Download Tony Davis, LLM Lecture 1 – Plate Techtonics

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 – 3.8
billion years old,
from northern
Quebec. Single
celled lifeforms
evolved around
hydrothermal
vents.
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. 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). 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. 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
[email protected]