Internal Structure of Earth and Plate Tectonics Chapter 2 Internal Structure of Earth and Plate Tectonics

Learning Objectives Understand the basic internal structure and processes of Earth Know the basic ideas behind and evidence for the theory of plate tectonics Understand the mechanisms of plate tectonics Understand the relationship of plate tectonics to natural hazards

2.1 Internal Structure of Earth Earth is layered and dynamic Internal structure of Earth By composition and density By physical properties Figure 2.2

Structure of Earth Inner core Outer core Solid 1,300 km (808 mi.) in thickness High temperature Composed of iron (90 percent by weight) and other elements (sulfur, oxygen, and nickel) Outer core Liquid 2,000 km (1,243 mi.) in thickness Composition similar to inner core Density (10.7 g/cm3)

Structure of Earth Mantle Crust Solid 3,000 km (1,864 km) in thickness Composed of iron- and magnesium-rich silicate rocks Density 4.5 g/cm3 Crust Outer rock layer of Earth Density 2.8 g/cm3 Moho discontinuity Separates lighter crustal rocks from more dense mantle

Lithosphere Cool, strong outermost layer of Earth Asthenosphere Below lithosphere Hot, slowly flowing layer of weak rock

Continents versus Ocean Basins Crust is embedded on top of lithosphere Ocean crust is less dense than continental crust Ocean crust is also thinner Ocean crust is young (< 200 million years old) Continental crust is older (several billion years old)

Convection Earth’s internal heat causes magma to heat up and become less dense Less dense magma rises Cool magma falls back downward Similar to pan of boiling water Figure 2.3

How We Know About Internal Structure of Earth Most of our knowledge of Earth’s structure comes from seismology Study of earthquakes Earthquakes cause seismic energy to move through Earth Some waves move through solid, but not liquids Some waves are reflected Some waves are refraction Information on wave movement gives a picture of inside of Earth

Figure 2.4

What We Have Learned About Earth from Earthquakes Where magma is generated in the asthenosphere The existence of slabs of lithosphere that have apparently sunk deep into the mantle The extreme variability of lithospheric thickness, reflecting its age and history

Plate Tectonics Large-scale geologic processes that deform Earth’s lithosphere Produce landforms such as ocean basins, continents, and mountains. Processes are driven by forces within Earth

What Is Plate Tectonics? Lithosphere is broken into pieces Lithospheric plates Plates move relative to one another Plates are created and destroyed

Figure 2.5a

Location of Earthquakes and Volcanoes Define Plate Boundaries Boundaries between lithospheric plates are geologically active areas Plate boundaries are defined by areas of seismic activity Earthquakes and volcanoes are associated with plate boundaries

Figure 2.5b

Seafloor Spreading Is the Mechanism for Plate Tectonics At mid-ocean ridges new crust is added to edges of lithospheric plates Continents are carried along plates Crust is destroyed along other plate edges Subduction zones Earth remains constant, never growing or shrinking

Sinking Plates Generate Earthquakes Sinking ocean plates are wet and cold Plates come in contact with hot asthenosphere Plates melt to generate magma Magma rises to produce volcanoes Earthquakes occur along the path of the descending plate

Figure 2.6

Plate Tectonics Is a Unifying Theory Explains a variety of phenomena Convection likely drives plate tectonics Figure 2.8

Table 2.1

Figure 2.9

Rates of Plate Motion Plate motion is fast (geologically) Plates move of few centimeters per year Movement may not be smooth or steady Plates can displace by several meters during a great earthquake Figure 2.12

Figure 2.11

A Detailed Look at Seafloor Spreading Mid-ocean ridges discovered by Harry H. Hess Validity of seafloor spreading established by: Identification and mapping of oceanic ridges Dating of volcanic rocks on the floor of the ocean Understanding and mapping of the paleomagnetic history of ocean basins

Paleomagnetism Earth’s magnetic field can be represented by dipole Forces extend from North to South Poles Caused by convection in the outer core Magnetic field has permanently magnetized some surface rocks at the time of their formation Iron-bearing minerals orient themselves parallel to the magnetic field at the critical temperature known as Curie Point Paleomagnetism is the study of magnetism of such rocks

Magnetic Reversals Volcanic rocks show magnetism in opposite direction as today Earth’s magnetic field has reversed Cause is not well known Reversals are random Occur on average every few thousand years

Figure 2.13

Magnetic Stripes Geologists towed magnetometers along ocean floor Instruments that measure magnetic properties of rocks When mapped, the ocean floor had stripes Areas of “regular” and “irregular” magnetic fields Stripes were parallel to oceanic ridges Sequences of stripe width patterns matched the sequences established by geologists on land

Figure 2.14

Seafloor Age Using the magnetic anomalies, geologists can infer ages for the ocean rocks Seafloor is no older than 200 million years old Spreading at the mid-ocean ridges can explain stripe patterns Rising magma at ridge is extruded Cooling rocks are normally magnetized Field is reversed with new rocks that push old rocks away

Figure 2.15

Figure 2.16

Hot Spots Volcanic centers resulting from hot materials from deep in the mantle Materials move up through mantle and overlying plates Found under both oceanic and continental crust Plates move over hot spots creating a chain of island volcanoes Seamounts are submarine volcanoes Example: Hawaiian Island Chain

Figure 2.17

Plate Tectonics, Continental Shape and Mountain Ranges Movement of plates is responsible for present shapes and locations of continents 180 million years ago there was the break-up of Pangaea Supercontinent extending from pole to pole and halfway around Earth Seafloor spreading 200 million years ago separated Eurasia and North America from southern continents; Eurasian from North America; southern continents from each other 50 Million years ago India crashed into China creating the Himalayas

Figure 2.18a, b

Figure 2.18c, d

Understanding Plate Tectonics Solves Geologic Problems Reconstruction of Pangaea and recent continental drift clears up: Fossil data difficult to explain with separated continents Evidence of glaciation on several continents

Figure 2.19

Figure 2.20

Driving Mechanism Two possible driving mechanisms for plate tectonics Ridge Push and slab pull Ridge push is a gravitational push away from crest of mid-ocean ridges Slab pull occurs when cool, dense ocean plates sinks into the hotter, less dense asthenosphere Weight of the plate pulls the plate along Evidence suggests that slab pull is the more important process

Figure 2.21

Plate Tectonics and Hazards Divergent plate boundaries (Mid-Atlantic Ridge) exhibit earthquakes and volcanic eruptions Boundaries that slide past each other (San Andreas Fault) have great earthquake hazards Convergent plate boundaries where one plate sinks (subduction zones) are home to explosive volcanoes and earthquake hazards Convergent plate boundaries where continents collide (Himalayas) have high topography and earthquakes

Internal Structure of Earth and Plate Tectonics End Internal Structure of Earth and Plate Tectonics Chapter 2