Kuliah 3 The Origin of Ocean Basin Plate

(Kuliah # 3) The Origin of Ocean Basin: Plate Tectonics Plate tectonics is the concept that the outer part of the earth is split up into a set of rigid, moving plates. These plates move because of slow convecting currents of hot rock inside the earth.

The Earth’s Structure To understand plate tectonic theory, we need to know something about the structure of the earth. Lithosphere - the outer rigid shell of the earth’s structure. Mantle - layer or shell of the earth below the lithosphere and above the core. Core - the central portion of the earth. It The outer core is molten, while the inner core, even though just as hot, is a solid because of the increased pressure.

Summary of Important Concepts • Earth is composed of layers. These layers have different chemical and physical properties. The main layers are the core, mantle, and two types of crust (oceanic crust and continental crust), and also the lithosphere and asthenosphere. • Elevations of different parts of the earth’s crust are controlled by isostatic equilibrium: the concept that the oceanic crust and the continental crust float buoyantly in the denser mantle beneath. • Earth’s internal layers are studied by observing how earthquake waves change as they pass through the earth. • In plate tectonic theory the Earth’s outer rigid surface -- the lithosphere -- is divided into moving segments called plates. These plates move away from one another, move toward each other, or slide side-by side past each other.

Summary of Important Concepts, continued • The theory of plate tectonics explains many important features of Earth’s surface, such as: - mid-ocean ridges and the earthquakes and volcanic activity there - ocean trenches and the earthquake and volcanic activity there - the young age of the ocean floor • One force that drives plate motion is heat-driven convection currents in the mantle. The heat is generated by the decay of radioactive elements within Earth. • Many separate pieces of evidence demonstrate that plate tectonic theory is correct.

A Layered Earth - Density is a key concept for understanding the structure of Earth. Density measures the mass per unit volume of a substance. Density = Mass Volume Density is commonly expressed as grams per cubic centimeter. Water has a density of 1 g/cm 3 Continental crust has a density of 2. 7 g/cm 3 Oceanic crust has a density of 2. 9 g/cm 3 The mantle has a density of 3. 3 to 4. 5 g/cm 3 The fact that the mantle is denser than either type of crust is important: The less dense oceanic and continental crust float buoyantly in the mantle. This is the concept of isostatic equilibrium (more on this is explained below). © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Layered Earth A cross section of Earth showing the internal layers. These layers can be described by their chemical and physical characteristics. You should learn the difference between the core, mantle, and crusts (oceanic & continental), and the differences between the lithosphere and the asthenosphere (explained below).

Layered Earth - Chemical Properties Note that Earth is density stratified, that is, each deeper layer is denser than the layer above. *************** The layers listed above are distinguished based on chemical composition and density. Another important layered aspect of the Earth is layers distinguished based on their physical properties, in particular whether they are stiff and rigid, versus able to flow slowly. (Next slide. )

Layered Earth - Physical Properties The outer ~100 kilometers of the earth includes both the crust and the upper part of the mantle. In this region the rock is cool, and therefore rigid (stiff and not easily deformed). This layer is called the lithosphere (crust plus uppermost mantle). In contrast, below this layer for several hundred kilometers within the mantle is a layer in which the rock is so hot that it flows slowly. This layer is called the asthenosphere. Keep these differences between the chemical layers and the physical layers in mind in the following sections.

Layered Earth - Isostatic Equilibrium Think for a moment about the crust. Why doesn’t it sink into the mantle? Why do the continents stick up above the ocean surface? How are features such as mountains supported? The concept of buoyancy is illustrated by a ship. The ship sinks until it displaces a volume of water equal to the weight of the ship and its contents. Earth’s continental and oceanic crust are supported on the denser underlying mantle in a similar manner. Both types of crust “float” in the mantle. Instead of buoyancy, the term isostatic equilibrium describes the way the two types of crust are supported on the mantle. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Isostatic Equilibrium - an example This figure shows how the continental crust adjusts itself to maintain isostatic equilibrium. A great weight, like the formation of a glacial ice cap, will cause the crust to slowly sag down into the mantle. After the ice melts, the crust will gradually rise back up. Several places on earth are presently rising upward this way, because Ice Age ice caps have been melting.

Layered Earth - Internal Heat Where does the heat within Earth’s layers come from? Heat from within Earth keeps the asthenosphere flowing. This allows the lithosphere to keep moving. The source of this heat is radioactive decay, given off when the nuclei of unstable forms of elements break apart. This heat causes the rock of the mantle to flow very slowly by convection. Hotter areas of the mantle (shown here in RED) are less dense, and so rise upward, while cooler areas of the mantle (BLUE) are more dense, and sink downward. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Evidence of Earth’s Layers What evidence supports the idea that Earth has layers? The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the internal layers of Earthquake waves bend, bounce off different layers, and change speed and direction as they pass through the earth. These changes reveal the layers you have been learning about. For example: S waves (above-left) cannot penetrate Earth’s liquid core, and P waves (above-right) are bent as they pass through the liquid outer core. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Wegener’s Theory of Continental Drift Alfred Wegener gathered evidence in the early 1900’s that the continents on either side of the Atlantic Ocean were once joined to form a single large continent he called Pangea. His evidence was based on similarities of fossils, and large areas of rock, on either side of the Atlantic. Animation © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Fossil Evidence Fossil remains of the same organisms can be found on different continents.

Lungfish Remains of the modern-day lungfish are present in fossil records stretching back 300 million years. These fish have lungs that allow them to survive dry periods by forming a “cocoon” in the mud and breathing quietly until the next rains. There are species of lungfish in Africa, South America, and Australia. The distribution of lungfish reflects their origins in Gondwanaland before continental drift separated these continents.

Alfred Wegener’s theory of continental drift was out of favor with the scientific community for decades. Eventually new technology provided evidence to support his idea. (Unfortunately this evidence did not come along until after his death -- sorry Alfred!!) - Radiometric dating of rocks revealed that the oceanic crust is surprisingly young compared to the continents. Oceanic crust is not more than about 200 million years old anywhere. - Echo sounders revealed the shape of the Mid-Atlantic Ridge. - Seismographs revealed that volcanoes and earthquakes occur mostly in narrow belts. (See next slide. )

Earthquakes show where plate boundaries are located, providing important evidence for movements of the earth’s plates. Notice on this figure that earthquakes occur in narrow zones on the earth. These areas correspond to the edges of tectonic plates. As the plates move against each other, they make earthquakes!

Seafloor Spreading - A Key Idea • An idea proposed by Harry Hess and Robert Dietz in 1960 explained the development of the seafloor at the Mid-Atlantic Ridge • Rising convection currents in the mantle force the sea floor apart at the ridge, causing it to grow and spread: a process called sea floor spreading. • As the sea floor spreads, the continents on either side drift apart

Thus the Mid. Atlantic Ridge conforms to the shape of the continents. The inset shows the center of the Mid-Atlantic Ridge. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Apa mekanisme yang menggerakkan sfl: Teori Covenction Cells

• Pemanasan material dalam bumi melebur menimbulkan suatu aliran. Jika aliran lapisan bergerak ke atas mencapai litosfer, aliran membelok dibawah lapisan tersebut dan selanjutnya mengalami pendinginan. • Material dingin menjadi pekat dan menurun ke arah pusat bumi. Selanjutnya material terangkat kembali ke atas karena proses pemanasan. • Terbentuk gunung api (menembus litosfer) • Terbentuk gerakan lateral (bergerak dibawah litosfer)

Creating new ocean crust Theory of Plate Tectonics begins to be accepted in the 1960 s

3 -3 Destructive margins Subduction zones Constructive margins Midocean ridges Driving Mechanisms for Plate Motions

Sea Floor Spreading Rising convection currents of hot rock in the mantle cause new oceanic crust to form and spread apart at mid-ocean ridges.

Rate of Seafloor Spreading For reasons yet unknown, the polarity of the earth’s magnetic field reverses periodically. These reversals are easily identified in the magnetic orientations of the basalt layer of the ocean floor. The length of time between reversals enables oceanographers to estimate the rate of seafloor spreading at about 1 -4 cm per year.

The Young Ocean Basins Sea floor spreading and subduction together explain the young age of the oceanic crust. New oceanic crust forms by sea floor spreading at midocean ridges. This crust is pushed away from the ridges by continuing sea floor spreading. Eventually the oceanic crust subducts below another plate at an oceanic trench and gets melted back into the mantle. Oceanic crust is continually created at mid-ocean ridges, and continually destroyed at oceanic trenches!

Age of the Ocean Floor A special research ship, the Glomar Challenger, was built that was able to conduct drilling experiments out in the deep sea on the abyssal plain on both sides of the Mid-Atlantic Ridge. The youngest rocks were at the base of the ridge, and the rocks became older as they moved away from the ridge, consistent with seafloor spreading.

The Theory of Plate Tectonics The ideas of continental drift and seafloor spreading were tied together in theory of plate tectonics. Main points of theory: • Earth’s outer layer is divided into moving lithospheric plates. • The plates move apart at mid-ocean ridges, in a process called sea floor spreading. Magma rising and solidifying at the ridge forms new oceanic crust. This crust spreads away from the ridge to make room for more magma to rise up and form more crust. This process causes many earthquakes at midocean ridges. • The plates come together at oceanic trenches, where one plate dives down beneath another one and gets melted back into the mantle: a process is called subduction. This process causes many earthquakes and volcanoes near oceanic trenches. • These plates move because of convection in the underlying asthenosphere, and also the downward pull of the subducting plate. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

The Major Lithospheric Plates The major lithospheric plates and their direction of relative movement are shown here. The boundaries between plates correspond to most of the earth’s earthquakes and volcanoes. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Batas Lempeng • Lempeng bagian litosfer berupa kerak benua atau laut atau sebagian. • Batas lempeng trenches, ridges dan faults. • Arah gerakan; menjauh (ridges); menyatu (trenches); sejajar mendahului (faults). • Wilayah mid-ocean ridges dan rise bergerak lateral pada faults dikenal dengan transform faults

Plate Boundaries Convergent Plate Boundaries - plates come together; further classified as: Oceanic crust subducting under continental crust - for example, the west coast of South America. Oceanic crust subducting under oceanic crust - occurring in the northern Pacific and much of the western Pacific. Continental crust colliding with continental crust – one example is the Himalayas Compression at convergent boundaries produces buckling and shortening. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Subduction The oceanic trench marks the location where one plate bends down and descends into the mantle beneath the other plate. Notice the earthquakes, and the formation of magma (and therefore volcanoes) resulting from the melting of the subducting plate.

Subduction As a plate travels toward a subduction zone, it may be carrying seamounts, islands, or even small continents. These objects may not be subducted, but rather scraped off and attached to the other plate! These scraped off pieces are called terranes. Sometimes a large slice of the oceanic crust or lithosphere can be scraped off and attached to the other plate. This type of terrane is called an ophiolite.

Formation of the Rocky Mountains

Plate Boundaries The lithospheric plates can either move apart from one another, toward one another, or slide side-by-side past one another. Divergent plate boundaries – plate move apart, further classified as: Divergent oceanic crust – for example, the Mid-Atlantic Ridge, and other oceanic ridges, where sea floor spreading is occurring. Divergent continental crust - for example, the Rift Valley of East Africa. This is an area where the continental crust is pulling apart, and may eventually form a new ocean basin! Extension of divergent boundaries causes splitting and rifting. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

The Wilson Cycle

Plate Boundaries Transform plate boundaries - plates move side-by-side past one another, for example, the San Andreas fault. Side-by-side motion at transform boundaries causes shearing. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Transform Fault

Conservative margins Transform faults

Confirmation of Plate Tectonics Paleomagnetism: strips of alternating magnetic polarity at spreading regions. The patterns of paleomagnetism support plate tectonic theory. The molten rocks at spreading centers takes on the polarity of the planet while it cools. When Earth’s polarity reverses (as it does periodically through time), the magnetic polarity of newly formed rock reverses too. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Confirmation of Plate Tectonics Evidence supporting theory of plate tectonics: Apparent Polar wandering: plate movement causes the apparent position of the ancient magnetic poles to appear to be in different places, unless the continents are all put back together in the configuration of Pangea. In this position the paleomagnetic fields in rocks on the different continents all point to a single magnetic pole! © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Confirmation of Plate Tectonics Hot Spots: Surface expression of plumes of magma. As a plate passes over a stationary hot spot (a stationary area of rising hot mantle rock), volcanic islands are formed in sequence. The volcanoes get progressively older in the direction of plate movement. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Hot Spots, continued The Hawaiian Islands and the Emperor Seamounts are a classic example of a line of volcanoes produced by a plate moving over a stationary hotspot. Notice how the age of the volcanoes changes in a regular way along the line.

Confirmation of Plate Tectonics Guyots were once volcanic peaks above sea level. They were eroded flat by wave action, and then gradually sank beneath the ocean surface as the plate below grew cooler and denser, sinking slowly into the mantle (another example of isostatic equilibrium). © 2002 Brooks/Cole, a division of Thomson Learning, Inc.