Lecture 4 Plate tectonics II
1. Rates and History of Plate Motion Ship towing a sensitive magnetometer Magnetic anomalies: seafloor areas of high and low magnetic values Mid-Atlantic Ridge high intensity low intensity
2. Rates and History of Plate Motion Iceland Mid-Atlantic Ridge Mid-Atlantic Ridge high intensity low intensity ● seafloor as a magnetic tape recorder
3. Rates and History of Plate Motion ● magnetic time scale developed
3. Rates and History of Plate Motion ● velocity of seafloor spreading = d / t
3. Rates and History of Plate Motion ● example area: mid-ocean ridge, south of Iceland
3. Rates and History of Plate Motion ● Velocity = 60 km / 3.3 mil. yr. = 18 km / mil. yr. (or 18 mm / yr)
3. Rates and History of Plate Motion Example relative plate velocities: East Pacific Rise (Pacific and Nazca plates) – 138 to 150 mm/yr South Atlantic (Mid-Atlantic Ridge) – 34 to 35 mm/yr Southern Ocean, south of Australia – 70 to 75 mm/yr Southern Ocean, south of Africa – 14 mm/yr
4. The Grand Reconstruction • Reconstructing the history of plate motions: • Assembly and breakup of the supercontinent Rodinia • Assembly and breakup of the supercontinent Pangaea
4. The Grand Reconstruction Earth’s geography 1 billion years ago. Let’s see continental motion!
ASSEMBLY OF RODINIA Late Proterozoic (750 Ma) Formed about 1.1 billion years ago; began to break up about 750 million years ago
ASSEMBLY OF PANGAEA Late Proterozoic (650 Ma) The distribution of continents and oceans between Rodinia and the assembly of Pangaea
ASSEMBLY OF PANGAEA Middle Ordovician (458 Ma) The distribution of continents and oceans about 458 million years ago
ASSEMBLY OF PANGAEA Early Devonian (390 Ma) The distribution of continents and oceans about 390 million years ago
ASSEMBLY OF PANGAEA Early Triassic (237 Ma) The distribution of continents and oceans about 237 million years ago; Pangaea is formed
BREAKUP OF PANGAEA Early Jurassic (195 Ma) The breakup of the super-continent about 195 million years ago; Pangaea is being rifted
BREAKUP OF PANGAEA Late Jurassic (152 Ma) The distribution of continents and oceans about 152 million years ago
BREAKUP OF PANGAEA Late Cretaceous-Early Tertiary (66 Ma) The distribution of continents and oceans about 66 million years ago; much like today in some ways
PRESENT DAY The distribution of continents and oceans as we know them today
Convection Currents • Hot magma in the Earth moves toward the surface, cools, then sinks again. • Creates convection currents beneath the plates that cause the plates to move.
5. Mantle Convection: The Engine of Plate Tectonics Upper mantle Theory 1: whole mantle convection 700 km Lower mantle Plate recycling extends to the core-mantle boundary. 2900 km Outer core
5. Mantle Convection: The Engine of Plate Tectonics Theory 2: stratified convection Boundary near 700 km separates the two different convection systems. The lower mantle convects more sluggishly than the upper mantle.
5. Mantle Convection: The Engine of Plate Tectonics spreading centers and hot spots
6. Theory of Plate Tectonics and the Scientific Method • Plate tectonics is not a dogma, but a confirmed theory whose strength lies in its simplicity, its generality, and its consistency with many types of observations. • This theory has survived so many attempts to prove it wrong and has been so important in explaining and predicting so many phenomena that geologists treat the theory as fact. • Reasons why proof and acceptance took so long: very cautious approach of many scientists studying this issue; global scale of the problem; and specialized technology required to gain data took time to develop.
Tectonic Plates There are a dozen large lithospheric plates (smaller plates not shown).Some plates have continents; some don’t. All are in motion.Question: What evidence is there for these plate boundaries?
Volcanoes and Plate Tectonics… …what’s the connection?
Pacific Ring of Fire Volcanism is mostly focused at plate margins
- Subduction - Rifting - Hotspots Volcanoes are formed by:
Pacific Ring of Fire Hotspot volcanoes
What are Hotspot Volcanoes? • Hot mantle plumes breaching the surface in the middle of a tectonic plate The Hawaiian island chain are examples of hotspot volcanoes. Photo: Tom Pfeiffer / www.volcanodiscovery.com
The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.
Earthquakes and Plate Tectonics… …what’s the connection?
As with volcanoes, earthquakes are not randomly distributed over the globe • At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Figure showing the distribution of earthquakes around the globe
Earthquakes There are thousands of small earthquakes every day “Strong” earthquakes (~M7) occur once a month. >M8 occur about once/year. Where are the deepest earthquakes? For earthquakes of the past 2 weeks, go to http://www.iris.edu/seismon/
Where do earthquakes originate? Figure showing the tectonic setting of earthquakes
Plate Tectonics Summary • The Earth is made up of 3 main layers (core, mantle, crust) • On the surface of the Earth are tectonic plates that slowly move around the globe • Plates are made of crust and upper mantle (lithosphere) • There are 2 types of plate • There are 3 types of plate boundaries • Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates