earth to cool even more. (Similarly, global warming is causing the polar ice caps to melt back, so more heat is absorbed by the Earth and the globe warms up even faster).
During the last ice age, the ice caps in central BC were as much as 2 km thick. At the end of the ice age, the glaciers retreated by simply melting away, leaving us here at sea-level about 13,000 years ago, and leaving most of the higher country by about 10,000 years ago. Shells of an arctic species of mussel that no longer lives in this area were found on Vargas Island (straight across from Tofino) and have been carbon-dated to 13,000 years old.
At the end of an ice age, the water melting from the glaciers returns to the oceans and causes sea-level to rise. However, the land-level also rises as the Earth’s crust relaxes from releasing the weight of all of that ice. In this area, sand from former beachlines is found well back into the forests, for example at Chesterman’s Beach and on Vargas Island. This shows that the shorelines were recently higher up in the forest, and indicates that here, the land is rising faster than is sea-level.
The most recent ice age has left many marks on Vancouver Island’s landscape. One is rock surfaces scraped smooth by the grinding ice. This can be seen on the shoreline here at the Gardens (stop 5). Another good place to see this is on the Tofino-Port Alberni Road, where some of the steep smooth cliff faces alongside the road show horizontal grooves, scratched into the rock as the glaciers flowed down the Kennedy River valley. Glacial deposits include clay, which is fine-grained rock flour that settled out of the glacial meltwater in quiet lakes, and glacial till, which is a mixture of clay, gravel, sand, and rounded pebbles and boulders. The best way to recognize till from any other sandy kind of deposit is the presence of pebbles and boulders: they are always rounded by the glacial action, and they include a huge assortment of different rock types gathered up along the glacier’s journey. There are some good exposures of glacial till along the edge of the driveway and parking lot, right across from the Cafe Pamplona (stop 2).
A Geological Tour of the Gardens
Stop 1. (across from the Café Pamplona entrance, outcrop near the orange shed)
This is outcrop is a mixture of mudstone and greywacke. (You can see the best contrast between the black mudstone and pale grey greywacke at the left end of the outcrop, away from the orange shed). “Mudstone” is pretty self-explanatory – it is a rock made out of mud, and it is the very fine-grained black rock you see here. Greywacke is a rock deposited in the deep ocean. It consists of a mixture of grain sizes (unlike mudstone or sandstone, which are formed of grains all of one size), and usually the grains are angular (unlike till) and of many different rock types (indicating a large source area for the grains). Greywacke is formed by giant submarine mudslides, which flow like avalanches, gathering up material from a huge area of the continental shelf and swooshing it down to the deep ocean floor. These mudslides are huge catastrophic events, perhaps occurring only every few thousands of years, and are set off by major disturbances such as earthquakes or giant winter storm waves.
Stop 2. (immediately S of previous stop, on edge of parking lot by bike rack)
This is not an outcrop of rock, rather it is an exposure of glacial till. The till is composed of all sorts of material: fine-grained mud and clay, sand, and pebbles mixed in. If this deposit had formed under water, like greywacke does, the bigger, heavier pieces would have sunk faster and would be at the bottom of the bed. The mud would be the last to settle out and would be at the very top. But here, the pebbles and sand and mud are mixed evenly throughout, a characteristic of glacial till. The till here has a orange colour because the high rainfall here is turning iron minerals into rust, staining the till.
Note at this location and the previous outcrop how thin the organic material of the forest floor is. In terms of Earth’s 4,500,000,000 year history, the 13,000 years since the end of the last ice age is a very short time. It took a long time for lichens and mosses and grasses to colonize the denuded and barren landscape, and even longer for there to be enough organic material to for the trees and ferns of the rainforest to survive. In most parts of the forest, this organic layer (which took over 10,000 years to grow) is only a foot or so thick, and underneath is bare rock or barren till. This is why the coastal rainforest here is so sensitive to logging, and why it is impossible to replant a rainforest after it has been logged.
Stop 3. (in herb garden, near doves’ cage)
Here is another outcrop of swirly mixed mudstone and greywacke. In the rockface you can see thin streaky lines in the rock that go down towards the left, and grey quartz veins which go down to the right. The streaky lines are called “foliation”, and are a result of the rock being squished, or deformed. This deformation probably occurred as the Pacific Rim Terrane was accreted on to the continent. The quartz veins are deposits of quartz that formed in fractures in the rock. The quartz was dissolved in hot water that passed through the rock when this rock was still at great depth, many kilometres below the present surface, and the quartz minerals slowly deposited and filled the fractures. You can tell that quartz is harder than the surrounding rock because it sticks out; the surrounding rock has eroded faster than the quartz veins.
Some veins formed in fractures that simply opened up (extensional fractures). Other veins form in fractures that slid, like mini-faults (shear fractures). If you look carefully at how the veins cut the foliation, you can see that these veins are shear fractures – they offset or move the foliation lines.
Stop 3 Quartz veins which crosscut and displace foliation in the mudstone and greywacke (e.g. to left of coin).
Stop 4. (in forest, on trail near tropical garden)
It is not always easy for a geologist to say what the name of a rock is. This rock almost definitely was a mixture of mudstone and greywacke, since that is what all of the surrounding rocks here are. But this rock has been affected by alteration. Alteration is a process, usually involving heat, hot fluids deep in the Earth’s crust, and pressure, which changes the composition of the rock. The hot fluids, often at temperatures of several hundred degrees Celsius (hydrothermal fluids), pass through the rock along fractures and through microscopic pores, dissolving some of the original minerals and adding other new ones.
Stop 5. (at end of boardwalk, on shoreline; note, this outcrop might not be accessible at high tides)
This is a quite massive outcrop of mudstone and greywacke. It is not particular altered and does not show any foliation like the previous outcrops. It is just plain old Pacific Rim Terrane. Many of these submarine sedimentary rocks are quite boring, and this outcrop is a good example of that.
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