Everybody Needs a Rock

Aggregates Manager Staff

January 1, 2008

Follow the journey of a pet rock, ‘born’ approximately 1.8 billion years ago.

Author’s notes:

(1) Geologic time is translated into a 24-hour day. The beginning of the earth (4.54 billion years ago) is 00(hrs):00(min):00(sec). The present is 24:00:00. Each second equals about 52,546 years; each minute about 3.2 million years; each hour about 189 million years.

(2) The following scientific abbreviations are used: Ga (billion years ago), Ma (million years ago), Ka (thousand years ago).

My 3-year-old granddaughter, Delaney, loves books. During our last visit we read Everybody Needs a Rock, by Byrd Baylor. The book begins, “I’m sorry for kids who don’t have a rock for a friend.” Well, don’t feel sorry for us. Delaney and I have a friend named Rocky that came from a quarry in Morrison, Colo.

Rocky was born 1.8 Ga (14:29:04) in a sea located between an ancient supercontinent named Laurentia and a collection of volcanic islands (see illustration). Lava and ash from the volcanoes, and sediments from Laurentia, piled up in the sea and, throughout time, were consolidated into rocks. Propelled by plate tectonics, Laurentia swept up the sedimentary rocks and volcanic islands. The rocks were heated and squeezed into gray metamorphic rocks, pushed up to form mountains, and intruded with molten magma that ultimately cooled as pink veins in the rocks. Geologists refer to these very old crystalline rocks as “basement,” and by 1.1 Ga (18:11:06), Rocky’s home in the Colorado basement was finished. Officially, Rocky is referred to as a gneiss (pronounced ‘nice’).

The star indicates Rocky’s birthplace in southwestern United States, 1.8 billion years ago.  Outlines of states are shown by red dashed lines.

Twice during Rocky’s lifetime, once between about 510 to 300 Ma (21:18:14 to 22:24:51), and more recently at 100 Ma (23:28:17), seas washed over Colorado, leaving behind vast areas of sandstone, limestone, and other sedimentary rocks. Some of these rocks subsequently were eroded away; others remain to tell their stories.

Continental plates on the earth’s surface have been drifting together and splitting up since before Rocky was born. Around 300 Ma (22:24:51), a number of plates collided to form a supercontinent named Pangea, and in doing so, pushed up the Ancestral Rocky Mountains. During the next 80 million years (25 min 22 sec), the Ancestral Rockies eroded and shed thousands of feet of sediment on the surrounding basement.

Pangea began to break apart around 180 Ma (23:02:55). From 72 to 40 Ma (23:37:10 to 23:47:19), the Rocky Mountains were thrust into the air, this time from tectonic activity on the western margin of the North American plate. But Mother Nature was not finished. Around 37 Ma (23:48:16), a massive volcanic eruption deposited ash 400 feet thick in the area just south of Rocky’s basement apartment. And to polish things off, glaciers began the final sculpting of the landscape around 1.8 Ma (23:59:26). When the Ice Age ended around 10 Ka (23:59:59.81), Rocky’s home looked much like it does today.

The next few columns will take a closer look at Rocky’s journey through geologic time. We will see how the North American plate wandered around the surface of the Earth; how those movements affected the climate; how climate changes affected life; and more. Hang on! Those articles might just stir the science in your soul!

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