Course:EOSC311/2024/How Vancouver’s Geological Features Affect the City’s Geography

Geology and geography are two sciences that are deeply intertwined. Geology is the study of the earth and the forces acting upon the land. Geography is the study of the relationship between people and their environment. By analyzing Vancouver’s geology, we can understand why and how the city’s geography developed the way it did. We can also be aware of what natural disasters the city is in risk of and implement measures to mitigate hazards.

Vancouver's Geology

Figure 1: Geomap Vancouver

Physiology

Vancouver is surrounded by water and mountains. To the north is the Coast Mountains, to the east is the Cascade Mountains, to the south is the Juan de Fuca Strait, and to the west is the Strait of Georgia. Greater Vancouver is made of three distinct geological areas: the Fraser Valley uplands, the Fraser River lowlands, and mountains. The Fraser Valley uplands is where the city of Vancouver is located and ranges from 15 metres to 250 metres above sea level. The uplands contain the sediments silt, clay, sand, gravel, and till that were deposited during the Ice Age in the Pleistocene Epoch 2 million to 11,000 years ago. The Fraser River lowlands are flat and only 15 metres above sea level, making the area prone to flooding. Sediments deposited here are silt, clay, peat, sand, and gravel that are considered modern because they were deposited during the Holocene, less than 10,000 years ago. There are two mountain ranges surrounding Vancouver: the Coast Mountains and the Cascade Mountains. The Coast Mountains extend 1700 kilometres from Fraser Valley to Alaska and are made of solid bedrock consisting of granite, sandstone, and volcanic rock. The mountains closer to Vancouver are made of 100 million year old granite while the mountains made of sandstone are younger at 85 to 35 millions years old. Originally created through compression heating, uplift, and subduction, today's features of the Coast Mountains were created by erosion. Streams and rivers eroded canyons and valleys into the mountains, and glaciers and ice sheets further modified these features to form fjords. The Cascade Mountains are also made of solid bedrock and consist of metamorphosed sedimentary and volcanic rock from the Mesozoic and Paleozoic ages.

Figure 2: Vancouver foundation cross section

Rock Foundation

The Coast and Cascade Mountains are made of the five major rock types found in Vancouver: granitic, metamorphic, sedimentary, volcanic intrusions, and younger volcanic rocks. Granitic rocks are formed from the cooling and crystallization of magma (melted rock) below the surface. Millions of years of uplift and erosion brought the granitic rocks to the surface to form mountains like Stawamus Chief in Squamish. Metamorphic rocks are formed when high heat and pressure from tectonic plates colliding causes deformation and recrystallization of crustal rocks into new metamorphic rocks. Located in the Coast Mountains, Whistler's foundation is made of 100-200 million year old metamorphic rock that has been shaped by glaciers and rivers to form the current landscape. The town has two types of metamorphic rock: granitic and mica-rich. Blackcomb Mountain consists of metamorphosed granitic rocks that are rich in quartz and feldspar while the foundation of Whistler is built on mica-rich metamorphic rock that is recognizable by shifts: flake-like fragments at the base of a cliff. Sedimentary rocks are formed when mountain sediments are buried in basins. As seen in Figure 2, the foundation of Stanley Park to the Fraser River is made of the sedimentary rocks sandstone and shale. Much of the sandstone at Stanley Park has been eroded away, making it easy to identify Prospect Point and Siwash Rock as being made of erosion-resistant volcanic rock. Volcanic intrusions are created when magma erupts and cools below earth's surface. When magma erupts above the surface, volcanoes like Mount Garibaldi and Mount Baker are built. Located in Squamish, Mount Garibaldi was formed by volcanic eruptions 15 to 20 thousand years ago when the valley was filled by a large glacier. While Mount Baker was also formed by volcanic eruptions, the eruptions occurred over the last 40,000 years. Mount Baker is an active volcano with the last eruption recorded during the 1800s.

Why Did People Settle in Vancouver?

Prior to European settlement, the area that is now Vancouver was inhabited by the Squamish, Musqueam, and Tsleil-Waututh First Nations, their ancestors having settled in the region over 8,000 years ago. Their close proximity to the coast led the Indigenous peoples to rely on salmon as their main food source, but they also ate other fish, shellfish, sea and land mammals, plants, and berries. The abundance of salmon and the ability to preserve it through drying or smoking allowed them to live in larger social groups and develop wealth, complex social organization, and elaborate cultural practices. The first European to make contact with the Vancouver region was Spanish explorer José Maria Narváez in 1791, but it wasn't until 1827 when the Hudson's Bay Company built Fort Langley that the first permanent European settlement was established. Originally a settlement called Granville, Vancouver got its name and city status in 1886 when it became the western terminus station of the Canadian Pacific Railway (CPR). The CPR extended the railway to Vancouver for a better harbour and by 1914 after the opening of the Panama Canal, the city was a prosperous port finding success in shipping grain and lumber to the United States and Europe.

Figure 3: Gold vein in quartz and granite

Other than Vancouver's advantageous location near the water, the city attracted an influx of population due to the gold rush. Vancouver experienced two gold rushes, the Cariboo Gold Rush in the1860s, and the Klondike Gold Rush the late 1890s. The city's luck with gold is the result of gold deposits in the province. British Columbia has three types of gold deposits: porphyries, Volcanogenic Massive Sulphides (VMS), and veins. Porphyry deposits are found at subduction zones of oceanic and continental plates. As the oceanic plate is dragged downwards, fluid is released and melts the mantle wedge to create magma. The magma migrates upwards to the crust and either erupts as lava, crystallizes to form plutons, or transforms into porphyry ore deposits. Porphyry deposits are the main source of gold in B.C. most likely because the province resides on the Cascadia Subduction Zone where the oceanic Juan de Fuca Plate is subducting under the continental North American Plate. VMS deposits form at or near the seafloor in submarine volcanic environments as a result of hot, metal-rich hydrothermal fluids discharged from seafloor hydrothermal convection. The ability to harvest gold from VMS deposits can once again be contributed to B.C.'s location because VMS deposits commonly occur in settings with tectonic activity. Vein deposits are a type of mineral deposit that form in the fractures or fissures of rocks and normally occur near fault zones where rocks are cracked and deformed. Historically, the main sources of gold in B.C. were found in epithermal and mesothermal vein deposits. Epithermal vein deposits are formed from hot, mineral-rich fluids that rose from deep in the crust but are located in the uppermost part of the crust. Mesothermal vein deposits are similar to epithermal vein deposits but form at higher temperatures and pressures and are located deeper in the crust.

Natural Disasters

Earthquakes

Figure 4: Vancouver's tectonic plates

Earthquakes occur when tectonic plates slide past or underneath one another, releasing energy and stress which causes the ground to shake and move. They are one of the major natural disasters Vancouver is in risk of due to the city's location in an active earthquake region. Vancouver is in proximity to three tectonic plates: the North American Plate, the Juan de Fuca Plate, and the Pacific Plate. The city resides on top of the North American Plate and near the Cascadia Subduction Zone: the boundary where the Juan de Fuca Plate is subducting below the North American Plate. This movement can cause massive 9.0 magnitude earthquakes called megathrust earthquakes. The region has been anticipating a megathrust earthquake for a while now, but there is no way of telling when it will occur. A less destructive and more frequent type of earthquake caused by the subduction are deep intraslab earthquakes, These earthquake occur deep in the Juan de Fuca Plate as it gets further below the Strait of Georgia and the mainland coast. A type of earthquake that poses high risk to Vancouver are shallow crustal earthquakes. These earthquakes are frequent, highly destructive, and since they happen not at plate boundaries but near the surface, occur closer to the city.

With the possibility of an earthquake happening any time, the city of Vancouver has taken measures to prepare for earthquake impacts. In 2014, the Earthquake Preparedness Strategy was created to be able to respond to an earthquake whenever, address risks, and take actions to minimize impacts. Further strategies have been introduced to address building collapse which is a major concern for certain areas like the Fraser Delta because of the foundation of silt and sand. While such a foundation is usually suitable for large buildings, it becomes prone to liquefaction when saturated with water. Liquefaction occurs when loose water-saturated silts and sands at shallow depth transform into a liquid due to the vibrations from earthquakes, causing structures built on top to fall over. The city of Vancouver conducted a simulated model of a 7.3 magnitude earthquake and found that over 150 buildings were likely to collapse with older buildings made of concrete, brick, and wood being the most severely damaged. More than 10,000 buildings would be unusable for an extended period of time and over 4,000 of those buildings would need to be demolished, leaving almost half of the city's population displaced. British Columbia's Building Code implements an earthquake resistant building design that considers soil type, building height, materials, and the seismic hazard of the site when designing a building's structural system and foundation. This ensures that the building's structural system is strong enough to withstand earthquake forces but also flexible enough to respond to ground motion without losing its strength. It is important to note that structures built under this code are not meant to be earthquake-proof, but rather meant to survive moderate earthquakes without structural damage and not collapse during major earthquakes so that there is enough time for people to escape. In areas at higher risk of experiencing earthquakes, structural design is based on anticipated peak horizontal ground acceleration and velocity value which estimates how quickly ground motion decreases based on distance from an earthquake. Ground motion is the vibration during an earthquake that causes damage, liquefaction, and landslides.

Landslides

Landslides are the downward movement of water and solid earth due to gravity and slope failure. They are triggered by seismic activity like earthquakes and volcanoes but mostly by an increase in water supply. Most landslides occur during the wet season when snow melts because there is an increase of groundwater levels, soil moisture, and surface flows. Human development can also play a part in the likelihood of landslides happening; water concentration or diversion by roads and loss of root strength due to logging are common causes. In Vancouver, areas with steep buffs and slopes of are at risk of landslides because slopes affect drainage and stability. Surface draining improves at steeper slopes while stability worsens. Most landslides in Vancouver occur

References

This Earth Science resource was created by Course:EOSC311.