Lecture 10
Introduction to Canada’s Geologic Journey
Focus on the last 4.5 billion years of geological history
Lecture divided into 5 parts, starting with the Precambrian period.
Table of Contents
Overview of topics covered:
Introduction
Learning Outcomes
Timeline
Module: The Canadian Shield
Virtual Field Trips exploring the Canadian Shield and its regions.
The Hadean
In the very beginning:
A “magma ocean” undergoing bombardment by meteorites
Segregation of elements by weights
Heavy things like metal would go down near the core, and light things like silicate would go to the surface
Outgassing of volatiles to make the atmosphere
Initially would be dense, but then would condense into ocean
Crust absorbs water if it makes contact with ocean
The first continent was a small volcano arc
Oceanic crust + water = continental crust
Magma that comes into contact with water would become wet magma
Form andesite and granite (both silica-rich)
Andesite makes crust that's a little too thick and makes it hard to subduct
Magma evolves from its parent rock each time it melts and recrystallizes
Basalt + water → andesite + partial melting → granite
All of these evolutions are lowering density and thickening crust
Oldest rocks ~ 4.03 billion years ago
Gneiss tells us that there is already a functioning ‘rock cycle’
Shale → slate → phyllite → shist → gneiss
Outline of the Precambrian Period
Characteristics of the Precambrian:
Lack of oceans, granites, and continents early in the period.
Craton: The ancient core of a continent (stable landmass).
Cratonization: Process of continent growth.
Provinces and Terranes: Distinct geological regions comprised of old crust embedded in current continents.
Orogeny: The process of mountain building, resulting in orogens (folded/metamorphosed rocks).
Greenstone Belts: Rich mineral formations indicative of gold (metamorphosed basalts).
Ophiolites: Remnants of ancient oceanic crust.
Grenville Orogeny: Key geological event occurring around 1 billion years ago.
Canadian Shield: A significant geological formation that is the bedrock of much of Canada.
Stromatolites: Evidence of ancient life, formed by bacterial colonies.
Ediacaran Fauna: Multicullular life forms appearing prior to the Cambrian explosion (540 million years ago).
Gneiss: High-grade metamorphic rocks commonly found in the Shield.
Geologic Time Scale Overview
Introduction to the Phanerozoic Eon and its divisions:
Quaternary (0-2 Ma): Current geologic time.
Tertiary (2-65 Ma): Includes the Paleogene and Neogene periods.
Cretaceous (K) (65-145 Ma): Period of dinosaur dominance.
Mesozoic and Paleozoic Era: Key timeframes for the development of early life.
Tracking the approximate ages of various geological periods aids in understanding Earth's history.
Geological Layers Below the Lecture Room
Precambrian Basement (> 1000 Ma): Foundation layer consisting of ancient rocks.
Phanerozoic Cover Rocks (600 Ma - 2 Ma): Surface layer containing younger sedimentary rocks.
Glacial Sediments (< 2 Ma): Related to more recent glacial activity.
'Built' Landscape and Historic Fill Materials: Recent construction and anthropogenic changes.
Early Earth Conditions - Hadean Eon (4.567 Ga)
Magma Ocean: Initial state of Earth, being bombarded by meteorites.
Differentiation: Formation of the Earth's layers based on elemental weight.
Outgassing: Emission of volatiles that contributed to the formation of the atmosphere and water bodies.
Subduction: Initiating when crust begins to fracture, leading to geological processes that shape the planet.
Formation of Continental Crust
Continental Crust Origins:
Created from volcanic arcs at convergent margins (rich in silica).
Resulting crust composition includes andesite and granite.
Divergent Margins: Involves the formation of basalt and gabbro in oceanic crust, leading to continental crust formation with the presence of water.
The original North American continent, Artica, which started to form about 2.5 billion years ago from smaller continents and was completed by about 1.9 billion years ago when old Archean cratons (e.g., Slave, Nain provinces) were weilded together by the Trans-Hudson Orogen and others.
Added to the North American continent during the formation of Nena about 1.8 billion years ago after the Penokean Orogeny.
Added during the formation of Rodinia about 1.3 billion years ago during the Greenville Orogeny.
Added during the formation of Pangea about 600 million - 300 million years ago.
Added after the breakup of Pangea about 250 million years ago.
Development of the Lithosphere
Lithosphere: Rigid outer layer of Earth containing continental crust above sea level.
The process includes erosion and sedimentation into basins as continental crust is created and then subjected to geological processes.
Magma Evolution
Partial Melting of Basalt: Leads to the creation of andesite and granite as magma evolves with each melting and recrystallization cycle.
Crust Thickening: Results in lower density rocks becoming dominant in continental crust formation.
Acasta Gneiss - Ancient Crustal Record
Acasta Gneiss (NWT): One of the oldest known rock formations, dating back to approximately 4.03 billion years, signifies early continental crust formation.
Geological Record of Metamorphism
Gneiss Characteristics: Indicates the presence of a functional rock cycle with increasing metamorphism.
Anatomy of Continents
Continental Structure: Cratons from the Archean and Proterozoic surrounded by younger geological formations, signaling the expansion and growth of continents.
Snowman Analogy for Continental Growth
Analogous representation of how terrane accretion contributes to the growth of continental landforms over geological time.This process illustrates the way in which smaller landmasses come together, much like building a snowman, resulting in the larger, more complex structures we see today.
Continents grow by collecting terrane overtime (largest snowball)
Sometimes those cratons collide to form a supercontinent (snowballs stacked on top of each other)
Cover rocks are sediments that accumulate over the continents (snowball decorations)
Possible that it contributes to continental growth
Paleozoic cover rocks drape over the carton
Terrane Accretion
Definition: Incremental growth of continents through the assemblage of fragments and geological formations over time, illustrating regionally specific tectonic interactions.
The Supercontinent Cycle - J. Tuzo Wilson
A conceptual framework to explore Earth’s historical geological events, understanding the dynamic interactions among tectonic plates and continental formation.
Geology of North America Overview
Contextualization of major geographic and geological features across North America, focusing on the significance of orogenic events in shaping the continent's structure over time.
Stages
Stage 0: Orginal Cratons > 2.5 Ga
Stage 1: Paleoproterozoic Tectonic Activity (2.5 - 1.6 Ga) - This stage marks the formation of the earliest continental landmasses, characterized by the stabilization of cratons and the development of greenstone belts.
All these cratons/provinces (at least most of them) had to have formed before 2.5 million years ago
Form form accretion process (this landforms is called Granite-Greenstone Belts)
Accretion creates these cratons that consist of alternating band of volcanic stuff
In between these cratons, there slivers of ocean floor.
Granite-Greenstone Belts: Basically, bands of ocean floor separated by bands of arc (volcanic) material.
Stage 1 & 2: Arctica and Nena And the Trans Hudson Orogen
Small pieces of content come together to form a first supercontinent (called Arctica) around 2 billion years ago
It’s basically a big mountain belt using 2 continents
Superior craton collided with the Chruchill craton to create the Trans-Hudson orgeny
Happened again 1.8 billion years ago, when another supercontinent formed, known as Rodinia, as various landmasses continued to merge due to tectonic forces, significantly altering the geography of the Earth.
Artica breaks up in between 2 billion and comes back 1.8 billion years ago
Lots of obduction happens
Obduction: A geological process where oceanic crust is thrust over continental crust, often resulting in the formation of mountain ranges and significant changes in the lithosphere.
What we now call Labrador was added to Canada just after 2 billion years ago
Metorite impacts hid Sudbury 1.8 billion years ao
Bolide: large meteor, 10-15KM in diameter
Fun Fact: Sudbury doesn’t have a lof nickel because it came from that large meteor, the meteor made a lot of hydrothermal circulation in the local rocks around it and ultimately changed the structure of them.
Stage 3
Rodinia is another supercontinent
Came from South America colliding into North America
Although we don’t know exactly what it looked like orogenic belts are similar in age prove that it existed
Structure of the Greenville Orogen
Laurentia (aka North America) collides with Amazonia (aka South America) and Amazonia gets thrust up over Laurentia.
Greenville Orogeny: The collision of Laurentia with South America (from supercontinent Rodinia)
Himilayen Orogeny: A modern analog for the Greenville progeny (basically, Greenville is the eroded version of the Himalayan mountains we know today)
Index minerals: metamorphic minerals that can act as paleothermometers (basically, we can use those metamorphic rocks to provide an estimate of what the temperature was like when those natural minerals formed)
Garnet: index minerals that forms at depths over 25km
Mylonite: the product of intense shear
Migmatite: partial melting of gneiss
Central Metasedimentary Belt Boundary Zone (CMBBZ): a really old big fault that still makes earthquakes
Stromatolites: layers of bacteria (from organisms 3 billion years ago)
The ‘Cambrian Explosion’
Rapid diversification
Hard-bodied animals
First appearance of organisms we evolved from
Burgess Shale has a lot of organisms there
Definitions: Shield vs. Craton
Shield: A landform presenting exposed bedrock.
Craton: A geological formation comprising ancient rocks older than 600 million years, with Paleozoic cover rocks overlaying it.