Today’s APOD

READ Chapter 5 – The EarthHomework 4 Due FridayQuiz on Friday

The Sun Today

A100 - Oct. 8 Shaping the Earth’s Surface

Writing the History of Earth

Surface formations visible today have emerged only very recently compared to the age of Earth.

Earth’s Interior

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 cools.

Motion in the Earth’s Interior

Heat generated by radioactive decay in the Earth creates movement of rock

This movement of material is called convectionConvection occurs because hotter material will

be less dense than its cooler surroundings and consequently will rise while cooler material sinks

Convection 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 tectonicsEarly 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

supercontinent

Total continental area has increased with time as new continental crust has formed

Earth’s Tectonic History

Plate TectonicsTectonic 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

Geological Features of Plate Tectonics 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 continents

When two continental plates collide• one can not subduct under the other • crust is pushed up to form mountain ranges• e.g. the Himalayas

Geological Features of

Plate Tectonics

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

Geological Features of

Plate Tectonics

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

earthquake

Rifting

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

SubductionSubduction

Cool material sinks, and drags crustal pieces together buckling them upward into mountains

If one piece of crust slips under the other, the process is called subduction

Plate tectonics is the dominant force that sculpt the Earth’s surface – it also triggers earthquakes and volcanoes

Geological activity mostly occurs on plate

boundaries

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.

Found above

hot spots

Fluid magma chamber, from which lava erupts repeatedly through surface layers above.

All volcanoes on Venus and Mars are shield volcanoes

Volcanism on Earth – Shield Volcanoes

Shield VolcanoesTectonic plates moving over hot spots produce shield volcanoes → Chains of

volcanoes Example: The

Hawaiian Islands

Earth’s AtmosphereOnly about 1/800 of the

Earth’s radius!

The Earth’s Atmosphere

The layer of gases around Earth constitutes its atmosphere

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 atmosphere

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 altitudes

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

Troposphere: Heated by greenhouse effect

Atmosphere gets colder at larger distance from heat sources.

Exosphere: Heated by UV and X-rays from spaceThermosphere: Heated by X-rays from space

Stratosphere: Heated by UV radiation from spaceOzone Layer

Top of Ozone Layer

The Ozone Layer

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 O3

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 & Mars?

• 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 (O3).• 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

stratospheres

ASSIGNMENTSthis week

Dates to Remember

READ Chapter 5 – The EarthHomework 4 Due FridayQuiz on Friday