Presentation on theme: "Module 7: Home Planet – the Earth Activity 1: The Earth as a Model Planet."— Presentation transcript:
Module 7: Home Planet – the Earth Activity 1: The Earth as a Model Planet
Summary In this Activity, we will investigate (a) why astronomers study the Earth, (b) the structure of the Earth, and (c) volcanism on Earth.
(a) Why astronomers study the Earth... This view of the Earth rising over the Moon was taken on the Apollo 8 mission on the 22 December * NASA Press releaseNASA Press release * Apollo pictures like this one presented for the first time direct visual imagery of Earth as one celestial body among others in our Solar System, rather than as an all-encompassing “world”.
Now that space missions travel further than our Moon, we have even more graphic pictures of our Earth as a planet - here photographed with the Moon, at a distance of about 6 million km, by the Galileo spacecraft on December * * NASA Press releaseNASA Press release Click here to see a simulation of the Earth- Moon system
The Earth is a planet, and as such is studied by astronomers as well geologists. Modern astronomy and geology study “comparative planetology” - comparing planets with each other to find similarities, which in turn might suggest theories to explain their formation and evolution.
Astronomers study the Earth because it is the planet about which we know the most. Earth acts as a model planet with which to compare the properties of other planets. In this Unit you will notice that we mostly quote vital statistics of other planets in terms of Earth. (For example, the mass of Mars is easier to conceptualise if we say that it is approximately 11% that of Earth, rather than that it has a mass of approximately 642,400,000,000,000,000,000,000 kg!)
If you think that Earth is too familiar a topic to be of interest, wait till you see it from an astronomer’s point of view - it may surprise you!
(b) The Structure of the Earth The Earth is affected by First we will investigate the Earth’s overall internal structure. astronomical influences from outside e.g. tidal forces between the Earth & Moon biological influences on the surface e.g. production of oxygen by plants geological influences from within e.g. volcanic outflows
It turns out that the average density of the Earth is about 5.5 times the density of water. The density of rocks on the surface of the Earth is only approximately half this value - therefore at least some of the interior of the Earth must be very dense (otherwise the average density would not be so high). To determine the structure of the Earth, geologists study the way earthquake waves travel through its interior (“seismology”). The average density of the Earth can be estimated by measuring its gravitational attraction on satellites (including our natural satellite, the Moon).
The large-scale model seismologists have come up with for the Earth’s internal structure looks basically like this: crust mantle liquid outer core solid inner core
The inner core’s radius is approximately 20 % of that of the whole Earth, with a density around 4.6 times that of the Earth’s crust. solid inner core The inner core is hot (temperatures up to nearly 5000 C), metallic, and rotates very slightly faster than the rest of the Earth.
The outer core extends out to almost one-third of the Earth’s radius, with density gradually decreasing until it drops at its outer surface to approximately 1.6 times that of the Earth’s crust. liquid outer core Seismological evidence suggests that the outer core is hot, liquid and metallic, but its exact composition is not known for certain.
Made up of solid silicate minerals, its density gradually decreases until it drops at its outer surface to only slightly more than that of the Earth’s crust. mantle The mantle extends out almost to the surface of the Earth. The crust, plus the relatively rigid outer part of the mantle (roughly the top 150 km) are collectively called the lithosphere.
mantle Below the lithosphere, the mantle is very hot (temperatures from around 2,200 C down to 1,200 C). It is still solid rock, as the high pressures acting on it due to its depth are enough to keep it from becoming liquid even at these high temperatures. In this region - called the asthenosphere - the mantle is hot enough to be ‘ductile’, i.e., to undergo a type of plastic flow, so that it moves in convective currents like those in water heated on a stove - only much, much more slowly!
By comparison, the crust of the Earth is only about 35 km thick under the continents (and approximately 5 km thick under the oceans), but together with the atmosphere it supports all the Earth’s known life forms, including us. crust
The convective currents in the ductile part of the Earth’s mantle (the asthenosphere) are driven by the considerable temperature difference between the hot core (approaching 5000 C) and the relatively cool crust. The convective currents in the asthenosphere very slowly drag along regions of the lithosphere, including the thin crust. These regions are called tectonic plates. The Earth’s crust is made up of a number of separate continental & oceanic plates, all supported by the mantle.
The crust is thinnest under the oceans, where it tends to be made up of heavy, plastic oceanic basalt (solidified lava). Where the convection currents under the oceans sink down, they drag down regions of crust, forming deep chasms called mid-ocean trenches. ocean crust mantle
Where convection currents in the asthenosphere “well up” under the ocean, with its relatively thin crust, the pressure can drop enough for the ductile mantle material to ‘melt’, turning into molten rock called magma. As the magma pushes upwards, it lifts up the crust to form mid-ocean ridges, such as the mid-Atlantic ridge. This upwelling of lava pushes the oceanic plates apart, causing continental drift - at a few centimetres per year. ocean crust mantle
When continental plates collide, they produce folded mountain chains. The Himalayan Mountains show intricate folding patterns resulting from the collision of the Indian & Asian continental plates.
When a continental plate collides with an oceanic plate, oceanic basalt continental granite mantle the heavy plastic oceanic basalt tends to slide under the light, brittle continental granite. The rising crust crumples up into coastal mountain ranges.
In the process, the basalt is likely to heat up and melt into magma and heat trapped gases and water, forming outflows called lava and releases of gases as volcanic activity - volcanism - and associated earthquakes. The Andes mountains in South America are the result of the Pacific Ocean floor slipping under the continental plate.
Where two tectonic plates collide, the two sections of the lithosphere meet each other at what is called a fault. As the two plates continue to drift, stress builds up at these faults, threatening to tear them apart temporarily. (c) Volcanism on Earth Earthquakes When this happens, the surrounding ground shifts suddenly to relieve the stress, and an earthquake occurs. Many faults have histories of repeated ruptures over thousands or millions of years.
The arrows indicate the directions of the stresses which build up across the fault. As oceanic plates try to ‘slip under’ continental plates, they can form one sort of fault (called technically a variety of ‘dip-slip fault’), represented schematically below:
Other sorts of faults also occur: More varieties of ‘dip-slip faults’: and ‘strike-slip faults’:
The famous San Andreas fault (which has many neighbouring faults) in California, USA, is a classic example of a strike-slip fault, brought about as the North American plate gradually drifts south-east, while the Pacific plate gradually drifts north-west. Pacific plate (moving north-west) North American plate (moving south-east) In the spectacular quake on this fault which devastated parts of San Francisco in 1906, the resulting relative displacement of one side of the fault to the other was up to roughly 6 metres (21 feet).
Here, looking south- east, the San Andreas fault runs down the centre of the photo, under the San Andreas lake and next to Interstate 280 in San Mateo county, Northern California.
This picture illustrates a section of the fault where it temporarily changes direction, setting up a region of compression. Relative motion between tectonic plates, detected by GPS (Global Positioning System) measurements, increases the strain on the fault until it ruptures, causing series of earthquakes.
This map shows the major faults in southern California (for full details, see The San Andreas fault is indicated in red: To see ‘virtual reality’ panoramas of earthquake faults in the Californian area, visit the Internet site
California’s spectacular fault system, lying as it does under high density urban areas, rightly gains a great deal of publicity, but earthquakes are a world-wide phenomenon. This map shows the location of 10 recent (June 2000) large earthquakes in the world.
Volcanoes Many of Earth’s volcanoes are stratovolcanoes (‘layered volcanoes’), raised up as releases of magma rise up through the crust. A typical stratovolcano is conical, built up from layers of solidified lava flows and ash and rock fragments from eruptions. Mt Fuji, Japan, photographed from Earth orbit (NASA)
Shield volcanoes are another variety of volcano which we will meet again when we study volcanoes elsewhere in our Solar System. Shown in this NASA image taken from Earth orbit, the Hawaiian archipelago of islands have been formed by this form of vulcanism.
Geologists explore the lava flow at Hualalai Volcano, Hawaii
The chain of Hawaiian islands, with their shield volcanoes, have formed as their tectonic plate drifts slowly over ‘hot spots’ in the underlying mantle - the volcanoes form like “scorch marks left on a sheet of paper that is moved across a candle flame”*. *S. Sieh & S. LeVay, “The Earth in Turmoil”, W.H. Freeman & Co, 1998 Once the motion of the tectonic plates is allowed for, hot spots in the Earth’s mantle are found to be stationary, or very nearly so. They are very long-lasting: the Yellowstone hot spot has been active for at least 15 million years. Geologists model hot spots as being due to mantle plumes:
A mantle plume is thought to occur when, once every few million years or so, a mass of ductile rock rises up under the influence of the convective currents in the asthenosphere, and breaks free and expands sufficiently to melt, forming magma as it rises. crust mantle Convective currents in the asthenosphere When the head of the plume reaches the Earth’s crust, widespread volcanic eruptions occur, covering large areas of the Earth’s surface with lava which cools to form flood-basalt provinces, extending for up to several million square kilometres.
This Landsat image (taken from Earth orbit) shows part of the Deccan Traps, a flood-basalt province which once covered more than one million square kilometers with an average thickness of 1 km. This scene shows a part of the surviving Deccan cover along the west coast of the peninsula section of India.
Watch our animation of a mantle plume event, and pay special attention to the narrow ‘conduit’ (like the string of a balloon) which trails after the plume. crust mantle Convective currents in the asthenosphere This narrow conduit (only a few kilometres across) may continue to transport magma to the surface for hundreds of millions of years, while the Earth’s tectonic plates move gradually across it.
crust mantle Convective currents in the asthenosphere In later Activities we will contrast this situation to that on Venus and Mars, where shield volcanoes occur but there is little if any evidence of tectonic plate movement. In the process, a chain of shield volcanoes is likely to be produced, especially in ocean crust, which is relatively thin and uniform - see, for example, the Hawaiian islands.
By studying evidence such as matches between now- separated shore lines, and tracks left by hot- spots on the Earth’s crust, geologists conclude that the present-day continents on Earth originally (2 or 3 hundred million years ago) made up a ‘super-continent’, called Pangea.
This NASA globe shows the present- day boundaries between some of Earth’s tectonic plates, with associated volcano & earthquake regions. Andes mountains
When we study other terrestrial planets in the Solar System we will find more evidence of volcanoes, but relatively little evidence of tectonic plate movement or earthquakes. The differences between Earth and other planets in this respect can teach us quite a deal about how planets form and evolve.
We haven’t discussed the outermost visible layer of the Earth yet - its inner atmosphere. We’ll study it in the next Activity.
Further out still (though not visible in optical images from space) are the upper atmosphere and the van Allen belts, regions containing charged particles from the solar wind trapped by the magnetic field of the Earth.
Image Credits NASA Photo AS : High-oblique view of Moon’s surface showing earth rising above horizon NASA Photo NUMBER p-41508c: Image of the Earth and Moon from Galileo NASA: View of Australia NASA: Volcanoes & Earthquakes NASA: The Western Himalayas (from the Shuttle Atlantis) NASA: Hawaiian Island Archipelago NASA: Deccan Traps, Landsat image
Now return to the Module home page, and read more about the structure of the Earth in the Textbook Readings. Hit the Esc key (escape) to return to the Module 7 Home Page
Press Releases: NASA Photo AS : High-oblique view of Moon’s surface showing earth rising above horizon File Name: jpg Film Type: 70mm Date Taken: 12/22/68 Description: High-oblique view of the moon's surface showing the earth rising above the lunar horizon, looking west-southwest, as photographed from the Apollo 8 spacecraft as it orbited the moon. The center of the picture is located at about 105 degrees east longitude and 13 degrees south latitude. The lunar surface probably has less pronounced color than indicated by this print. Click here to return to the Activity!
Press Releases: NASA Photo p-41508c: Image of the Earth and Moon from Galileo GALILEO December 22, 1992 P Eight days after its encounter with the Earth, the Galileo spacecraft was able to look back and capture this remarkable view of the Moon in orbit about the Earth, taken from a distance of about 6.2 million kilometers (3.9 million miles), on December 16. The picture was constructed from images taken through the violet, red, and 1.0-micron infrared filters. The Moon is in the foreground, moving from left to right. The brightly-colored Earth contrasts strongly with the Moon, which reflects only about one-third as much sunlight as Earth. Contrast and color have been computer-enhanced for both objects to improve visibility. Antarctica is visible through clouds (bottom). The Moon's far side is seen; the shadowy indentation in the dawn terminator is the south-Pole/Aitken Basin, one of the largest and oldest lunar impact features, extensively studied from
Galileo during the first Earth flyby in December The Galileo project, whose primary mission is the exploration of the Jupiter system in , is managed for NASA's Office of Space Science and Applications by the Jet Propulsion Laboratory. Click here to return to the Activity!