Hotspot in the hot seat

Jennifer Chu, MIT News Office

The Hawaiian archipelago, and its chain of active and extinct volcanoes, has long been viewed as a geological curiosity. While most volcanoes arise at the boundaries of shifting tectonic plates, the Hawaiian chain lies smack in the middle of the Pacific plate, nowhere near its borders.

Now a study by researchers at MIT and Purdue University, published this week in Science, paints an unexpected picture of what’s beneath Hawaii. Using a new imaging technique adapted from uses in oil and gas exploration, MIT’s Robert van der Hilst and colleagues produced high-resolution images that peek hundreds of kilometers below the Earth’s surface.

They found a hotspot — but not where many scientists had thought it would be. Instead, the MIT team found evidence of hot mantle activity some 600 kilometers deep and 2,000 kilometers wide, in an area far west of the “Big Island” of Hawaii.

Many geologists had thought the Hawaiian Islands resulted from a stationary plume of white-hot material rising from the Earth’s lower mantle, spewing out masses of magma in fits of volcanic eruption. This theory held that the massive Pacific plate, moving slowly northwestward, carries newly formed volcanoes away from the hotspot, forming the Hawaiian island chain seen today.

According to the theory, the Big Island, the newest formation in the chain, sits directly over the blistering plume. Scientists have attempted to characterize this hotspot for decades, believing that if a plume exists, it may be a window into the Earth’s deep processes that could help quantify how the Earth loses heat from its core.

“The implication [of this new work] is that there is no simple, deep plume directly beneath Hawaii,” says Van der Hilst, the Cecil and Ida Green Professor of Earth and Planetary Sciences at MIT, and director of the Earth Resources Laboratory. “So the textbooks on Hawaii will have to be rewritten.”

Heat wave

The team developed a new deep-Earth imaging technique using seismic- and mineral-physics data to determine the temperature of the Earth at various depths. Extreme temperature profiles, they reasoned, might suggest plumes or hotspots.

Seismic waves travel through the Earth’s interior at speeds that are primarily influenced by temperature: The higher the temperature, the slower the waves. For years, seismologists have used seismic wave speeds to create — much like CAT scans — 3-D views of the Earth’s internal structure. This tomographic technique works well near earthquake sites or below vast networks of seismographic sensors. But Hawaii, as Van der Hilst observes, is in a no-man’s land of seismic data, far from any tectonic upheaval and adequate seismograph arrays.

Van der Hilst  — along with co-authors Qin Cao, an MIT graduate student; mineral physicist Dan Shim, associate professor of earth, atmospheric and planetary sciences at MIT; and Maarten de Hoop of Purdue University — came up with a new technique, combining seismic data and mineral physics to map temperatures in the Earth’s mantle. The team first collected all available seismic data from the Incorporated Research Institutions for Seismology Data Management Center, based in Seattle, which collects and distributes seismic information to the research community. This amounted to more than 100,000 records of seismic waves from more than 5,000 earthquakes in the last 20 years.  Much of the data came from the so-called “Ring of Fire,” a massive horseshoe of seismic and volcanic activity surrounding the entirety of the Pacific Ocean.

The team then modified a technique used in the oil and gas industry. Typically, companies such as Shell and Exxon Mobil create seismic shocks, and then listen to the echoes that bounce back. The seismic reflection creates a map of the underlying rock compositions, and clues to where oil and gas might lie.

Instead of creating shocks, Van der Hilst’s team took advantage of Earth’s natural shocks — earthquakes — and analyzed seismic waves as they reflected off the rocks underneath Hawaii. By analyzing seismic reflections, the team determined mineral compositions at various depths, noting the boundaries at which minerals changed. Knowing at which pressures and temperatures such boundaries occur in laboratory simulations, they were able to map out the temperatures deep beneath Hawaii.

Seismic shift

Cao, the lead author of the study, developed an algorithm that worked the massive amount of seismic data into deep-Earth temperature maps, revealing the newfound hotspot west of Hawaii. Van der Hilst says the discovery of this 2,000-kilometer-wide anomaly refutes the popular theory of a narrow, pipe-like plume rising straight up to Hawaii from the core-mantle boundary — a finding he anticipates will shake up the geodynamical and geochemical communities studying mantle convection.

Yang Shen, a professor of seismology and marine geophysics at the University of Rhode Island, says the new imaging technique provides much higher-resolution images of the Earth’s mantle than previous techniques, and may change the conventional wisdom on Hawaii’s hotspots.

“The observation is intriguing because it does not fit nicely within the current plume model,” Shen says. “So I think the paper will force us to rethink … mantle plumes and convection.”

Cao is now refining the mapping algorithm, and plans to make it accessible to other researchers in the next few months. As countries set up more earthquake monitors in the coming years, Van der Hilst says the new imaging technique will allow seismologists to draw up higher-resolution images of deep-Earth processes.

“I think this could be the technique of the future,” Van der Hilst says. “The receiver networks are exploding, and in the next five to 10 years we can probably do even more spectacular things.”

http://web.mit.edu/newsoffice/2011/hawaii-hotspot-0527.html

博友vividfn 在六一儿童节给出了参考译文，谢谢他了！

    长久以来，地质学者们都对夏威夷群岛及其活火山、死火山带兴趣十足。绝大总份火山都出现在漂浮板块的边缘地带，然而夏威夷火山带刚矗然出现在太平洋板块的中部，而非边缘。
    本周，《Science》刊登了近期一些MIT、普渡大学的研究人员绘制的夏威夷地下图像。MIT的Robert van der Hilst与他的同事们运用油气勘探中的成像技术，绘制出了地下数百公里的高分辨率图像。
他们发现了一个火山热点，虽然其他学者不以为然。此外，MIT的研究小组发现有迹象表明在夏威夷大岛的西部约600KM深处，有近2000KM宽的热地幔活动。
    许多地质学家认为夏威夷群岛是在下地幔的白热物质在稳定羽流下，由火山喷发出的大量岩浆形成的。该理论认为,巨大的太平洋板块向西北方缓慢移动,带动新形成的火山远离原热点,形成了我们今天看到的夏威夷火山带。
该理论表明，这个火山带上后期发育出夏威夷大岛，正好直接落在了炙热的羽流上。在过去的几十年间，科学家们都在尝试描述这一热点区域，他们认为：如果真的存在这个羽流，那么我们可以透过它，来窥见地心是如何通过深部的运动来释放热量的。
    “这一全新的工作可能意味着夏威夷地区的正下方并不存在一个简单的深层羽流，”Van der Hilst——MIT地球和行星科学教授;地球资源实验室主任说道，“所以，教科书上关于夏威夷的内容可能将要改写。”

    热波
    该团队开发了一种新的深层成像技术：运用地震与地矿数据，来确定不同深度的温度。他们推论：极限温度剖面可能预示着羽流或热点。
    影响地震波传播速度的首要因素就是温度：温度越高，速度越低。多年以来，地震学家都是用地震波来制作地球内部结构的三维图像，就像造影扫描图一样。该层析技术在地震区域或大量地震感应仪器的覆盖区域下是十分有效的。但是，如 Van der Hilst观察所得：夏威夷地区没有人工地震数据，远离构造运动，也没有足够的地震记录资料。
    Van der Hilst 与麻省理工学院硕士研究生Qin Cao，矿物物理学家Dan Shim——地球大气和行星科学副教授，普渡大学的Maarten de Hoop，共同研发了一项新技术，结合地震资料与矿藏物理，绘制出地幔温度图。
首先，他们从地震数据管理中心（该机构总部设立于西雅图，负责向各研究单位收集、分发地震信息）收集可用的地震数据——过去20年中，超过5000次的地震中记录的超过10万道地震波记录。它们绝大多数来自于环绕整个太平洋的马鞍形的太平洋火圈。
    然后他们再利用油气勘探中的相关技术：通常,如壳牌和埃克森美孚等公司会在勘探中激发地震波，再记录其反射波，由此生成地下岩性结构图，再以之找寻油气。
    不同的是，Van der Hilst他们利用天然地震或冲撞的反射波来分析夏威夷地下的岩层。通过分析反射波，他们确定不同深度的岩石成份，并划分界面。通过在实验室中模拟而得的不同压力、温度下的界线，他们就可以标出夏威夷地下不同深度对应的温度。

    地震引发的变形
    这项研究的主要发起者Qin Cao改进了一种算法，使大量的地震数据得以转化为深度-温度图，从而揭示了夏威夷西面新发现的热点区域。 Van der Hilst说该发现反驳了现在的流行观点：夏威夷地下存在一个形成于地心与地幔边界处的狭窄管状羽流。他认为这一发现将震动研究地幔对流的地球动力学与地球化学组织。
罗德岛大学地震学和海洋地球物理学教授Yang Shen说：“与先前技术想比，这一技术对地幔成像的分辨率更高，也许会改变人们对夏威夷地区热点的传统看法。”
   “观察结果十分有趣,因为它没有很好地符合当前的羽流模型,”Yang Shen说，“所以,我想,该论文将迫使我们重新思考羽流和对流…。”
    Qin Cao目前正在改进映射算法，并计划在接下来的几个月里使其被其他研究人员接受。Van der Hilst 表示：将来，随着国家设立更多的地震检测点，这一新技术能使地震学家们绘出高精度的深层图像。
    Van der Hilst 表示：“我认为这一技术在将来会被广泛运用，接收网络的铺设工作正在迅速展开，在未来5到10年内，我们能做的可能会更多。”

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