The Earth's Surface
- Why the substantial differences between the Earth and the Moon?
- Size matters
- Planets start out internally hot due to the heat of accretion and radioactive decay.
- Over time they cool. The larger the planet
the longer it retains its internal warmth and thus its
geological activity. Resurfacing continues.
- The Moon's crust thickened to the point of terminating geologic activity more than three billion years ago.
- In the absence of erosion/resurfacing its surface preserves the record of billions of years of cratering.
- The Earth is still geologically active. Despite being cratered as heavily as the Moon (the atmosphere does little to stop large asteroids), most of those blemishes have been erased by geologic activity.
- In addition, the Earth is large enough to retain an atmosphere which further erases craters through erosion.
- Earth's defining features
- Few impact craters -- implying active resurfacing.
- Continents and Basins
- Continental areas lie a few kilometers above the
- Unlike Venus, for example, there is very little land at intermediate elevation!
- Oddly, there appears to be just enough water on
Earth to fill up the basins.
- If there was substantially less water the
Earth would be covered with lakes and puddles.
- If there was substantially more water the
land would be completely submerged.
- This apparent coincidence may have grand implications for the development of complex life on Earth.
- Long mountain chains cover the Earth
- The mountain chains seem to prefer the centers of oceans and
edges of continents.
- Mountain chains and boundaries between continents
and basins are associated with earthquakes and volcanoes.
- Continental outlines fit together like a jigsaw puzzle.
- The Earth's crust is thin and fragile. The Earth's
interior is still warm and active.
- Motions in the Earth's interior crumple and break apart the
thin crust and permit fresh molten material to reach the surface.
- The Earth's crust is broken into about 20 crustal
"plates" which are driven about by currents in the mantle below.
- These plates grind against one another, split, and collide producing many of the geological features we see on the Earth's surface today.
- This drifting, colliding, and separating of blocks of crustal
material is called plate tectonics
- Europe and North America move a few centimeters further apart
every year opening the Atlantic Ocean.
- This movement is measured with radio telescopes and
with the Global Positioning System (GPS).
- Two square kilometers of new ocean crust are created each year! It takes only 200 million years to replace the seafloor -- no craters!
- Old crust is destroyed where plates collide as it
subducts back into the Earth's interior.
- Mountain ranges are built where plates collide.
- It takes only 100 million years for erosion to
erase a mountain range.
- If mountains were not continually built by plate
collisions there would be no mountains on the Earth.
- Earthquakes occur where plates move relative to one another.
- In southern California two plates are grinding past one another.
- In about 20 million years Los Angeles will be an island suburb of San Francisco.
- The distribution of continents will continue to change into the distant future
- There are few craters on the Earth, in part, because the Earth is being
continually resurfaced by plate tectonics (and by atmospheric
- The ocean basins are "young" surfaces relative to the age
of the Earth -- the average age is a few hundred million years. Since they are so young they should
be relatively crater free.
Revised November 7, 2007