We know much more about the interior of the Earth than the other planets. By observing variations in the speed of seismic waves as they travel through the Earth after earthquakes, we can determine the structure of our planet.
The terrestrial planets share a common structure produced by heating up and melting of these planets very early in their history. During this period, metallic iron and nickle settled into the center of these planets to form a very dense core. These dense cores are detectable in all the terrestial planets by abberations in their gravity and rotation.
Smaller planets like Mercury and our Moon cooled completely within the first billion years or so (give or take a few days!). Mars, which is bigger, developed a stable crust that has remained relatively unchanged for more than 3 billion years. However, the presence of extremely large volcanoes like Olympus Mons, which formed in the last 30 million years, shows that the interior of the planet remains hot. Venus and Earth are the largest of the rocky planets and have very active surfaces. The entire surface of Venus appears to have turned over catastrophically and reformed about 500 million years ago. Another good reason not to live on Venus (not to mention the super hot dense carbon dioxide atmosphere capable of crushing cars).
The Earth is unique in that the ocean crust that makes up 75% of the surface all less than 250 million years old. In fact, most of the ocean crust is very young and crust is being constantly created along huge submarine ridges that circle the globe.
We know much more about the interior of the Earth than the other planets. By observing variations in the speed of seismic waves as they travel through the Earth after earthquakes, we can determine the structure of our planet.
The outer shell is a cold, rigid layer that we call the crust. There are two types of crust: oceanic and continental. Continental crust is generally old (> 1 billion years) and thick (30-40 km). Continental crust is formed mainly of lighter elements silcia, aluminum, potassium, and sodium. Continental crust is mostly "granitic" and is about 2.6 times denser than water. Oceanic crust is all young and thin (5-10 km). However, oceanic crust contains large amounts of the denser elements iron, magnesium, and calcium. Oceanic crust is "basaltic" and is about 2.9 times denser than water. This means that oceanic crust can sink back into the interior of the Earth, whereas continental crust floats like a cork.
Beneath the crust is a 3000 km thick layer called the mantle. The mantle is made up mostly of very dense magnesium-iron minerals called olivine and pyroxene (olivine is the bright green mineral found in Hawaiian basalt). This zone is about 3.3 times as dense as water. Most of this zone is hot, but solid. The convection currents cause the mantle to slowly flow in a plastic manner (like taffy, silly putty, or roofing tar).
The last 3000 km to the center of the Earth is made up by the iron-nickel core. The outer core is molten. Strong convection in the outer core is thought to be responsible for the Earth's magnetic field (the thing that makes your Boy Scout compass work!). Because of extreme pressure, the inner core is solid iron and nickel.
The classical idea is that decay of radioactive elements inside the Earth keeps it hot. However, some scientists now think that frictional drag along the mantle core interface may also produce a large amount of heat. The source of energy for this heat is the rotational energy of the Earth.
Plate tectonics seems to be unique to Earth. Mercury appears to be a "dead" planet, while recent information from Venus suggests that periodically the entire surface is destroyed and recreated (the last time was about 500 million years ago), much like an overturn on an active lava lake. In contrast, Mars has the largest volcanoes in the solar system perched on very thick crust that has not moved in over 3 billion years.
The Earth has active plate tectonics which produces obvious features that are readily visible from space. These include long chains of composite volcanoes marking subduction, ridges snaking around the planet where plates pull apart and fresh magma wells up to the surface, and great mountain ranges where continents have collided, and crumpled against each other. These boundaries are marked by the yellow lines on the relief map of the world.
Plates may be entirely oceanic or a combination of ocean and continental crust. Both the outer part of the crust and mantle are cool and fairly rigid beneath both oceans and continents. This cool outer zone called LITHOSPHERE varies from about 40 km thick beneath oceans to 50-100 km thick beneath some continents. Directly beneath the lithosphere is a zone of partially molten mantle called the ASTHENOSPHERE. The lithospheric plates are able slide a bit over the underlying convecting mantle at this boundary. Spreading ridges vary in rate of opening from about 2-8 cm per year.
There are three principal types of plate boundaries:
SUBDUCTION ZONES: When an ocean plate collides with either a continental plate or another ocean plate, one of the ocean plates will be forced back into the mantle. These zones are marked by chains of stratovolcanoes (like Mt. Ranier or Mt. St. Helens). Volcanism at subduction zones is produced by frictional heating as the cold crust is forced beneath the continent and by release of water from the ocean crust into the overlying mantle. The resulting magmas contain higher amounts of silica and water than mid-ocean ridge magmas. Because the magmas are very viscous, water cannot escape as the molten rock comes to the surface and instead the expanding steam bubbles cause highly explosive eruptions. Any magma that does erupt is very sticky and forms thick flows and domes that make up the steep sides of stratovolcanoes.
Large mountain ranges along the edge of continents commonly form where ocean crust is subducted beneath continetal crust. The Andes along the western edge of South America are an excellent example of this type of range. Where ocean crust is subducted beneath ocean crust volcanic island chains called island arcs form. Classical examples of island arcs are the Aleutian Island, the Marianas Islands, and Japan.
CONTINENTAL COLLISIONS: When two continents collide, neither is subducted. Instead large mountain ranges form in the interior of continental masses. The Himalayas, the highest mountains on Earth, formed this way when India began colliding with China about 40 million years ago. This collision is continuing today as India continues to push northward and the Himalayas continue to grow higher.
Though the San Andreas Fault is the most famous of these, transform faults on mid-ocean ridges far outnumber continental transform faults. Only the area between the two mid-ocean ridge segments is a transform fault (separating oceanic plates moving opposite directions). Outside of this zone, these become large fracture zones within oceanic plates. The best example of a mid-ocean transform fault can be seen on the figure just south of Iceland in the northern Atlantic ocean. The best fracture zones can be seen in the SE Pacific Ocean. This figure shows only submarine topography (the continents are gray) with high elevations in red grading to the lowest elevations in purple.
Interestingly, the highest portions of ocean basins seem to be in the center of the oceans, just opposite of what you might think. And the lowest parts of many of the oceans are near the margins. This is due to plate tectonics. Subduction zones produce the deepest part of the ocean (off the Marianas the ocean is 37000 ft deep!), while mid-ocean ridges make high mountains that can even emerge above sealevel (like at Iceland).
The Hawaiian Islands stand out as a pronounced anomaly on the ocean crust. They form a high ridge that cuts the NW Pacific Basin and appear unrelated to either spreading ridges or transform fracture zones. It is the unique formation of the Hawaiian Islands that we will focus on in this course.
If you have comments or suggestions, email me at kenhon@hawaii.edu