加州地質簡史 — A Brief Geological History of California

（如果您喜歡這篇文章，請至下列網頁按〝like〞 ，以後就可以比較快得知此部落格的新動態。）
https://www.facebook.com/gliding.cloud  April 23, 2013

 

(Please scroll down for a rephrase in English.)

      最近我在離家不遠的一座公園找尋菟絲花，以便拍攝照片和大家分享時，附近有一塊在附圖一所顯示的裸岩，看起來並不起眼，但是卻透露出加州地質歷史的許多故事。這塊裸岩的不尋常點，在於它的岩層走向幾乎是垂直的。像這樣垂直走向的沉積岩層，通常只有在地層曾經被劇烈扭曲過的地方才見得到。但是這塊裸岩所在的地方很接近舊金山海灣的東南水濱，周圍是一片開闊的沖積地，並沒有地層被劇烈扭曲的歷史。這到底是怎麼回事呢？

 

      在過去有一段很長的時間，有一個巨大的、被名為〝Farallon 〞的海洋板塊，緩慢地從西邊被擠壓到北美大陸板塊之下。這個期間很長，也造成了北美洲大陸西部很大的變化。我把這個期間快結束時的情況，幫大家在附圖二的上半部，作了一個簡化了的描繪。

 

      當Farallon海洋板塊被擠壓到一定深度之後，它會被熔回成岩漿。而板塊被壓入所帶來的額外壓力，則會迫使岩漿找岩石間的縫隙往上竄昇。有些岩漿終於找到出路，竄出地面而形成火山。有些則到了半途，就被阻在一些岩漿室裏。當時這些岩漿，經年累月地形成了一個巨大的岩漿室，因為它距離地表不遠，而慢慢地冷卻成為一塊巨大的花崗岩。後來這塊花崗岩，由於其它的地層活動而被抬高。在表土被侵蝕掉之後，就成了今天位於加州與內華達州交界的內華達山脈 (Sierra Nevada)。 (也可以意譯成〝雪的山脈“。)

        Farallon海洋板塊的南端被用光了，而代之而來的新鄰居，是〝太平板洋海洋板塊〞。它的走向相對於北美大陸板塊，則是由南至北。所以地層運動的情況，就變成如我在附圖二的下半部所示。這兩個 phases 之間的轉換時間，在南加州大約是兩千五百萬年前，(英文資料上常會簡寫成 25 MYA, 〝25 million years ago〞。) 而大約在一千五百萬年前到達舊金山灣區，今天則已經快到達加州和奧瑞岡州的邊界了。如今在奧瑞岡州和華盛頓州的外洋底下，還有幾小片Farallon海洋板塊的殘餘。其中一片〝Juan de Fuca 海洋板塊〞比華盛頓州的面積還大。這幾片殘餘板塊還正被推擠到北美大陸板塊之下，這種推擠正是造成 1980 年華盛頓州Mount St. Helens 火山爆發的原因。
 

 

       Farallon海洋板塊被推擠到北美大陸板塊之下的過程中，有些在Farallon海洋板塊比較突出的表層，會被刮下而黏附到北美大陸板塊上，這些畸零岩層岩塊，有時會凸出地表成為地標。像舊金山海灣裏的著名觀光景點〝惡魔島〞 (Alcatraz Island) ，和做為海灣大橋 (Bay Bridge) 中途著地點 的〝薄荷島〞 (Yerba Buena Island) ，以及兩處海灣水濱兀然凸起的小丘 Albany Hill 和 Coyote Hills， 都是這樣來的。我所附的裸岩照片，就是 Coyote Hills 的一部分。

      這些岩塊雖然看起來並不起眼，它們可是歷時悠久，大概與眾恐龍們同壽（老於六千六百萬年前）。而且它們形成的地方，通常是千里之外的太平洋洋底。所以它們可以提供一些寶貴的遠古大氣和海洋的資訊。Marin County 和 Mt. Diablo 是探訪這些遠來耆宿的好地方。

       在附圖二，我刻意地把大陸板塊和海洋板塊畫得不一樣厚度，因為事實上就是如此。一般來說，大陸板塊不僅比海洋板塊要來得厚，比重也比較輕。所以當兩者一起飄浮在熔岩層上時，大陸板塊的上表面就會明顯高出海洋板塊的上表面。所以地球表面那裡會是陸地？那裡會是海洋？ 其實並非偶然，而是有一些根本性的原因的。

        A couple of weeks ago when I was looking for dodders to photograph for Facebook friends, I passed by a rock outcrop as shown in the first attached picture. Although this rock appears to be ordinary, it betrays the long history of California geology. The unusual feature of this outcrop is that its layers are virtually aligned vertically. A sedimentary rock with such a layer arrangement usually can only be seen at sites where there had been violent geologic twisting and folding. However, the outcrop in the picture is located near the southeastern shore of San Francisco Bay and is surrounded by wide-open alluvial plane and tidal mud flat. There is no history of violent geologic twisting. What is going on?

        Over a very long period of time in the past, an oceanic plate called “Farallon” has been slowly pushed underneath the North American continental plate. This period lasts for eons, and ends up with huge geometrical changes to the western part of North American continent. To depict the situation near the end of this period, I have made a simplified drawing in the upper half of the second attached picture.

        When the Farallon oceanic plate is subducted to a certain depth, it will be turned back into magma. The extra pressure brought in by the subduction forces magma to find fissures in North American continental plate to rise up. Some magma managed to find its way out in the form of volcano. Other can be trapped in magma chamber on its way up. At that time, there is a gigantic magma chamber formed, and subsequently slowly cooled into granite. This piece of granite was subsequently lifted by other geological activities. After the top soil was eroded, it became the Sierra Nevada we have today.

        Eventually, the southern end of Farallon oceanic plate was completed consumed. The new neighbor, Pacific oceanic plate, mainly moves from south to north. As a result, the geologic process becomes what I depicted in the bottom half of the second attached picture. The transition between these two phases began in Southern California about 25 million years ago (25 MYA). The transition reaches San Francisco Bay Area around 15 MYA. Today, the transition point has almost reached the border between California and Oregon. Beneath the ocean off Oregon State and Washington State, there are still few remnant pieces of Farallon oceanic plate. One of them, the “Juan de Fuca” oceanic plate, is larger than Washington State. These remnant pieces are still being pushed underneath the North American continental plate. This subduction is the cause for the eruption of Mount St. Helens volcano in 1980.

        In the subduction process of Farallon oceanic plate, the more prominent portions of its surface layers can sometime be scrapped away and accreted onto North American continental plate. Occasionally, a small piece can protrude out of surface and become a salient geographic feature. Examples in San Francisco Bay include the well-known tourist attraction, the Alcatraz Island, and Bay Bridge’s mid-point anchor, the Yerba Buena Island, as well as two hills jutting out near the Bay’s shore, the Albany Hill and the Coyote Hills. The first picture I attached here was taken at Coyote Hills.

In spite of their ordinary look, these rocks are extremely ancient, being about the same age as dinosaurs. (> 66 MYA). Furthermore, they were usually formed thousands of miles away as well as deep beneath Pacific Ocean. Hence, they can provide useful information on the archaic atmosphere and ocean. In San Francisco Bay Area, Marin County and Mt. Diablo are two best places to visit these ancient guests coming from far, far away.