People and Planet Earth 1. Plate Tectonics: Shifting Continents and Changing Geographies

This series of lectures focuses on environmental history –our changing relationship with our environment But before we do this we need perspective - a long-term (geologic) context We need to know what’s normal before we evaluate our impacts

A few weeks ago the world’s oldest fossils were identified – 3 A few weeks ago the world’s oldest fossils were identified – 3.8 billion years old, from northern Quebec. Single celled lifeforms evolved around hydrothermal vents.

Lifeforms such as these are called extremophiles Lifeforms such as these are called extremophiles. They persist today in a range of severe environments – in very hot, dry, cold, and chemically harsh environments. Their history is long, but ours is short. Our genus Homo evolved about 3 MY ago. Our species, Homo sapiens, only 200,000 years ago.

Our impact on environment was limited until about 12,000 years ago when we started to domesticate plants and animals. Our history and that of all life has been ultimately determined by two major natural constraints – internal convective processes (plate tectonics) and the supply and processing of external energy (solar radiation).

The Earth is special, at least in this solar system The Earth is special, at least in this solar system. It’s useful to consider how and why.

How and why is the Earth different from the other planets? 1.It has an atmosphere composed predominantly of nitrogen (78%) and oxygen (20%). It produces a modest greenhouse effect because of small, but significant amounts of carbon dioxide, methane and other gasses. How stable is this composition? How much has it changed through time? What are the causes and consequences of changing atmospheric chemistry?

2. It has lots of water (a hydrosphere) 2.It has lots of water (a hydrosphere). This water can occur as liquid, solid and gas and readily exchanges between the three states. Energy exchanges accompany changes in state (e.g. hurricanes as power plants) . At present 70% of the Earth’s surface is water-covered. How has this changed through time? What roles do oceans play in driving environmental change?

3.The Earth has a biosphere. Much of it is microbial. How does the biosphere interact with climate? How does it contribute to the greenhouse effect? How has the biosphere changed through time and why?

4. The Earth is dynamic and mobile, a remnant of its origins 4.The Earth is dynamic and mobile, a remnant of its origins. The processes are all part of plate tectonics. The rock record tells us that PT has been going on for perhaps 4 billion of the Earth’s 4.5 billion year history.

The Earth has been called ‘The Goldilocks Planet’ –not too hot, not too cold, not too wet, not too dry; just right. The key is the availability of water and its ability to exist in liquid, gaseous and solid forms.

The Earth characteristics outlined above are products of internal and external energy. Internal energy reflects the Earth’s origins. That energy operates through plate tectonics. Atmospheric and surface terrestrial processes are driven by external energy; solar radiation. Today we’ll look at internal energy.

Plate tectonics What is plate tectonics? Why does it happen? How long has it been going on? How do we know about the pattern and speed of the process? What are its impacts?

Internal processes

Tectonic plates

Building blocks

The tectonics of North America

Paleomagnetism and seafloor spreading

Patterns in the Past Although we focus on the current ‘cycle’, we know that the process has been going on ever since the Earth started to develop a crust – about 4 billion years ago. From that time a series of supercontinents have been formed and dismantled. Present movement reflects the dispersal of the supercontinent Pangaea over the last 250 million years.

Former supercontinents

Pangaea/Pangea – the last agglomeration

50 million years ago

The world 250 million years ahead

What impact does PT have on Earth systems? It changes global geography by the agglomeration and dispersal of continental masses. It produces distinctive global patterns in the distribution of mountains, volcanoes and earthquakes. It exerts a major control on global climate on a range of time scales. It is responsible for north-south peculiarities in flora and fauna and the behaviour of island ecosystems.

Earthquakes and Volcanoes

Mountains and plate tectonics

India and the Himalayas

Island chains and arcs

Hawaiian hotspot

Plate tectonics and climate; PT influences climate by (a) changing the locations of continents, (b) changing their topographies and by (c) influencing atmospheric chemistry. Continental location and topography determine atmospheric circulation. They also exert a major control on oceanic circulation .

Oceanic circulation today and 100 million years ago

PT and the Greenhouse Effect; In the long term, PT controls global climate by determining the size of the Earth’s Greenhouse Effect. This is always around and is NOT a human artifact. Today it’s modest; about 30C. PT does this by its control of the carbon cycle. Most of the Earth’s CO2 is stored in rocks. PT cause C02 to be released or stored.

Climate over the last 600 MY

On one or two occasions the Earth may have been completely glaciated (Snowball Earth)

Short term impacts of PT on climate Large single volcanic eruptions can cause significant cooling events. The largest historical volcanic eruption is that of Tambora in 1815. The global dust veil reduced mean temperature in 1816, ‘The Year Without Summer’. That event caused considerable loss of life, mostly from starvation.

Mt. Pinatubo, 1991

Spread of stratospheric dust from Pinatubo

Global temperature changes

Threats to life Volcanic eruptions may cause major loss of life through lava flows and ash falls. On volcanoes with ice caps there may be rapid flows of water and ash (lahars and jokulhaups). Large volcanic eruptions and earthquakes can trigger tsunamis.

Eruption in Iceland

Jokulhaup, Iceland

Pompeii and Vesuvius 79AD

Earthquakes and tsunamis – Japan, 2011

Large eruptions may have huge societal impacts – the demise of the Minoan civilization may have been caused by the eruption of Thera (Santorini) about 3500 years ago.

The caldera of Thera (above). The buried city of Akrotiri (right)

PT and biogeography Plate tectonics plays a major role in determining global biogeography Much of the Earth’s fauna and fauna have evolved over the last 65 million years (after the major extinction event at the Cretaceous-Tertiary boundary) Although our biosphere is being ‘globalized’ there are still some obvious regional differences that can be attributed to plate tectonics

The northern and southern hemispheres have some distinct biological differences. In general the NH is more modern. The SH retains many ancient elements. e.g. Marsupials and monotremes Gymnosperms and angiosperms Ratites

Large flightless birds, the ratites

Kiwi, ostrich and moa

Isolated islands and their biotas Perhaps the most distinctive biological impact of plate tectonics is related to oceanic islands and their unusual flora and fauna. Most oceanic islands are volcanic. The more remote ones occur over hotspots. Island biotas are peculiar and very vulnerable. More later.

The Galapagos Islands

Marine iguana (endemic)

Flightless cormorant(endemic)

Galapagos penguin (endemic)

Lonesome George - RIP 2012

PT, human evolution and our global diaspora In our third lecture, we’ll see how climatic changes driven by PT constrained human evolution, and our movements out of Africa and across the globe.

Next week we’ll take a look at the external energy (solar radiation) and its effects on Earth systems If you have a need for clarification, more discussion, etc. feel free to contact me at davis@geog.utoronto.ca