Planet Earth

Opmerking: onderstaande informatie is ook – op verzoek – beschikbaar in het Nederlands. Information below is based on lessons given by my geography teacher mr. BTS

Contents

  • Plate tectonics and volcanism
  • Köppen climate system
  • Air circulation and pressure areas
  • Geological history of the earth

Plate tectonics and volcanism

  • The Earth consists of the following layers:
    Inner core – outer core – inner mantle – outer mantle – earth’s crust
    The core is solid and mainly contains iron and nickle
  • Below the earth’s crust lies the asthenosphere (part of the earth’s mantle). Through heat from the core, matter becomes fluid and ascends. The higher it comes, the colder it gets. The lower it gets, the warmer it becomes. That’s how a convection current comes into existence.
  • The earth’s crust consist of different plates. Under these plates lies the asthenosphere, hence the plates move along with the convection currents.
  • There are three different plate movements:
    1. Convergence: plates move towards one another/collide
    2. Divergence: plates move from each other. Subsequently, magma rises in oceans, water solidifies, and island come into existence; or volcanoes and mountain areas.
    3. Transversal: plates move next to each other in contrary directions
  • Continents have granite as foundation stone, oceans have basalt. Granite weighs less than basalt, so an oceanic plate dives under the continental plate – this is subduction. If subduction takes place with two oceanic plates, the eldest dives under the other. In case of two continental plates, they both rise up. Subduction can cause trenches (V-shaped sea floor), volcanism (due to rising magma), earthquakes (as they clash with one another) and mountain ranges (either uplifted or corrugation in case of two continental plates)
  • If an oceanic plate dives under a continental plate, the plates first grind against one another; the spot where they grind past each other is called a hypo centre. In case of a sea quake, the hypo centre is at shallow depth. Under water one speaks of a hypo centre; on land it’s called an epicentre.
  • The Krakatau volcano is situated between Sumatra and Java. In 1883, the vulcano erupted. Much ash spread in the atmosphere through winds. Hence,the average temperature on earth decreases for some time, as the ash layer blocked sunbeams. The sound of the eruption was heard in East Africa and Australia
  • Pumice: light volcanic rocks with ash particles
  • Aerosols = coloured ash particles in the air, required for condensation, otherwise clouds couldn’t exist
  • Sumatra and Java are situated on a break line, so there’s a large chance of endogene natural disasters. There are also trenches near the coast due to subduction of the Indian-Australian and Eurasian plate.
  • The 2004 tsunami: there was a sea quake, with a magnitude of 8/9 on the scale of Richter, which caused high waves. Within eight hours, waves reached East Africa. Most deaths occurred near the epicentre on Sumatra.
  • Types of volcanoes
    — Stratovolcano: consists of layers, emerged through the eruption of various materials. In South East Asia much eruption occurs; the top layer of a volcano is blown away, and a new crater emerges
    – Then, a caldera volcano emerges. When the volcano dies, there is still a channel that can push up magma, causing an eruption and the existence of a new volcano
    – Characteristics of a stratovolcano: steep slope, thick fluid lava; explosive; bombs and lapilli (little stones), lahars (mud streams), ash, blast struck / pyroclastic streams (transparent steam with high temperature from a crater, causing a sea of fire
    — Shieldvolcano: eruption with thin lava; the lava flows far away before it solidifies – so a gentle slope emerges. This is an effusive volcano (not so explosive).
  • The Pinatubo is a stratovolcano. Its eruption caused ash clouds high up in the sky, and rains of ash. Ash that reached jets was spread, so the average temperature decreases. The steam that emerged condensed and in combination with a typhoon, there was an ash/mud rain

Köppen climate system 

The German climatologist distinguished between a few climate zones on the planet, characterized by temperature, precipitation and vegetation. The A-climates are the warmest, the E-climates the coldest. Isotherms form the borders.

[north pole]

E-climates [pole climates]
———————————— 10 °C in the warmest month
D-climates [continental climates]
———————————— –3 °C in the coldest month
C-climates [maritime climates]
———————————— 18 °C in the coldest month
A-climates [tropical climate]
___________________________________ [equator]
Reflection of the northern hemisphere,
with the exception of D-climates

Comments

  1. An isotherm is a line that spots with the same average temperature connects
  2. The first letter says something about the temperature, the second letter something about precipitation:
    f = precipitation all year
    s = dry summer
    w = dry winter
    S = steppe, W=dessert, T=tundra, H=high mountains, F=snow
  3. The B-climates have in common that there is little precipitation. It thus concerns dry climates, of which there are two: BS- and BW-climate. They occur in the zones of the A-, C- and D-climates. The temperature in B-climates is not to be considered.
  4. As for the E-climates, we don’t use a lowercase letter as second letter; due to low temperatures there is little precipitation
  5. The D-climates don’t occur on the southern hemisphere, as there are relatively more oceans than continents – the Ds climate doesn’t exist anywhere

Air circulation and pressure areas

  • Due to warmth of the sun, air ascends at the equator. Also at 60° N/S air ascends. At these two spots there are low-pressure areas, while at 30 and 90° N/S there are high-pressure areas
  • Thermal minimum: a minimum driven by warmth, hence on the equator
  • Dynamic minimum: a minimum existing through colliding air streams, so at 60° N/S
  • At 90° N/S it is cold, so air descends, which causes a high-pressure area. The wind that subsequently returns to the equator, collides at 60° N/S with the wind moving from 30° to 90°
  • Through an accumulation of air, air streams diverge. If there is a shortage of air, air is attracted; hence, a circulation comes into existence
  • In case of high pressure, air descends – this is at a maximum
  • In case of low pressure, air ascends – this is a minimum
  • The Law of Buijs Ballot:
    1) Air always moves from a maximum to a minimum
    2) On the northern hemisphere, the wind has an deviation towards right, and on the southern hemisphere towards left – this is the coriolis force caused by the rotation of the earth’s axis
  • Wind between
    90-60° N/S = pole wind
    60-30° N/S = westerlies / anti-trades
    30 N/S – 0° = easterlies / trade winds
  • Imagine air circulation in a spiralling circle, a circle each for a different latitude – 30, 60 & 90° – three circles on a diagonal from bottom left to upper right – spiralling from the center to periphery (convergent) or the other way around (divergent)
  • Air streams on the northern hemisphere have a deviation towards right. At 60° air ascends, at 30 and 90° it descends. At 30° the wind goes spiralling from the center to the periphery and hence moves to the right. Once the wind is at the periphery, the wind moves to 60°, where it moves in a spiral towards the center.
  • The wind that moves from 90 to 60°, also wants to deviate to the left, which it does, and the wind arrives at 60°. There, the wind makes a convergent movement in the spiral towards the center. That is how the winds from 30 en 90° meet and how air ascends.
  • 1013 Mb = 1 hPa. This is an average air pressure. A higher value means there is a high pressure, a lower value than 1013 Mb means there is a low pressure.
  • One can recognize a low-pressure area (depicted as a spiralling circle) because:
    1. The values decrease towards the center
    2. The values are lower than 1013 Mb
    3. The spiral makes a convergent movement
    If the spiral makes a divergent movement, then it’s not a low-pressure area.
  • On the southern hemisphere it is summer when the sun directly overheads the southern tropic (Tropic of Capricorn, 23° S); on the northern hemisphere on the Tropic of Cancer. However, the sun never overheads the same exact spot, hence the low-pressure area moves along the sunbeams. As it is harder for the sea surface to warm up, this warming up doesn’t occur in a straight line.
  • The vertically positioning of the sun around the equator, is called the Intertropical Convergence Zone (ITCZ); this is the ‘line’ that moves between the tropics.
    At 90° there is a polair maximum
    At 60° there is a sub polair minimum
    At 30° there is a sub tropic maximum
    At 0°/equator is the equatorial minimum / ITCZ
  • Monsoons are winds moving from the equator towards the ITCZ. These concern westerlies/anti-trades, coming from the sea, wet and warm (this applies to South East Asia during summer)
  • Trade winds are winds moving from the ITCZ towards the equator. These winds are coming from land, are easterlies and dry

Geological history of the earth 

The climate changes in the past were caused by factors such as:

  • Plates tectonics
  • Astronomical processes
  • Developments in the atmosphere
  • Geomorphology (formation of landscapes)

Geological time scale – division of the history of the earth in certain geological eras, going back millions of years in time. The human timescale is thousands of years back in time.


Eras

Precambrian: before 540,000 million years BP

Paleozoic: 540,000-250,000 million years BP

Mesozoic: 250,000-70,000 million years BP

Cenozoic: 70,000 million years BP-now

Quaternary: 2.5 million years BP-now

Pleistocene: 2.5 million years – 10K years

Holocene: 10K years – now

Elstar glacial: 465-420K years

Saale glacial: 150-100K years

  • Last time there was land ice in The Netherlands, causing some relief patterns.

Weichsel glacial: 115K-12,000

  • Sand from the North Sea was taken by the wind. The coarse sand ended up in the west of the Netherlands, the fine sand (löss) in the southeast.

Ways to reconstruct climates

Paleoclimatology: the science that researches previous climates on earth and the mechanisms causing change. Scientists often work with proxy indicators – indicators that indirectly tell us something about a phenomenon, but not always accurately. Some of the methods used are:

  • Looking at isotopes: atoms with the same chemical characteristics, but with different atomic weight. These atoms are from various elements found in drillings. 16O evaporates faster than 18O. With low temperatures, 18O is less likely to evaporate. Hence, when researchers find much 18O in calcifications, that seems to imply there was a cold period.
  • The 14C method: this determines the old age of organic material, by looking at the extent of decrease of the radioactive carbon isotope 14C in dead organic material. When there is a high concentration, it points out relatively young dead organic material.
  • Geomorphology focuses on describing and explaining certain forms within the landscape, like river valleys and lateral moraine. Their presence tells us something about the climate during the time they came into existence. This climate can also be determined based on physical and chemical composition of old soils.

Methods that are suitable for research on climate from more recent past are:

  • Palynology: this is the analysis of pollen (grains) which differ for all trees and other plants. Pollen have a waxy layer, hence they are often well preserved. This analysis shows how vegetation has changed and how the climate developed. Though it is possible that pollen were taken by the wind, eroded from older sediments and moved to younger sediments.
  • Dendrochronology: research concerning the thickness of the year rings of trees. These rings show under which circumstances, especially the amount of precipitation, a tree grew up. The thicker, the better the circumstances – likely warmer with more precipitation.
  • Historical written sources: the reliability of those cannot be confirmed always.

The distant past

There’s little known about the climate in the Precambrian (which started five billion years BP). Based on research of sediment rocks it is only possible to make claims about the average temperatures of the entire earth. It was probably between a 0 and 80 degrees Celsius. There were periods with glaciations, caused by movements of the former continents near the poles.

Around 700,000,000 BP there was a situation called White Earth: everything was completely covered with snow and ice. There was little solar activity, so the average temperature decreased, causing a lot of snow and ice. The Albedo effect effect caused glaciation to move to lower latitudes. Sunlight was reflected, so the ice sheet further expanded. It was a case of a positive feedback loop: the process strengthened itself.

During the Precambrian there was also a divergent movement in the earth’s crust, causing new gorges and hence volcanoes. Due to volcanic eruptions, CO2 spread in the air, being part of the greenhouse effect. The atmosphere warmed up, and there were warmer periods with vegetation growth.

During the Carboniferous period (360-300 billion years BP), Western Europe was situated close to the equator, and there was dense vegetation. As the sea level was rising, there were many floodings, leaving sediments on the land. The plants were hence sealed airtights. The pressure increased, making fluids and gases leave, temperatures rose, and coal came into existence (a hardened layer of carbonized plant-based material).

There’s more known about the climate during the Permian period (299-251 million years BP) due to research concerning fossils, rocks and reconstructions of tectonic movements. Movement of continents and changing sea levels prevailing then, influenced the climate.

Paleomagnetism is a form of research in which one can date layers of the seafloor and research how fast the seafloor moved at some points. During Permian, the continents were united in the supercontinent Pangea. There were large and deep ocean basins, causing the sea level to be low. No new basins were formed due to little tectonic movements. Quite some rock was vulnerable to weathering and erosion. 

Rivers took the erosion material to the ocean, where flora and fauna received nutrients and started growing. This process demanded much CO2 from the atmosphere. A low CO2 concentration let to a decrease of the greenhouse effect, causing the earth to cool down and ice sheets to grow. The sea level even descended further.

During Permian, The Netherlands was located in a lowland at the current location of the Sahara desert. This area flooded regularly. Each time after the sea retreated, salt was left behind. That is why the current Dutch soil contains a salt layer.

The period from 145.5 until 65.5 million years BP is also called Greenhouse Earth. The Cretaceous period is the warmest period in the Mesozoic. The supercontinent Pangea functioned as a covering layer of the earth’s mantle. As a result, the earth was unable to release a part of its internal heat. Convection streams – streams of plastic rocks in the mantle – turned and broke up the Pangea. The continents then drifted apart.

Because of the large amount of magma from volcanic eruptions, there was less space for water, and the oceans became less deep. Meltwater from ice sheets also disappeared in the ocean. That is the reason the sea level was 300 meters higher than during Permian. The magma also released a lot of CO2, which strengthened the greenhouse effect. During the Cretaceous period there was the highest level of carbon dioxide, something that makes that period interesting in today’s world. Based on insights of that period, we can draw claims about the present, and the other way around. Actuality principle: this principle assumes that all natural process in the past and present occur the same way, and is used when explaining geological phenomena. 

Meteorite impact?

There is a lot of discussion about what happened between the Cretaceous period and Tertiary age. It is certain that an abrupt climate change took place, and 70% of flora and fauna went extinct. At the end of the 1970s a theory was founded about a meteorite impact, as in clay from marine sediments iridium was found. Iridium is rare on earth, but present relatively a lot in meteorites.

Later there were more signs: alien materials were found, and carbon in sediments were discovered, probably caused by the huge fires after the strike on the earth. The strike also brought a lot of nitrogen oxides in the atmosphere, that made the ozone layer thinner. Also, much particulate matter in the air blocked the sunlight. The temperatures declined on earth and the process of photosynthesis was disrupted. Entire food chains were hence destroyed.

Aside from the meteorite impact, there is another theory on the abrupt climate change and extinction of dinosaurs. The highland of Deccan (India) is a hotspot. A hotspot is a remnant of a mantle plume, a large amount of magma from the deep earth mantle that rises and breaks through the crust. It is likely that poisonous substances entered the air at this location. When those toxic substances spread, many animals died.

The temperature decline after the Cretaceous period also has tectonic causes:

  1. India collided with Asia, resulting in the Himalayas and the Tibetan Plateau. Behind the Himalayas there was no influence from sea anymore, making it colder. Hence, ice expanded with the Albedo effect. 
  2. Antarctica became detached from Australia and South America. Consequently, a sea current started around Antarctica. The warm sea current that used to move from the equator to Antarctica, was blocked by the new current, further contributing to cooling down.
  3. North and South America collided, causing relief in the United States (Colorado Plateau and the Rocky Mountains). There was now also a blockage for the warm equatorial sea current that used to pass by between the continents. The northern hemisphere cooled down afterwards.

Quaternary

Typical for the Quaternary are the changes between glacial and interglacials (warmer periods between de glacials of the Pleistocene). The variation in warmer and colder periods cannot be attributed to a single cause. It concerns a complex interaction of multiple factors during different time scales.

The preconditions for an ice age should be right, including the conditional factors – preconditions required for something to happen.The most important factor is movement of continents. Another condition is the presence of much land near the poles, as ice sheets only start existing on continents.

The temperature changes during the Pleistocene are not directly the results of continents movement. Variations in the orbit of the earth around the sun, and the tilt of the earth’s axis, are the causes. These variations are directing mechanisms in the existence of these ice ages. Directing mechanisms are factors that contribute to an occurance, but are not the preconditions. They influence the distribution of solar radiation during summer and winter, and over continents and oceans.

Milankovitch

The intensity of the solar radiation is linked to the seasons, but also to the changing position of the earth in her orbit around the sun. The scientist Milankovitch did research on this. He thinks three cycli are responsible for these climatologist variations:

  • Eccentricity: the elliptical orbit of the earth around the sun is sometimes more like a circle, and sometimes like an elliptic – which changes every 100 to 400,000 years.
  • Tilting: the tilt of the earth’s axis compared to the orbit of the earth around the sun, changes in a period 41,000 years, moving between 21.5 and 24.5 degrees.
  • Precision: a fluctuation in the earth’s axis (period of 19,000 – 230,000 years), causing winter and summer to occur at another point when the earth finishes its orbit around the sun.

Changes in these three entities cause variations in the amount of radiation the earth receives. Small fluctuations can lead to a global temperature increase or decline of 5%, through greenhouse gases also play a role. The way it works exactly is still unclear.

Not only variations of radiation are responsible for ice ages in the Pleistocene. The climate is also determined by winds, precipitation, evaporation and sea currents. There are also positive and negative feedback loops. In the Pleistocene, the concentration of CO2 and CH4 in the atmosphere also varied. A low CO2 concentration leads to lower temperatures (positive feedback loop).

The current patterns of sea currents didn’t exist in the last ice age. As a result, there was no supply of heat to the Dutch latitude. Why currents changed is unknown. The temperature on earth can also change as the radiation of the sun changes. The amount of sunspots is not a constant: when there are few, the sun is less warm at the surface, and on earth it’s also colder. This theory has not been confirmed though.

Finally, volcanic activity also influences the temperature. Large eruptions bring dust particles in the air, that float around the atmosphere. These particles reflect the sunlight, whereupon light doesn’t reach the earth fully, and hence makes it cool down.

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