Geology and Sea Level Change

Examples of the different volcanic flow units and evidence of past higher sea-levels and possible future sea-level rise impacts



Tim Webster, PhD

Applied Geomatics Research Group

COGS, Middleton

timothy.webster@nscc.ca

825 5475



The North Mountain is comprised of three volcanic flow units and was erupted from fissure volcanoes during the Triassic period, 200 Million Years ago. The three volcanic flow units are quite distinct and have variable resistance to erosion. As a result the morphology (shape) of the North Mountain reflects these differences in erodability of the flow units. The lower flow unit (oldest) is exposed along the south face of the North M Mountain and is quite thick, massive with columnar joints and very resistant to erosion. The resistance to erosion of this unit is in part why we have such a steep slope on the north side. The lower flow unit is overlaid by the Middle Flow unit which is comprised of several thin volcanic flows that are highly vesicular (gas bubbles). These air bubbles have been subsequently in filled with zeolite minerals as a result of ground water circulation through the rocks. Stilbite, Nova Scotia’s mineral, is one such mineral that infill’s these voids in the rocks. Zeolites have a unique crystal structure and grow radially outward. The Middle Flow unit is less resistant to erosion and is often referred to as “rotten rock” by local construction operators. The Upper Flow unit overlies the Middle Flow Units and is similar to the lower flow unit in that it is resistant to erosion and often outcrops along the Bay of Fundy Coast.



The flow units have been mapped with the aid of a new remote sensing technique known as LiDAR, Light Detection & Ranging. We use a laser onboard an aircraft to precisely measure the earth’s topography (lay of the land) to an accuracy of 15 cm in the vertical. The measurements are then used to construct a continuous surface known as a Digital Elevation Model (DEM). We then use these DEM in the computer to better visualize the subtle topographic changes of the land surface. As a result we can see the contacts between the flow units and other interesting landforms including raised terraces along the Bay of Fundy that represent higher sea-levels. After deglaciation 12-15,000 years ago, the sea-level rose faster than the earth’s crust rebounded. Evident of this high sea level is found at several locations along the coast in the form of terraces. Recent tide gauge records from Saint John, NB indicate relative sea-level is rising by 22 cm per century. This is a combination of crustal subsidence and global sea-level rise. We have also used these new LiDAR elevation models to map the town of Annapolis Royal and project possible flood limits based on storm surges and future projections of higher sea-levels. The Groundhog Day storm of 1976 was used a bench mark storm and the flood extent was mapped using the LiDAR DEM. Future sea-level rise projections were then used with this storm to predict what areas are at risk if the storm were to reoccur in the future.



The field trip will begin with a presentation of the sea-level history of the area, followed by an explanation of LiDAR and the improvements to topographic mapping that it provides. Flood maps of Annapolis Royal during the Groundhog Day storm and possible future flooding considering sea-level rise will be shown. Maps of the North Mountain will be displayed and the contact between the flow units identified. In addition we will examine these raised beach terraces that have resulted from past higher sea-levels. We will then depart to visit the different volcanic flow units and contacts in the field and the raised terraces along the coast.


Tim Webster, Ph.D.
Research Scientist, Applied Geomatics Research Group (AGRG)
Centre of Geographic Sciences (COGS), Nova Scotia Community College



Chair, Halifax Branch, Canadian Institute of Geomatics

Adjunct professor, Acadia University, Dept. of Earth and Environmental Sciences

Adjunct professor, Dalhousie University, Dept. of Earth Sciences