What keeps the continents from eroding away?

Question

I know that, after a few million years, mountain ranges and islands erode down to nothing. After four and a half billion years, why haven't the continents eroded away into the oceans, or been destroyed by subduction? What geological mechanism is at work?

Answer 1

As the mountains are eroded, the sediment is carried away and deposited off shore. The weight of this sediment causes surrounding lands to spring up - a process known as uplift. This creates new mountain ranges. So, basically it is simply a process of moving material from one place to another.

Answer 2

It is easy to explain why old continental crust has not been subducted--it is too buoyant. But, I think the other part of your question has not been adequately answered--specifically, why is there still some Precambrian continental crust that has not been eroded away? Even though mountains don't actually "erode down to nothing" in only a "few million years", hundreds of millions of years is sufficient to plane down a mountain range. However, at some point, after being eroded down to a fairly flat surface, Precambrian rocks formed the basement beneath shallow seas and great flat river basins. In other words, they became buried by sediment and, therefore, protected. You can imagine this by thinking of all the sedimentary strata covering Precambrian rocks on the Colorado Plateau. We know that older Precambrian rocks lie beneath, because they are exposed deep in the bottom of the Grand Canyon. The Precambrian shields of the continents must have been covered during most of Phanerozoic time.
Mountain building around the edges of the continents, caused by tectonic processes, kept adding buoyant rocks to the perimeter of continents. In some cases, great overthrusts covered Precambrian rocks, protecting them from later erosion.

Answer 3

Erosion is only one part of the equation. Land is also uplifted by the activity of tectonic plates. So, as the old mountain ranges wear away, new ones form.

A good example of this on a small scale is what happened to the island of Simeulue after the 2005 earthquake.

"BANGKOK, Thailand - A 2005 earthquake off the coast of Indonesia raised an island nearly four feet out of the water..."
http://news.yahoo.com/s/ap/20070413/ap_on_sc/indonesia_coral_deaths;_ylt=Anhl2kQUszT4.l03qFhDNL1SbA8F

Besides tectonic plate activity, you also have volcanoes raising the bar, so to speak. The Hawaiian Islands are a good example of that process. The islands are raised by volcanic activity, then slowly eroded as they move away from the hot spot.

JMB

Answer 4

Eroding Away

Answer 5

First of all, the tectonic plates are not what is characterized as a "Total Loss" system. They rise and fall according to the mantle's density, plus the weather's ambient temperature determines the coastline: ice ages lower the sea level.
Also sediment from rivers deposit in deltas and lowlands and change the landscape from sea to land.
The Appalachian mountains are older than the Rockies, that are older than the Sierras, and the heights are lower according to their age. They're all inland mountains for the most part.
There's a helluva lot of mechanisms at work, and erosion is just one part.

Answer 6

Plate tectonics is at work. There is nothing that keeps the continents from eroding away. Given enough time the continents we know and recognize will erode, join together, pull apart and form new continents.

Answer 7

Geologic activity builds them up. For example, an earthquake in California some years ago was reported to have added a foot to the height of the local mountains.

Answer 8

Uplift, orogeny. E.G. crustal plates colliding (Himalayas), upwelling of magma (Hawaiian Islands).

Answer 9

volcanoes create land mass

Answer 10

study up on isostacy...this will help out