The Most Hated Word in Geology

Hated WordOk, so I’m sure that not everyone hates this word. But, I sure do. I see many kids get frustrated with the subject of rocks just because this word is confusing and teachers often use it without explaining how easy the word really is to understand.

What’s the word? The word is detrital. Its synonym, clastic, is a close second on my “words I hate list.” Number three on my list has nothing to do with geology.

The other night my son was doing his science homework, which happened to be on sedimentary rocks. He got super frustrated trying to find the meaning of the word detrital. He didn’t understand what his teacher said in class and the handouts didn’t really explain the word either. Sometimes I think that we should just ban this word until kids get into college because it throws them off. The word sounds much more complicated than it is. To make it really simple: detrital means that the rock is made out of grains. These rocks are all sedimentary. The most common rocks that you probably know of are sandstone, siltstone, shale, conglomerate and breccia.

There are three general types of sedimentary rocks. The first type is made of grains, like I just said. The second is made from chemical precipitation of minerals from water like limestone, chert, dolostone and rock gypsum. The third type is organic, like coal, which forms from once-living materials.

Geologists classify detrital sedimentary rocks based on the size of the grains. This table shows the different grain sizes and the names of the some of the common rocks that they form. The largest grain sizes are on top and the smallest on the bottom.

Rock Name

Grain Size

Sediment Type

Conglomerate (rounded grains)

Breccia (angular grains)

256 mm
64 – 256 mm
4 – 64 mm
2 – 4 mm

Boulder
Cobble
Pebble
Granule/Gravel

Sandstone

1/16 to 2 mm

Sand

Siltstone

1/256 – 1/16 mm

Silt

Shale

<1/256 mm

Clay

So, if you are teaching sedimentary rocks to kids remember that the words detrital and clastic are hard for kids to grasp. Explain that it just means that individual grains are glued or compressed together to make the rock. If you can think of a more descriptive word than detrital, I’d love to hear it. All I can think of so far is grain rocks and that isn’t good either…

 

Save

Save

Leave a Reply

You must be logged in to post a comment.