The Earth

A100 - Oct. 8
Shaping the
Earth’s Surface
READ Chapter 5 – The Earth
Homework 4 Due Friday
Quiz on Friday
Today’s APOD
The Sun Today
Writing the History of Earth
Surface formations visible today have emerged only very recently
compared to the age of Earth.
Earth’s interior is
hot in the center
(6500K, as hot as
the Sun’s surface)
Heat flows from the core to the surface as Earth slowly
Motion in the Earth’s Interior
Heat generated by radioactive decay in the Earth
creates movement of rock
This movement of material is called convection
Convection occurs because hotter material will be
less dense than its cooler surroundings and
consequently will rise while cooler material sinks
 Convection in the Earth’s interior
 The crust and mantle are solid rock, although when heated,
rock may develop convective motions
 These convective motions are slow, but are the cause of:
earthquakes, volcanoes, the Earth’s magnetic field, and
perhaps the atmosphere itself
Plate Tectonics
Earth’s crust is fractured into more
than a dozen plates
 These plates “float” on top of the Earth’s mantle
 Convection in the mantle cause the plates (and continents)
to move
The shifting of large blocks of the
Earth’s surface is called plate
Early researchers noted that South
America and Africa appeared to fit
together and that the two continents
shared similar fossils
It was later proposed (1912) that all of
the continents were once a single
supercontinent called Pangaea
The Earth’s surface is continually
building up and breaking down over time
scales of millions of years
Earth’s Appearance: Past, Present, & Future
 Plates (and continents) move at 2 cm/year
• that’s 2,000 km in 100 million years
 We can project the motion of continents into the past or future.
• 200 million years ago all continents were connected into one
Total continental area has
increased with time as new
continental crust has formed
Earth’s Tectonic History
Plate Tectonics
Tectonic plates move with respect to each other
Where plates move toward each
other, plates can be pushed
upward and downward → formation
of mountain ranges, some with
volcanic activity, earthquakes
Where plates move away from
each other, molten lava can rise
up from below → volcanic activity
Features of Plate
 Continents are shaped by:
• volcanism
• stresses from plate tectonics
• erosion
 Subduction zones cause:
• volcanic eruptions which form
mountain ranges
• islands to be scraped off
seafloor plates onto
 When two continental plates collide
• one can not subduct under the other
• crust is pushed up to form mountain ranges
• e.g. the Himalayas
Features of
 When two continental plates pull apart
 a rift valley is created
 mantle convection causes eruption of basalt from valley floor
 a new zone of seafloor spreading is created
 e.g Arabian peninsula detached from Africa to form the Red Sea
Features of
 When two plates slip sideways against each other
 rough grinding of plates builds up pressure along the crack
between them
 this crack is called a fault
 pressure eventually breaks, causing a sudden shift, or
Hot, molten material rises from deep in the
Earth’s interior in great, slow plumes that work
their way to the surface
Near the surface, these plumes spread and
drag the surface layers from below
The crust stretches, spreads, and breaks the
surface in a phenomenon called rifting
Cool material sinks,
and drags crustal
pieces together
buckling them
upward into
If one piece of
crust slips under the
other, the process is
called subduction
Geological activity mostly
occurs on plate boundaries
Plate tectonics is the dominant force that
sculpt the Earth’s surface – it also triggers
earthquakes and volcanoes
Volcanism on Earth – Subduction Zones
Volcanism on Earth is commonly found along
subduction zones (e.g., Rocky Mountains).
This type of volcanism is not found on Venus or Mars.
Volcanism on Earth – Shield Volcanoes
above hot
Fluid magma
chamber, from
which lava erupts
through surface
layers above.
All volcanoes on Venus and Mars are shield volcanoes
Shield Volcanoes
Tectonic plates moving over hot spots produce
shield volcanoes → Chains of volcanoes
Example: The
Earth’s Atmosphere
Only about
1/800 of the
The Earth’s
 The layer of gases around Earth constitutes its
 Relative to other planetary atmospheres, the Earth’s
atmosphere is unique
 However, studying the Earth’s atmosphere can tell us
about atmospheres in general
Composition of the Earth’s Atmosphere
Primarily nitrogen (78.08% by number) and
oxygen (20.95% by number)
The remaining gases (about 1%) include:
carbon dioxide, ozone, water, and argon
This composition is unique relative to the
carbon dioxide atmospheres of Mars and
Venus and the hydrogen atmospheres of the
outer large planets
Sources of Atmospheric Gas
 Outgassing: Release of gasses bound in compounds in
the Earth’s interior through volcanic activity
 Later bombardment with icy meteoroids and comets
Loss of Atmospheric Gas
 Chemical reactions in the oceans
 Energetic radiation from space (in particular, UV)
The early
was different
 Earth’s original atmosphere formed from gasses captured during
formation of Earth
 Contained much more methane (CH4) and ammonia (NH3)
 Atmospheric composition changed through a combination of
several processes:
 Solar UV was intense enough to break out hydrogen atoms from
CH4, NH3 , and H2O leaving carbon, nitrogen, and oxygen behind
while the hydrogen escaped into space
 Ancient plants further increased the levels of atmospheric
oxygen through photosynthesis
Structure of the Earth’s Atmosphere
Atmosphere extends to hundreds of
kilometers becoming very tenuous at high
The atmosphere becomes less dense with
increasing altitude
Half the mass of the atmosphere is within the
first 4 kilometers
The atmosphere eventually merges with the
vacuum of interplanetary space
The Temperature Structure of Earth’s Atmosphere
Top of Ozone Layer
Ozone Layer
Heated by UV and
X-rays from space
Heated by X-rays
from space
Heated by UV
radiation from
Troposphere: Heated
by greenhouse effect
Atmosphere gets colder at larger distance from heat sources.
The Ozone
Oxygen shields against solar UV radiation
O2 provides some shielding, but O3, or ozone, provides most of it
Most ozone is located in the ozone layer at an altitude of 25 km
Shielding is provided by the absorption of UV photons by oxygen
molecules (both O2 and O3) and their resultant dissociation
 Single O atoms combine with O and O2 to replenish the lost O2 and
 Life probably could not exist on the Earth’s surface without the
ozone layer
The Greenhouse Effect
 Visible light reaches the
Earth’s surface and is
converted to heat
 As a result, the surface
radiates infrared energy,
which is trapped by the
atmosphere at
infrared wavelengths
 This reduces the rate of
heat loss and makes the
surface hotter than it
would be otherwise
The CO2 cycle is a feedback mechanism
which regulates Earth’s climate
Stability of Earth’s Climate
 Plate tectonics causes the relative stability of Earth’s climate.
• plate tectonics makes the CO2 cycle work
• it takes about 40,000 years for the CO2 cycle to restore balance
 There have been temporary episodes of extreme cooling and
heating in Earth’s history.
• these ice ages & hothouse period have their own feedback mechanisms
Earth’s Atmosphere is Unique
 Outgassing from volcanoes on Venus, Earth, & Mars
released the same gasses:
• primarily water (H2O), Carbon dioxide (CO2), and Nitrogen (N2)
 So why did Earth’s atmosphere end up so different?
• why did Earth retain most of its H2O – enough to form oceans?
• why does Earth have so little CO2 in its atmosphere, when it
should have outgassed just as much CO2 as Venus?
• why does Earth have so much more Oxygen (O2) than Venus &
• why does Earth have an ultraviolet-absorbing stratosphere?
Why Earth’s Atmosphere is Unique
 Earth’s H2O condensed because of the temperature
oceans formed, in which the CO2 gas dissolved
chemical reactions bound the C of CO2 into rocks like limestone
low level of atmospheric CO2 causes moderate greenhouse effect
temperatures on Earth remain where H2O can be a liquid
 There was once liquid H2O on Mars and maybe Venus.
 before CO2 could dissolve out, temperatures fell/rose so that
 oceans boiled away on Venus and froze out on Mars
 Earth’s O2 was not outgassed by volcanoes.
 O2 is a highly reactive chemical
 it would disappear in a few million years if not replenished
 no geologic process creates O2
Life makes Earth’s atmosphere unique
 Earth’s O2 was created through the evolution of life.
• plants & microorganism release O2 via photosynthesis
• they convert CO2 into O2
 In the upper atmosphere, O2 in converted into ozone
• via chemical processed involving Solar ultraviolet light
• O3 absorbs Solar UV photons which heats the stratosphere
 Venus & Mars lack plant & microbial life.
• so they have no O2 in their atmospheres and no
this week
READ Chapter 5 – The Earth
Homework 4 Due Friday
Quiz on Friday